Laboratory Manual Communications Laboratory Ee 321
LABORATORY MANUAL COMMUNICATIONS LABORATORY EE 321
© Khosrow Rad Revised 2011
DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING CALIFORNIA STATE UNIVERSITY, LOS ANGELES Lab-Volt Systems, Inc
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BRIEF CONTENTS
Experiment 1 Introduction to Analog Communications 4 Exercise 1-2 (Familiarization with the AM Equipment) 11 Exercise 1-3 (Frequency Conversion of Baseband Signals) 15
Experiment 2 The Generations of AM Signals 22 Exercise 2-1 (An AM Signal) 29 Exercise 2-2 (Percentage Modulation) 35 Exercise 2-3 (Carrier and Sideband Power) 45
Experiment 3 Reception of AM Signals 55 Exercise 3-1 (The RF Stage Frequency Response) 61 Exercise 3-2 (the Mixer and Image Frequency Rejection) 65
Experiment 4 Reception of AM Signals 69 Exercise 4-1 (The IF Stage Frequency Response) 73 Exercise 4-2 (The Envelope Detector) 77
Experiment 5 Single Sideband Modulation –SSB 84 Exercise 5-1 (Generating SSB signals by the Filter Method) 91
Experiment 6: Fundamentals of Frequency Modulation 96 Exercise 6-1 (FM Modulation Index) 102 Exercise 6-2 (POWER DISTRIBUTION) 107
Experiment 7 Generation of FM Signals 117 Exercise 7 Direct Method of Generating FM Signals 121
Experiment 8 Exercise 8 (Indirect Method of Generating FM Signals) 126
MATLAB 132
These laboratory note are reproduced in part from the Lab-Volt 2
EE321 Dr. Rad
Experiment 1
Part 1: Exercise 1-2 (Familiarization with the AM Equipment) Part 2: Exercise 1-3 (Frequency Conversion of Baseband Signals)
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INTRODUCTION TO ANALOG COMMUNICATIONS
OBJECTIVE At the completion of this unit, you will be able to describe the basic principles of analog radio communications, the ANALOG COMMUNICATIONS circuit board, and the balanced modulator.
UNIT FUNDAMENTALS
A block diagram of a communication system is shown.
Communication is the transfer of information from one place to another.
A bidirectional communication system operates in opposite directions. The receiver can respond to the sender.
Radio communication uses electrical energy to transmit information. Because electrical energy travels almost as fast as light, radio communication is essentially instantaneous.
A radio transmitter converts audio (sound) signals to electrical signals that are sent over wires or through space.
A radio receiver converts the electromagnetic waves back to sound waves so that the information can be understood.
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The transmitted information is the intelligence signal or message signal
In this course, the term message signal refers to the transmitted information. Message signals are in the Audio Frequency (AF) range of low frequencies from about 20 Hz to 20 kHz.
The Radio Frequency (RF) is the carrier signal. Carrier signals have high frequencies that range from 10 kHz up to about 1000 GHz.
A radio transmitter sends the low frequency message signal at the higher carrier signal frequency by combining the message signal with the carrier signal.
Modulation is the process of changing a characteristic of the carrier signal with the message signal. In the transmitter, the message signal modulates the carrier signal.
The modulated carrier signal is sent to the receiver where demodulation of the carrier occurs to recover the message signal.
The three principal forms of modulation are the following:
1. Amplitude Modulation (AM) 2. Frequency Modulation (FM) 3. Phase Modulation (PM)
FM and PM are types of angle modulation.
In modulation, the message signal changes the amplitude, frequency, or phase of the carrier signal.
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To prevent interference, every radio communication transmits at its own frequency.
The designations for carrier frequency ranges are shown.
The ANALOG COMMUNICATIONS circuit board contains the following circuit blocks:
• VCO-LO • AM/SSB TRANSMITTER • PHASE MODULATOR • QUADRATURE DETECTOR • VCO-HI • AM/SSB RECEIVER • PHASE-LOCKED LOOP
These circuit blocks permit you to configure transmission and reception circuits for amplitude, frequency, and phase-modulated signals.
On the ANALOG COMMUNICATIONS circuit board, a versatile Integrated Circuit (IC) called a balanced modulator performs the following functions:
• amplitude modulation • Double-Sideband (DSB)modulation • mixer • product detector • phase detector
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NEW TERMS AND WORDS audio - signals that a person can hear. electromagnetic waves - the radiant energy produced by oscillation of an electric charge. intelligence signal - any signal that contains information; it is also called the message signal. message signal - any signal that contains information; it is also called the intelligence signal. Audio Frequency (AF) - frequencies that a person can hear. AF signals range from about 20 Hz to 20 kHz. Radio Frequency (RF) - the transmission frequency of electromagnetic (radio) signals. RF frequencies are from about 300 kHz to the 1,000,000 kHz range. carrier signal - a single, high-frequency signal that can be modulated by a message signal and transmitted. Modulation - the process of combining the message signal with the carrier signal that causes the message signal to vary a characteristic of the carrier signal. demodulation - the process of recovering or detecting the message signal from the modulated carrier frequency. Amplitude Modulation (AM) - the process of combining the message signal with the carrier signal and the two sidebands: the lower sideband and the upper sideband. Frequency Modulation (FM) - the process of combining the message signal with the carrier signal that causes the message signal to vary the frequency of the carrier signal. Phase Modulation (PM) - the process of combining the message signal with the carrier signal that causes the message signal to vary the phase of the carrier signal. angle modulation - the process of combining the message signal with the carrier signal that causes the message signal to vary the frequency and/or phase of the carrier signal. balanced modulator - an amplitude modulator that can be adjusted to control the amount of modulation. Double-Sideband (DSB) - an amplitude modulated signal in which the carrier is suppressed, leaving only the two sidebands: the lower sideband and the upper sideband. mixer - an electronic circuit that combines two frequencies. product detector - a detector whose audio frequency output is equal to the product of the Beat Frequency Oscillator (BFO) and the RF signal inputs. phase detector - an electronic circuit whose output varies with the phase differential of the two input signals. envelopes - the waveform of the amplitude variations of an amplitude modulated signal. sidebands - the frequency bands on each side of the carrier frequency that are formed during modulation; the sideband frequencies contain the intelligence of the message signal. AM - an amplitude modulated signal that contains the carrier signal and the two sidebands: the lower sideband and the upper sideband.
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bandwidth - the frequency range, in hertz (Hz), between the upper and lower frequency limits. harmonics - signals with frequencies that are an integral multiple of the fundamental frequency. Beat Frequency Oscillator (BFO) - an oscillator whose output frequency is approximately equal to the transmitter's carrier frequency and is input to a product detector EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the AM / DSB / SSB Generator and the AM / DSB Receiver, as well as terminology used in amplitude modulation.
Discussion Modulation is the process of adding information, also called intelligence, to a high frequency radio wave for communication over long distances. This process depends on the type of modulation used, but in general, the amplitude of the information signal is used to vary the amplitude, phase, or frequency of the radio wave. In this manual, the information signal will be referred to as the message, which is usually a low frequency audio signal in the 20 Hz to 2O kHz range. The radio frequency (RF) signal is known as the carrier, and the frequencies of the message and the RF carrier are symbolized by f m and f S respectively. In amplitude modulation, the amplitude of the carrier wave is made to vary in accordance with the message signal. The waveform of a typical AM signal is shown in Figure 1-1. lt represents a high frequency carrier modulated by a sine wave. Notice the dashed curve drawn through the peaks and valleys of the AM waveform. This is called the envelope and tl is identical to the waveform of the message signal.
Figure 1-1. A typical AM signal.
Familiarization with the AM Equipment When the RF carrier wave is amplitude modulated, sidebands (or sideband fre- quencies) are produced. For a 2-kHz tone that modulates a 1000 kHz (1 MHz) carrier, the sideband frequencies are .f c + fm = 1 002 000 Hz, and fc – fm = 998 000 Hz. Figure 1-2 shows the frequency components of the AM signal.
Figure 1-6. Frequency components of an AM signal.
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1. A system for AM transmission and reception is built from components shown in figure 1.7.
Fig 1.7 Lab-Volt modular communications lab components.
Equipment Required
RF NOISE FREQUENCY GENERATOR COUNTER
AM/DSB/SSB AM/DSB GENERATOR RECEIVER
Function DUAL AUDIO Generator A AMPLIFIER OSCILLOSCOPE
Function POWER SPECTRUM Generator B SUPPLY ANALYZER
Brief description of the equipment: 1) Function generator A and B: These two function generator is used to generator message signals.
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2) AM/DSB/SSB GENERATOR: It is used to for amplitude modulation and generator either double side band or single side band waveforms for transmission. 3) RF Noise Generator: This can be used to generate RF noise. 4) Power Supply: It is used to supply power to equipment 5) Dual Audio Amplifier: Use for amplifier signals from the receiver to a proper listening level. 6) AM/DSB Receiver: This equipment receives the amplitude modulated signals and demodulate it before sending it to audio amplifiers 7) Frequency Counter: This equipment can be used for monitoring frequency of signals. 8) Spectrum Analyzer: This equipment is used for displaying signals in frequency base for analysis. 9) Oscilloscope: Use to display waveforms in time domain. .
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Part 1: Exercise 1-2 (Familiarization with the AM Equipment) Purpose: The purpose of this part of the experiment is to get familiar with the AM/DS/SSB Generator and the AM/DSB Receiver. It is also the purpose to get familiar with some terminology used in AM modulation.
Step 1: This step is to set up the equipment as follow:
FUNCTION FREQUENCY GENERATOR A COUNTER
Function AM / DSB Generator B RECEIVER
AM/DSB/SSB GENERATOR OSCILLOSCOPE
DUAL AUDIO POWER SUPPLY AMPLIFIER
Set up the equipment in a manner where it will be easy to counter the appropriate modules together. We need to make sure all the level and gain control is set to the minimum to avoid any incidents.
Step2: Observe the variation as the result of the RF tuning from the AM/DSB/SSB generator. This can be accomplished by looking at the output of the generator at the oscilloscope as the RF tuning frequency is varied. The test is done with the gain and level control at the maximum level.
Step 3: Find out the upper and lower limits in terms of frequency of the RF Tuning control of the AM/DSB/SSB generator. This can be accomplished by turning the control knob fully counterclockwise to get the lower limit, and fully clockwise to obtain the upper limit. Result: flower= fupper=
Step 4: Set the carrier frequency to 1000kHz and observe the results of the output due to changes from the FR Gain control and the Carrier Level control. The frequency can be set by connecting the output of the AM/DSB/SSB generator to the frequency counter.
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The effects the controls have on the waveform can be observe by the display of the oscilloscope.
Step 5: Set a message signal of 2kHz with amplitude of 400mVp-p. It can be accomplished by connecting the output of the signal generator to the oscilloscope, adjust the amplitude and the frequency, use the oscilloscope to verify the settings.
Step 6: Inject the message signal from step 5 into the Audio Input of the AM/DSB/SSB generator and observe the result of the carrier modulated by the message signal from the oscilloscope.
AM waveform
Step 7: Vary the RF Gain and observe the result of the waveform displayed on the oscilloscope. .
Step 8: What happens when the carrier level is varied. It can be accomplished by varying the Carrier Level control between min and max position.
Step 9: Observe the change of the modulated waveform as we vary the frequency of the message signal. .
Step 10: Adjust the message signal back to 2kHz and set the RF Gain to a one-quarter turn clockwise.
Step 11: Connect the output of the AM/DSBRF OUTPUT of the Am generator to the RF INPUT OF THE AM/DSB Receiver. Connect the Audio Output of the receiver to the Audio Amplifier. Set the listening level to a comfortable level using headphone. The connections were done by using BNC cables linking the generators to receivers, receivers to amplifiers.
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Step 12: Determine the frequency of the Local Oscillator of the receiver. This can be done by varying the RF tuning control until the signal is the loudest on the headphone. Result Step 13:
Determine the flo for a carrier frequency of 1510kHz. From the relationship flo = fc + fIF, fIF from Step 12 is determined to be 457kHz from fIF = flo-fc. .
Step 14: Adjust fc to 1100kHz. This step can be done by connecting the output of the AM generator and temporary disconnect the Audio Input. Adjust the RF Tuning control to obtain 1100kHz display on the frequency counter.
Step 15: Reconnect the message signal to the AM generator, readjust he RF gain to the one-quarter counter-clock wise. Retune the receiver to pick up the new broadcast and obtain the Local Oscillator frequency.
Step 16: Calculate flo – fc for steps 12 and 15. Result: Step 12: Step 15: .
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Review Questions: 1) What is amplitude modulation:
2) Sketch an AM waveform, as well as its representation in the frequency domain, label clearly the carrier, envelope, USB and LSB.
3) What are the USB and LSB frequencies for a 960-kHz carrier modulated by a 4- kHz wave? fUSB = fLSB = 4) What are the two equations showing the relationships between fLO, fc, and fIF? 1. fLO = 2. fLO = 5) Which is more useful for the analysis of communications signals, time domain observations or frequency domain observations? Explain. .
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Part 2: Exercise 1-3 (Frequency Conversion of Baseband Signals) Purpose: The purpose of this part of the experiment is to use the AM communications modules and the Spectrum Analyzer to demonstrate frequency conversion (translation) of baseband signals.
DISCUSSION lf you have already seen telecommunication installations, you will have noticed that there are many kinds of antenna structures. They vary in size from small to very large and yet, they are all used to perform the same function - communication using radio frequency signals. To be effective, the size of an antenna should be at least one-half the wavelength of the radio frequency. This means that a 1000 Hz signal having a wavelength of 300 km, would require an antenna 1b0 km long - not a very practical size. One way of avoiding this problem is to move (translate) the frequency contents of the message to a higher place in the frequency spectrum. Thus, a 1000-Hz signal that is converted to 1000 kHz before transmission only requires an antenna 150 meters long. As a general rule, the higher the radio frequency, the smaller the antenna.
A mixer (multiplier) (Figure 1-3 (a)) can be used to perform the process of frequency translation. Figure 1-3 (b) shows the effect of combining two signals through a mixer. The frequency contents of the message signal (f-) are displaced in the frequency spectrum to a position centered around the RF carrier frequency (fs). The sidebands are a result of the frequency conversion process, which causes duplication of the frequency contents of the message on each side of the carrier frequency. Mathematically, this corresponds to multiplying the message signal by the carrier signal. Note that in Figure 1-3 (b) the carrier is shown as a dotted line. This is because the theoretical output of a balanced mixer does not contain a carrier component.
Figure 1-3. Frequency translation of a message signal.
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Step 1: Set up the equipment as follow:
RF NOISE FREQUENCY GENERATOR COUNTER
AM/DSB/SSB AM/DSB GENERATOR RECEIVER
Function DUAL AUDIO Generator A AMPLIFIER OSCILLOSCOPE
Function POWER SPECTRUM Generator B SUPPLY ANALYZER
Step 3: Set the function generator A to frequency of 2kHz, peak-peak amplitude of 200mV, signal generator B to frequency of 3kHz and peak-peak amplitude of 300mV. This can be accomplished by connecting the output of each function generator to the oscilloscope. Set the desire frequencies and amplitude. Verify the setting from the oscilloscope.
Step 4: Set the AM/DSB/SSB Generator output frequency to 1100kHz. This frequency will be the carrier frequency. This step can be accomplished by the following setting. Carrier Level and RF Gain controls are set to the maximum level. The carrier Level knob is pushed into the Linear Overmodulation position. Use the frequency counter to verify the frequency of the output.
Step 5:
Determine the expected fLSB(lower sideband frequency) and fUSB(upper sideband frequency).
Step 6: Step is to verify the lower sideband frequency and the upper side band frequency using the spectrum analyzer.
fLSB =
fUSB =
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Step 7: Observe a combine 2kHz message with a of 3kHz message in the spectrum analyzer. The signal can be combined through a T-connector and send them into the Audio Input of the AM generator. The output of the AM generator is then connected to the spectrum analyzer. .
Step 8: Show the spectrum response of the frequency spectrum of the two different messages.
Spectrum Analyzer view of 2 KHz & 1.1 MHz 1. clearly the carrier, envelope, USB, and LSB.
CARRIER
LSB USB
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Spectrum Analyzer view of the 1.1 MHz unmodulated signal
Step 9: Remove all the connection of the AM/DSB/SSB Generator, the cable on the spectrum analyzer, and the cables for the function generator B which produced the 3kHz message.
Step 10: Set up a 3kHz Noise Source using a carrier of 1100kHz. The noise source can be created by connecting the 3kHz since wave from step 9 to the AMPLITUDE MODULATION INPUT of the RF/Noise Generator module. The RF carrier frequency is setup by adjusting the FREQUENCY ADJUST knob.
Step 11: Install the telescope antennal at the AM/DSI RF OUTPUT of the AM generator module. Step 12: Display the signals from the AM generator and the RF/Noise Generator. To accomplish this, the receiving antenna must be install first at the input of the spectrum analyzer. A wire antenna is then wrap around the receiving antenna.
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Step 13: Set up the wireless transmission and receiving between the AM generator module and the AM/DSB Receiver module. This can be accomplished by the following steps: 1) Establish the wireless receiving by installing a telescope antenna on the RF Input of the AM/DSB Receiver module. 2) Send the audio signal to the audio amplifier by connecting the Audio Output of the receiver to one of the inputs of the Dual Audio Amplifier. 3) Connect the headphones to one of the jacks on this module.
Step 14: Fine tune the local oscillator frequency to 1555kHz. This is the frequency of broadcast for the 2kHz message signal. This can be accomplished by connect the Frequency Counter to the local OSCILLATOR OUTPUT on the receiver, and turn the RF TUNING knob to obtain a reading of 1555kHz on the Frequency Counter.
Step 15: Find the shadow frequency to the right of the message signal. This can be accomplished by turn the RF TUNING knob on the receiver slightly to the right until a tone is heard. .
Step 16: Adjust one signal toward another signal to observe what happens what they start to interfere with other signals. This can be accomplished by adjust the Frequency Adjust knob on the RF/Noise Generator module to move the 1100kHz signals toward the 1110kHz signal to observe interference.
Step 17: Determine the minimum distance between carriers before interference occurs. To accomplish this, turn the FREQEUNCY ADJUST control knob slightly until the next tone can be heard, turn the FREQUENCY ADJUST control knob slightly back so the tone is not heard. The distance between this point and the first tone is the minimum distance between carrier frequencies before interference.
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Review Questions
1. What happens when messages signal is combine with a carrier signal through a mixer? . 2. What are two reasons for frequency translation? . 3. What is meant by the term baseband?
4. A 1600_kHz carrier is modulated by a baseband signal containing frequencies between 400 Hz and 4kHz. What are the frequency limits for the USB and the LSB?
5. Different AM baseband signals can be broadcast at the same time. What essential condition must be respected to allow an ordinary AM receiver to select each baseband signal individually.
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Experiment 2 (The Generations of AM Signals)
Part 1: Exercise 2-1 (An AM Signal) Part 2: Exercise 2-2 (Percentage Modulation) Part 3: Exerise 2-3 (Carrier and Sideband Power)
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The Generation of AM Signals
EXPERMENT OBJECTIVE when you have completed this unit, you will be able to use an oscilloscope and a spectrum analyzer to analyze AM signals in the time and frequency domains.
DISCUSSION OF FUNDAMENTALS Communication by means of radio waves over long distances requires that we perform certain operations or alterations on the electrical signal which carries the information, before it is transmitted. Upon reception, the reverse operations are applied in order to recover the information. In this unit, the generation of amplitude modulated signals will be studied. ln AM, the amplitude variations of the message signal cause corresponding amplitude variations in the radio wave carrier. This produces a modulation envelope such as you have seen in Experiment 1 (Figure 1-1). when the message signal is a sinusoidal tone, the frequency spectrum of the modulated carrier consists of three components- the carrier frequency (fc), the USB frequency (fc + fm), and the LSB frequency (fc - fm).This is shown in Figure 1-3 of Experiment 1. when the message signal is a more complex waveform, such as voice, the frequency spectrum is correspondingly more complex and contains many more frequency components: This means that a wider frequency space (bandwidth) is necessary to transmit the information. Since the frequency spectrum is limited and there are many users, limitations on bandwidth, carrier frequency spacing, and power output have been developed. These limitations are designed to allow diverse groups and individuals to communicate using radio waves without causing interference to each other. Government and regulatory agencies allocate frequencies and ensure adherence to regulations for a variety of civilian and military communications systems. commercial AM broadcasting, which is usually in the frequency band from 540 kHz to 1600 kHz, is permitted a 10-kHz bandwidth between stations. AM baseband signals, which include voice and music, are limited from about 'loo Hz to 5 kHz. Power limitations vary, depending on the time of day, the season and the distance between stations. ln fact, most commercial AM stations are required to lower their power output or modify their radiation pattern after sunset. This is because nighttime favors AM communications - the radio waves travel much further.
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An AM Signal
EXERCISE OBJECTIVE When you have completed this exercise, you will be able to use the AM / DSB / SSB Generator to demonstrate and explain an AM signal in both the time and frequency domains.
DISCUSSION There are many ways to produce an AM signal, but all of them must allow the amplitude variations of the message signal to be impressed onto the carrier. Figure 2-1 shows a simple modulator that we can use to understand the concept of AM a little better.
Figure 2-1. A simple modulator.
The input to the potentiometer is a fixed-amplitude high frequency sine wave (the carrier). The amplitude of Vout depends on the position of the wiper. lf we move the wiper up and down sinusoidally, we obtain the AM waveform shown in the figure. The sinusoidal movement of the wiper (the message) has been impressed onto the carrier.
The block diagram of Figure 2-2 shows how an AM signal is produced by the AM / DSB / SSB Generator. A dc level (for the carrier level) is added to the message signal. The resulting signal is mixed with the RF carrier to frequency translate the message signal, and is then amplified with the RF amplifier. Figure2-3 shows the waveforms and frequency spectra for an 1100-kHz carrier modulated by a 1O-kHz sine-wave.
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Figure 2-2. Block diagram tor generating an AM signal.
a) Time domain waveforms b) Frequency domain spectra
Figure 2-3. Waveforms and spectra tor the AM signal of Figure 2-2.
Notice that the information (message) has been impressed onto the carrier and that the envelope of the AM signal is an exact copy of the message signal. Also, the envelope varies at the same frequency as the message signal. The effect on the AM signal produced by different message signal amplitudes and frequencies is shown in Figure 2-4.
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a) AM modulation index = 0.40 b) AM modulation index = 0.80 c) AM modulation index = 0.20
Figure 2-4. AM waveforms for different message signal conditions. Amplitude modulation (AM) produces a modulation envelope that has the same waveform as the message signal.
If the message signal were a square wave, the modulation envelope of the AM signal would be a square wave.
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When the message signal is a sine wave, the frequency spectrum of the modulated carrier signal (AM signal) consists of three frequency components.
the carrier (fc) the upper side-band (USB) (fc + fm) the lower side-band (LSB) (fc - fm)
When the message signal is a more complex waveform, such as voice, the frequency spectrum is correspondingly more complex and contains many more frequency components.
Therefore, a wider bandwidth (frequency space) is necessary to transmit the information.
Commercial AM broadcasting, which is usually in the frequency band from 540 kHz to 1600 kHz, has a 10 kHz bandwidth.
AM baseband signals, which include voice and music, are limited from about 100 Hz to 5 kHz.
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NEW TERMS AND WORDS bandwidth - the frequency range, in hertz (Hz), between the upper and lower frequency limits. baseband - the band of frequencies occupied by a message signal. frequency translate - the process of displacing the frequency contents of a signal to another place in the frequency spectrum. modulation index (m) - the ratio between the amplitudes of the message signal and the unmodulated carrier signal. percentage of modulation (% Mod.) - the modulation index expressed as a percentage (m x 100). overmodulation - the term used when the modulation index is greater than 1. It occurs when the peak amplitude of the message signal is greater than the peak amplitude of the unmodulated carrier signal. transmission efficiency - fraction of the total AM signal power that is contained in the sidebands. splatter - adjacent-channel interference (fake sideband frequencies) due to overmodulation of carrier signal by abrupt peak message signals.
2. The components for all three parts of experiment 2 are shown in figure below
Lab-Volt modular communications lab components.
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Equipment Required
RF NOISE FREQUENCY GENERATOR COUNTER
AM/DSB/SSB AM/DSB GENERATOR RECEIVER
Function DUAL AUDIO Generator A AMPLIFIER OSCILLOSCOPE
Function POWER SPECTRUM Generator B SUPPLY ANALYZER
Brief description of the equipment:
4) Function generator A and B: These two function generator is used to generator message signals. 5) AM/DSB/SSB GENERATOR: It is used to for amplitude modulation and generator either double side band or single side band waveforms for transmission. 6) RF Noise Generator: This can be used to generate RF noise. 4) Power Supply: It is used to supply power to equipment 5) Dual Audio Amplifier: Use for amplifier signals from the receiver to a proper listening level. 6) AM/DSB Receiver: This equipment receives the amplitude modulated signals and demodulate it before sending it to audio amplifiers 7) Frequency Counter: This equipment can be used for monitoring frequency of signals. 9) Spectrum Analyzer: This equipment is used for displaying signals in frequency base for analysis. 9) Oscilloscope: Use to display waveforms in time domain.
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Part 1: Exercise 2-1 (An AM Signal) Purpose: The purpose of this part of the experiment to familiarize with the using the AM/DSB/SSB Generator to demonstrate and explain an AM signal in the time and frequency domain.
Connections and setup to investigate AM (Part 1 of Experiment)
Connections and setup to calculate Modulation Index (m) (Part 2 of Experiment)
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Connections and setup to make AM power measurements (Part 3 of Experiment)
Step 1: This step is to set up the equipment as follow:
AM/DSB/SSB FREQUENCY GENERATOR COUNTER
SPECTRUM FUNCTION ANALYZER GENERATOR A
FUNCTION OSCILLOSCOPE GENERATOR B
POWER SUPPLY DUAL AUDIO AMPLIFIER
This step is to set up the equipment in a manner where it will be easy to counter the appropriate modules together. We need to make sure all the level and gain control is set to the minimum to avoid any incidents.
Step2: Adjust the function generator A to produce the following signal: A. set the signal to a sine wave B. Adjust the frequency to 10kHz C. Set the output level to get a signal of 400mVp-p
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Step 3: Set the carrier frequency to 1100kHz. This can be accomplished by connect the AM/DSB Output terminal to the frequency counter. Adjust the RF Tuning knob to get the frequency to be 1100kHz display on the frequency counter. fc=
Step 4: Connect the output of the AM/DSB Output terminal to channel 1 of the oscilloscope.
Step 5: Display the signal from function generator A to the oscilloscope and connect it to the Audio Input on the AM/DSB/SSB Generator. This can be done by splitting the signal from function generator A using a T connector. Connect one to the oscilloscope and the other one to the Input on the AM/DSB/SSB Generator.
Step 6: Change the signal of function generator A from a sine wave to a square wave and observe the waveform.
Step 7: Used different forms of message signal from function generator an observe the result waveform such as triangular and saw-tooth.
Step 8: Set the message signal back to a sine wave, and observe the change in the modulated waveform to the change in frequency to the message signal.
Step 9: Observe the change in the modulated waveform as the amplitude of the message signal is varied.
Step 10: The change of the amplitude level of the information signal in step 9 corresponds to the variation of the modulation index.
Step 11: Disconnect the oscilloscope and set up the spectrum analyzer. Set the center frequency of the spectrum analyzer to 1.1MHz. Span. 50-200 KHz
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Step 12: Display the modulated signal to the spectrum analyzer. This can be accomplished by connecting the AM/DSB Output to he Input of the Spectrum Analyzer.
Step 13: Determine the difference between the display of the frequency response shown in step 12
Step 14: Observe the changes of the modulated signal in the spectrum analyzer as the frequency of the message signal is varied.
Step 15: Observe the changes of the modulated signals in the spectrum analyzer as the amplitude of the message signal is varied.
Step 16: Determine the effects of changes in the modulation index to the frequency spectrum.
Step 17: Observe the change in the frequency spectrum when the message signal is changed from a sine wave to a square wave. Dr. Rad 32 EE321
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Review Questions:
6) Base on the results of this exercise, which signal produces a frequency spectrum could be compared to a complex message signal, the sine wave or the square wave? Explain. 7) Indicate in a simple sketch how changes in the frequency and in the amplitude of a message signal are reflected in the frequency spectrum of an AM signal.
8) If the modulation index of an AM signal is increase, what effect does this have on the envelope of the AM waveform?
9) What happens to the envelope of the AM signal when the frequency of the modulating signal is increased? 5) When the message signal frequency increases, does the modulation index increase or decrease?
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Part 2: Exercise 2-2 (Percentage Modulation) Purpose: The purpose of this part of the experiment is to show the percentage of modulation either with an oscilloscope or a spectrum analyzer.
DISCUSSION
As you have seen in Exercise 2-1, increasing or decreasing the amplitude of the message signal causes higher or lower peaks and valleys in the envelope of the AM signal. This corresponds to changing the percentage modulation which is the term used when the modulation index m is expressed as a percentage. Percentage modulation is equal to m multiplied by 100%.
The modulation index is an important parameter in AM. lt is defined as the ratio between the amplitudes of the message signal and the unmodulated carrier. The AM modulation index is measured using a single-tone sine wave as the message signal.
Figure 2-8 shows how the modulation index is defined and measured. The figure shows a sine wave message signal with a peak amplitude of 200 mV, while the peak amplitude of the unmodulated carrier is 600 mV. The modulation index is therefore O.2 / 0.6 = 1/3 and the % modulation is 1/3 x 100% = 33 1/3%.
Figure 2-8. The AM modulation index
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Step 1:
AM/DSB/SSB FREQUENCY GENERATOR COUNTER
SPECTRUM FUNCTION ANALYZER GENERATOR A
FUNCTION OSCILLOSCOPE GENERATOR B
POWER SUPPLY DUAL AUDIO AMPLIFIER
Step 2: This step is to set the message signal to the following: A) set the signal to a sine wave B) adjust the frequency to 10kHz C) set the amplitude level to get 400mVp-p
Step 3: Set the carrier frequency to 1100kHz. This can be accomplished by connect the AM/DSB Output terminal to the frequency counter. Adjust the FR Tuning knob to get the frequency to be 1100kHz display on the frequency counter.
Step 4: Measure the peak to peak amplitude of the carrier Vc. This can be accomplished by measuring the signal in the oscilloscope. Vc =
Step 5: Determine the percentage of modulation base on the results of step 3 and step 4. The result can be determine using the relationship % Mod. = m * 100%=(Vm*/VC)*100% .
Step 6:
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Display the modulated signal on the oscilloscope. Use the result from the graph to calculate the modulation index using m =(A-B)/(A+B). A is the peak to peak amplitude of the envelop and B is the amplitude between valleys of the envelop. This can be accomplished by the following step: A) Use a T connector to split up the message signal from function generator A and connect it to the Audio Input on the AM generator. B) Set the largest usable display to show the waveform.
Step 7: This step is to calculate the percentage of modulation using the results from the oscilloscope in XY mode and display it as follow:
Step 8: Obtain modulation index 75% using the trapezoidal pattern by varying the level of the modulating signal. This step is tricky as level control on the message signal and the Gain control of the AM/DSB Generator must be varied back and forth to get the 75% modulation. Once B is determined, we can used the Level Control on the AM/DSB generator to obtain A for the other modulation levels.
Step 9: Remove the cables from the oscilloscope and set up the spectrum analyzer and center the frequency at 1.1MHz. Span 50-200 KHz
Step 10: This step is to connect the output of the AM/DSB/SSB Generator to the input of the spectrum analyzer.
Step 11: Fine tune the center frequency button of the spectrum analyzer to center the modulated signal to the center of the display.
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Step 12: Calculate the modulation index from the spectrum analyzer. Use the relationship between sideband and carrier from the following figure 2-10 to calculate the modulation index.
Example of carrier and its sidebands ∆ = %Mod=
Step 13: Compare the result in step 12 to the result from step 8.
Step 14: Vary the amplitude of the modulating signal to obtain two intermediate values between 20% and 70% for the modulation index. This process can be done by using Figure 2-11 and obtaining the values of ∆.
Step 15: Overmodulate the signal by max out the output from function generator A, and observe the result in the spectrum analyzer when the AM/DSB/SSB Generator is at the NONLINEAR OVERMODULATION position.
Step 16: Observe happens when the Output Level on Channel A is varied between Min and Max so the modulation index varies above and below 1.
Step 17: Connect the AM/DSB output of the AM generator in the oscilloscope (normal mode) when we repeat the same procedure as step 17.
Step 18: Set the CARRIER LEVEL knob in the AM/DSB generator to LINEAR OVERMODUALTION position, and repeat step 16 and compare the results with step 17.
Step 19: Use the XY mode of the oscilloscope obtain the trapezoid patter, use it to determine the overmodulation level.
Step 20: Sketch the trapezoidal pattern obtained when the modulation index is greater than one
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Step 21: What happens to the small triangle on the right of the trapezoid pattern when the CARRIER LEVEL knob is pulled out to the NONLINEAR OVERMODUALTION position.
Review Questions
6. The following trapezoidal patterns are obtained for different AM signals. Determine the modulation index in each case.
7. The amplitude of a sine-wave message signal is 500mVp-p, and the amplitude of the unmodulated carrier is 500mV peak. What is the theoretical modulation index?
8. What is mean by overmodulation, and why is it undesirable?
9. A technician uses the trapezoidal method to determine the modulation index of an AM signal. The resulting oscilloscope display shows the presence of a small triangle on the right side of the figure. What does this indicate?
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Experiment 2-3 (Carrier and Sideband Power)
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Equipment Required
AM/DSB/SSB FREQUENCY GENERATOR COUNTER
SPECTRUM FUNCTION ANALYZER GENERATOR A
FUNCTION OSCILLOSCOPE GENERATOR B
TRUE RMS POWER DUAL AUDIO VOLTTMETER/POWER SUPPLY AMPLIFIER METER
Brief description of the equipment: 1) Function generator A and B: These two function generator is used to generator message signals. 2) AM/DSB/SSB GENERATOR: It is used to for amplitude modulation and generator either double side band or single side band waveforms for transmission. 3) TRUE RMS VOLTMETER/POWER METER: This equipment can be used to determine AC power. It has the option of display the signal level in terms of voltage, or display the signal strength in terms of dBM. 4) Power Supply: It is used to supply power to equipment 5) Dual Audio Amplifier: Use for amplifier signals from the receiver to a proper listening level. 6) AM/DSB Receiver: This equipment receives the amplitude modulated signals and demodulate it before sending it to audio amplifiers 7) Frequency Counter: This equipment can be used for monitoring frequency of signals. 8) Spectrum Analyzer: This equipment is used for displaying signals in frequency base for analysis. 9) Oscilloscope: Use to display waveforms in time domain.
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Exercise 2-3 (Carrier and Sideband Power) Purpose: The purpose of this experiment is to learn to use the modulation index to determine the sideband power and transmission efficiency for AM signals.
Discussion In the previous exercise, you saw that varying the modulation index caused the power level of the sidebands to change, while the carrier power remained constant. Since the useful information contained in the RF signal is located in the sidebands, it is desirable to maximize the sideband power levels. In AM however, the modulation index must not be greater than 1 or distortion and interference will occur.
The total power (PT) in an AM signal is the sum of the carrier power (Ps), and the lower and upper sideband power (PLSB +PUSB). ln equation form, PT = Pc + PSB , where PSB = PLSB + PUSB. For AM signals the upper and lower sideband powers are equal.
The fraction of the total power that is contained in the sidebands is a measure of the transmission efficiency (µ). ln equation form this can be expressed as µ= P SB / P T .
Since PSB is directly related to the modulation index (m), the ratio PSB / PT, and the theoretical efficiency, can be determined from the modulation index using the Following equation:
The following example illustrates the use of these equations.
An AM station transmits an average arrier power of 40 kW. The modulating signal is a sine wave and m equals O.7O7 (√2 /2). We want to find the total average power output of the station, the average power in each sideband, and the transmission efficiency.
Since we know m, we can find µ. Thus,
Therefore, the ratio (PSB/ PT) = 1/2, or PSB = 0.2 PT .
The total power can be calculaled using PT = Pc + PSB and substituting the values for Ps and PSB. This gives
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The total sideband power PSB is the difference between the total power pr and the canier power PC , and is equal to 10 kW (50 - 40 =10 kW). Since the upper and lower sideband powers are equal for AM signals, each sideband contains 5 kW. Figure 2-18 can be used to find the transmission efficiency directly, once the modulation index is known. Since m ≤1 for AM, the maximum efficiency that can be obtained is 33 1/3%. You can verify this by substituting rn = 1 into the equation relating µ and m.
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Figure 2-18. Graph of transmission efficiency vs the modulation index
Step 1 Set up the equipment as follow:
AM/DSB/SSB FREQUENCY GENERATOR COUNTER
SPECTRUM FUNCTION ANALYZER GENERATOR A
FUNCTION OSCILLOSCOPE GENERATOR B
TRUE RMS POWER DUAL AUDIO VOLTTMETER/POWER SUPPLY AMPLIFIER METER
Set up the equipment in a manner where it will be easy to counter the appropriate modules together. We need to make sure all the level and gain control is set to the minimum to avoid any incidents.
Step 2 Set up the message signal to the following conditions: 1. f=10kHz 2. amplitude = 200mV 3. sine wave It can be accomplished by adjusting Function Generator A to the above condition.
Step 3 Adjust the carrier frequency to 1100kHz. This can be accomplished by connecting the output of the AM/DSB/SSB/Generator to the Frequency Counter, adjust he RF Tuning control until the Frequency Counter reads 1100kHz..
Step 4 Connect the output of the AM/DSB/SSB Generator to channel 2 of the oscilloscope.
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Step 5 Connect the message signal to both the AUDIO INPUT on the AM/DSB/SSB Generator and channel 1 of the oscilloscope. This can be accomplished by connect a T connector to the output of Function Generator A, connect one of the split signal to the oscilloscope and the other one to the AUDIO INPUT on the AM/DSB/SSB Generator.
Step 6 Set the oscilloscope in X-Y mode. The X-Y mode is then used to determine the modulation index.
Step 7 Set the modulation index to .50. This can be accomplished by utilizing the trapezoidal pattern in step 6, adjust the output level of the message signal to obtain proper value of A and B to get modulation index .50 use the relationship m=(A-B)/(A+B).
Step 8 Measure the rms voltage of the message signal. This can be accomplished by disconnect the message signal from the AUDIO INPUT of the AM generator and connect it to the True RMS Voltmeter/Power Meter. Result: Vrms=
Step 9 Measure the rms voltage of the message signal for modulation index of .75. It can be accomplished by repeating step 7 to obtain m=.75, then repeat step 8 to measure the rms voltage for the message. Result: m=.75 Vrms=
Step 10 Measure the rms voltage of the message signal for modulation index of 1.0. This can be accomplished by repeating step 9 for the modulation index of 1.0. Result: m=1.0 Vrms= Step 11: Set up the Spectrum Analyzer to center at 1.1MHz.
Step 12: Connect the output of the AM Generator to both the TRUE RMS VOLTMETER/POWER METER and the Spectrum Analyzer. This can be accomplished using a T connector at the output of the AM Generator to split up the signal, connect one output to the TRUE RMS VOLTMETER/POWER METER and the other to the Spectrum Analyzer.
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Step 13: Fine tune the spectrum analyzer to center the signal as close to the center as possible. This can be accomplished by varying the Tuning knob on the Spectrum Analyzer until the carrier signal is at the center of the spectrum analyzer.
Step 14: Determine the unmodulated carrier power using the TRUE RMS VOLTMETER/POWER METER. To accomplish this, the message signal must be removed from the Audio Input of the AM Generator. Make sure the Power Meter is in dBm, record the reading off the meter. Result: Pc(unmodulated) =
Step 15: Make sure the modulation index is still at 1.0. This can be done by using the trapezoidal pattern.
Step 16: Determine the power of the modulated signal. To accomplish this, connect the message signal to the Audio Input of the AM Generator. Record the reading from the TRUE RMS VOLTMETER/POWER METER. Result: PAM =
Step 17: Determine the amplitude of the carrier and its lower and upper sidebands from the spectrum analyze. The measurements can be done by placing the marker at the center of the carrier, record the value for Pc. Adjust the marker at the fLSB, which is the sideband to the left of the carrier frequency. Record the value as PLSB. Next, adjust the marker to the fUSB, which is the sideband to the right of the carrier frequency. Record the value as PUSB. Result: Pc = PLSB = PUSB =
Step 18 & step 19: Repeat the previous steps to find all the values to fill up the following table for modulation index of .75 and .50. Use figure 2-20 to convert dBm values to mW.
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Step 20: Make sure table 2-3 is complete. Result: TRUE RMS VOLTMETER SPECTRUM ANALYZER RESULTS PT = Pc + m PSB/PT Vaudio Pc(unmodulated) PAM(modulated) PSB PLSB PUSB M2/(m2+2) % Vrms (mV) dBm mW dBm mW dBm mW dBm mW dBm mW
0.50
0.75
1.00 Table 2-3
PSB/PT = (PAM-PC)/PAM (in mW) form columns 6 and 8 PT = PC+PLSB+PUSB (in mW) form columns 12, 14 and 16 Step 21: Compare the theoretical value with the transmission efficiency for the three different modulation indexes. Result: % of M PSB/PT difference m2/(m2+2) % %
0.50
0.75
1.00
Step 22: Compare the values of PAM and PT for the 3 different modulation indexes. Result: difference PAM(modulated) PT = Pc + PSB = PT - PAM dBm mW dBm mW dBm
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Step 23: Compare the sideband power obtain with the True RMS Voltmeter results with the corresponding values obtained using the Spectrum Analyzer results.
Result: Psb Psb (spectrum analyzer) (True RMS Meter) % of difference M =PUSB +PLSB (dB) =PAM-Pc (dB) (dB) 0.50 0.75 1.00
.
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Converting dBm to mW and RMS voltage.
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Review Question: 1. Write in equation form the expression for 1) the total power of an AM signal. 2) the relationship between and m.
2. The maximum power transmission efficiency that can be obtained in AM is 33%. Explain.
3. Spectrum analyzer measurements of an AM signal show that Pc = 0dBm and PLSB = PUSB = -6dBm. Determine the modulation index.
4. An AM signal is attenuated by 100 before measurements are made with a spectrum analyzer. The spectrum analyzer measurements show that Pc = 0dBm, and PLSB = PUSB = -6dBm. Determine PT, the actual power of the AM signal.
5. An AM station transmits an average carrier power of 10 kW. Spectrum Analyzer measurements show that the difference between carrier and sideband power is 8 dB (∆ = 8dB). Determine m, , PT, PLSB and PUSB.
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Experiment 3 (Reception of AM Signals)
Part 1: Exercise 3-1 (The RF Stage Frequency Response) Part 2: Exercise 3-2 (the Mixer and Image Frequency Rejection)
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DISCUSSION OF FUNDAMENTALS
In the previous units you became familiar with the generation and appearance of AM signals- You learned how AM radio waves are used to communicate information, and you observed AM signals in the time and frequency domains. ln this unit you will learn how the transmitted information is recovered from the RF carrier wave.
The process of amplitude modulation is accompanied by frequency translation of the message signal to a position in the frequency spectrum that is centered at the carrier frequency. This means that several stations with different carrier frequencies can broadcast messages at the same time in the commercial AM band. Figure 3-1 illustrates this and shows the frequency spectra of four different AM Station
Figure 3-1. The frequency spectra of four different AM stations.
An AM receiver has to be able to single out or select the desired station, and then recover the information that is being broadcast. This idea is illustrated in Figure 3-2. The station broadcasting at 950 kHz is the desired station, and the basic operations required are: 1) filtering so that only the frequency contents centered at 950 kHz are selected for demodulation and 2) returning the frequency contents of the message signal to their original place in the frequency spectrum. When you tune in a particular AM station, this is essentially all that happens, since the theoretical process involved is mainly frequency translation.
Figure 3-2. Recovering the information being broadcast.
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The kind of receiver that you will use in the Analog communications Training system is called a superheterodyne receiver. A simplified block diagram of this receiver is shown in Figure 3-3.
Figure 3-3. Simplified block diagram of a superheterodyne receiver.
The basic operation of the superheterodyne receiver is as follows. The incoming RF signals are filtered to select the desired station by adjusting the LO frequency with the RF Tuning control. This changes the canter frequency of RF Filter and can be compared to displacing a “window" (the RF Filter) in the frequency spectrum. When the “window" is positioned at the desired carrier frequency, station selection is complete and the selected RF signal is then amplified before mixing with the LO signal. The local oscillator is designed so that LO frequency is 455 kHz (the intermediate frequency) above the station frequency, and the RF Tuning control adjusts the Lo frequency at the same time as tire center frequency of the RF Filter. This results in a fixed lF for all stations (fLO = fc + fIF). Tuning LO above the carrier frequency, instead of below, results in more linear tuning the local oscillator. This is because it's easier to make oscillators which are linear over a 1 to 2 MHz range (a frequency ratio of 2), than it is to make them linear over a 0.1 to 1.1 MHz range (a frequency ratio of 11).
Mixing the selected RF signal with the LO signal produces sum and difference frequencies (of both input signals) at the output of the mixer. The IF filter allows only the difference frequency (fIF) to pass through, and the filtered signal is then processed by the detector. to remove the original information. The Automatic Gain control (AGC) circuit helps maintain a fairly constant output lever by controlling the gain of the RF amplifier. The AGC circuit in some receivers also controls the gain of the lF amplifier.
NEW TERMS AND WORDS
Automatic Gain control (AGc) - the circuit or process used to maintain the out- put volume of a receiver constant, regardless of variations in the RF signal strength applied to the receiver. demodulation - the process of removing the information contained in a modulated RF signal; also called detection. detector - the circuit or device used to perform the process of demodulation. IF stage - that section of a receiver contained between the mixer and the detector stage. This stage operates at the fixed intermediate frequency and it is here that most of the amplification and filtering takes place. intraband image frequencies - image frequencies that lie inside the allocated frequency band for the type of communications involved. ln AM, intraband image frequencies occur at both ends of the commercial band from 540 to 690 kHz and from 1450 to 1600 kHz.
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image frequency (fIMAGE) - in heterodyne receivers, an undesired input frequency equal to the station frequency plus twice the intermediate frequency (fIMAGE = fc +2fIF). The image frequency results in two stations being received at the same time, thus producing interference. (For receivers in which the LO is tuned below the station, fIMAGE = fc - 2fIF). image frequency rejection ratio - of a superheterodyne receiver, the ratio of the response at the desired frequency to the response at the image frequency.
RF stage - the first input stage of a receiver,-in which primary selection, filtering and amplification of the input RF signal is performed. selectivity - a measure of how well a receiver rejecls adjacent channel signals when tuned to a particular station. The components for both parts of experiment 3 are shown in figure .
Lab-Volt modular communications lab components.
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Connections and setup to investigate RF Filter (Part 1 of Experiment)
Setup to calculate Image frequency & its rejection (Part 2 of Experiment)
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AM Carrier after it passes through RF filter of AM/DSB Receiver
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Equipment Required
AM/DSB/SSB FREQUENCY GENERATOR COUNTER
SPECTRUM FUNCTION ANALYZER GENERATOR A
FUNCTION OSCILLOSCOPE GENERATOR B
TRUE RMS POWER DUAL AUDIO VOLTTMETER/POWER SUPPLY AMPLIFIER METER
Brief description of the equipment: 4) Function generator A and B: These two function generator is used to generator message signals. 5) AM/DSB/SSB GENERATOR: It is used to for amplitude modulation and generator either double side band or single side band waveforms for transmission. 6) TRUE RMS VOLTMETER/POWER METER: This equipment can be used to determine AC power. It has the option of display the signal level in terms of voltage, or display the signal strength in terms of dBM. 4) Power Supply: It is used to supply power to equipment 5) Dual Audio Amplifier: Use for amplifier signals from the receiver to a proper listening level. 6) AM/DSB Receiver: This equipment receives the amplitude modulated signals and demodulate it before sending it to audio amplifiers 7) Frequency Counter: This equipment can be used for monitoring frequency of signals. 9) Spectrum Analyzer: This equipment is used for displaying signals in frequency base for analysis. 9) Oscilloscope: Use to display waveforms in time domain.
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Exercise 3-1 (The RF Stage Frequency Response)
Step 1: Set up the equipment as follow:
AM/DSB RECEIVER
AM/DSB/SSB TRUE RMS GENERATOR VOLTMETER/POWER METER
FREQUENCY SPECTRUM COUNTER ANALYZER
POWER DUAL AUDIO SUPPLY AMPLIFIER OSCILLOSCOPE
Set up the equipment in a manner where it will be easy to counter the appropriate modules together. We need to make sure all the level and gain control is set to the minimum to avoid any incidents.
Step2: Set up the spectrum analyzer. Connect the RF Output of the AM/DSB Receiver to the input of the spectrum analyzer.
Step 3: Set up the Local Oscillator frequency to 1405 kHz at the OSC OUTPUT on the AM/DSB Receiver. The IF frequency is 450kHz fLO = fc + fIF, fc = fLO – fIF =
Step 4: Disable the local oscillator in the AM/DSB Receiver. This can be by opening the top panel of the module, and placing the FLT 2 switch under the small hinged cover in the 1(active) position.
Step 5: Set up the AM/DSB/SSB Generator module so the carrier frequency can be continuously monitored with the frequency counter. This can be done by opening the top panel of the AM/DSB/SSB Generator, connect TP 13 of with clip wire on the generator. Connect the other end of the lead wire to the frequency counter.
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Step 6: Connect the output of the AM/DSB/SSB Generator to the Input of the AM/DSB Receiver. This can be done by setting eh CARRIER LEVEL on the AM/DSB/SSB Generator to the max position, make sure it is in the LINEAR OVERMODULATION position. Connect the output to the input of the AM/DSB Receiver.
Step 7: Observe what happens when the RF TUNING on the AM/DSB/SSB Generator to get 950 kHz.
Step 8: Center the frequency to the center of the spectrum analyzer. This can be done by varying the TUNING control of the spectrum analyzer until the signal is at the center of the spectrum analyzer display.
Step 9: Adjust the height of the signal to the -10 dBm level. This can be done by adjust the RF GAIN control on the AM/DSB/SSB Generator until the signal level reaches -10 dBm.
Step 10: Trace out the response of the filter in the spectrum analyzer. This can be done by selecting the “max hold” position on the trace, then slowly vary the RF TUNING control on the AM/DSB/SSB Generator to both sides of 950kHz to obtain the filter response.3
Step 11: Calculate the 3dB, 10dB and 20dB bandwidth from the result of step 10. This can be accomplished by setting the marker on the top of the filter, note the amplitude. Adjust the marker to the right of 950kHz until it is 3 dB lower than the first marker, note this frequency. Adjust the marker to the left side of 950kHz until it is 3 dB lower than the amplitude at 950kHz, note this frequency. The difference between the right side and the left side is the 3 dB bandwidth. Repeat the same process for the 10 dB and 20 dB bandwidth. Result:
f Low Bandwidth f High (kHz) (kHz) (kHz)
3-dB BW
10-dB BW
20-dB BW
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Step 12: Disconnect the input of the spectrum analyzer and place it to the input of the TRUE RMS VOLTMETER/POWER METER. Adjust the zero on the voltmeter module and switch the MODE to dBm.
Step 13: Readjust the RF GAIN and the RF TUNING on the AM/DSB/SSB Generator to obtain a reading of -10dBm at 950kHz.
Step 14: Generate a table to characterize the low pass filter using the TRUE RMS VOLTMETER/POWER METER. Vary the RF Tuning to obtain the frequencies. Record and fill out the corresponding columns. Result: Frequency (kHz) 850 870 890 910 930 950 970 990 1010 1030 1050
dBm Reading
Step 15: Calculate the corresponding dB values and sketch the frequency response using the data. Result: Frequency (kHz) 850 870 890 910 930 950 970 990 1010 1030 1050
dBm Reading
Relative dB * 0dB Reference= MAXIMUM dBm Reading *Relative dB=(dBm Reading) – (0 dB Reference)
Step 16: Determine the 3-dB, 10-dB and 20dB bandwidth from the results of step 15.
Result: Bandwidth (kHz)
3-dB BW
10-dB BW
20-dB BW
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Step 17: Compare the result from step 11 and step 16. % of BW from step 11 BW from step differerence (kHz) 16 (kHz) (%)
3-dB BW
10-dB BW
20-dB BW
Step 18: Determine if the 3-dB BW of the filter is wide enough for a 5kHz message signal base on the fact the 3-dB BW must be twice as wide as the message signal.
Step 19: Return the FLT 2 to the 0 (inactive position).
Review Question
1. What kind of receiver is the AM/DSB Receiver? . 2. What are the four principle operations involve in AM reception?
3. If a message signal contains frequencies up to 3kHz, what is the minimum bandwidth required for the RF stage if the message is to be demodulated correctly?
4. What is the result of mixing the incoming RF signal with the local oscillator signal?
5. The difference frequency of 455kHz in the AM/DSB Receiver is call:
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Exercise 3-2 (The Mixer and Image Frequency Rejection)
EXEFCISE OBJECTIVE When you have completed this exercise, you will be able to demonstrate the role of the mixer in a superheterodyne receiver, and explain how image frequency is related to this functional element . DISCUSSION
The role of the mixer is to join the RF stage to the lF stage and to perform the necessary operations to convert (or translate) the RF signal to the fixed intermediate frequency. As you have learned in earlier exercises the output signal of a theoretical mixer contains both the sum and the difference frequencies of the two original input signals. The output of actual mixers, however, often contains frequency components corresponding to the input signals. Figure 3-7 shows an example of this for a 950-kHz carrier modulated with a 5-kHz sine wave. since the RF input signal to the mixer contains three frequency components, the mixer output signal will contain sum and difference frequencies for the three RF components. This results in 4 groups of frequency components in all.
Figure 3-7. Frequency components an the output of a mixer. Generally, the presence of these extra frequencies does not adversely affect the operation of the receiver. This is because the mixer output signal goes through am series of filters and amplifiers in the lF stage that are all tuned to the fixed lF 455 kHz in most receivers. Also, the power levels of these frequencies are much lower than those of the main difference components situated at 455 kHz.
One of the major disadvantages of the superheterodyne receiver is the problem of image frequency (fIMAGE). Figure 3-8 illustrates how the image frequency arises and the manner in which the mixer is involved.
Figure 3-8. The image frequency for 950 kllz ln this example, the carrier frequency is 690 kHz. The local oscillator frequency (f1e) must be 1 1 45 kHz in order to receive the station, since 1 145 kHz - 690 kHz results in the 455 kHz lF al which the receiver operates. Now, suppose that another station operating at 1600 kHz is located in the same region, and the LO frequency remains at 1145 kHz. The ditference between 1600 kHz and 1145 kHz is 455 kHz, and this EE321 65 Dr. Rad
new station will pass lhrough the lF stage and interfere with the 690-kHz station. Note that for commercial AM broadcasting, image frequency problems from other AM stations occur only at both ends of the band - between 540 - 690 kHz and 1450 - 1600 kHz. However, the problem could be caused by other types of communications facilities outside the commercial band. lntraband image frequencies can only be eliminated when the lF frequency is such that fC ± 2fIF falls outside the band of operation. This is because the image frequency is equal to the station frequency plus (or minus) twice the intermediate frequency.
Because of the problem caused by image frequencies, one of the more important criteria for a superheterodyne receiver is the image frequency rejection ratio, a parameter that you will experiment in this exercise. The RF stage is responsible for most of the image frequency rejection in a superheterodyne receiver. For this reason the bandwidth of the RF stage should be no larger than is necessary for the message signal and adjacent station rejection. ln this way the image frequency is greatly attenuated before the RF signal is mixed with the local oscillator sig
Step 1: Setup the equipment as follow:
AM/DSB/SSB FREQUENCY GENERATOR COUNTER
SPECTRUM FUNCTION ANALYZER GENERATOR A
FUNCTION OSCILLOSCOPE GENERATOR B
TRUE RMS POWER DUAL AUDIO VOLTTMETER/POWER SUPPLY AMPLIFIER METER
Set up the equipment in a manner where it will be easy to counter the appropriate modules together. We need to make sure all the level and gain control is set to the minimum to avoid any incidents.
Step 2: Adjust the carrier frequency to 690kHz and adjust the fLO to 1145kHz. This can be accomplished by the following steps: 1. Connect TP 13 from the AM/DSB/SSB Generator to the frequency counter. Adjust the RF Tuning to get 690kHz. 2. Connect the output of the AM/DSB/SSB Generator to the 50 input of the AM/DSB Receiver. 3. Connect the output of the AM/DSB Receiver to the frequency counter and adjust the RF Tuning to get 1145kHz.
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Step 3: Prepare the spectrum analyzer for measurement. This is done by center the frequency to 1MHz, select 0 dBm and FREQUENCY SPAN of 2MHz/V.
Step 4: Set up the message signal to 5kHz, and amplitude of 200mV.
Step 5: Verify fc is 690kHz and fLO is 1145kHz. This step can be accomplished by connecting each output to the frequency counter and readjust if necessary.
Step 6: Set up the spectrum analyzer as follow:
Step 7: Measure fLO, fc, fLO = fc and fLO – fc from the set up from step 6. Result:
fLO = fc =
fLO + fc fLO - fc =
Step 8: Determine the amplitude difference between fLO and IF component at 455kHz. .
Step 9: Connect the IF OUTPUT of the AM/DSB Receiver to the NPUT of the spectrum analyzer and observe the display.
Step 10: Adjust the RF GAIN on the AM/DSB/SSB Generator until we obtain -10dBm for the IF component.
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Step 11: Measure the amplitude of the AM signal using the TRUE RMS VOLTMETER/POWER METER.
Step 12: connect the AM/DSB output to the RF INPUT of the AM receiver, and adjust he RF TUNING of the AM/DSB/SSB Generator to obtain a carrier frequency of 1600 kHz.
Step 13: Readjust the RF GAIN to obtain -10dBm on the spectrum analyzer, and repeat step 12..
Step 14: Determine the image frequency rejection ratio. This can be done by PAM (1600kHz) – PAM (690 kHz).
Review Questions
1. An AM receiver tuned to a station at 600kHz uses a 455 kHz IF and the local oscillator operates above the station frequency. What is the image frequency for this station?
2. The image frequency of an AM station is 2500kHz. What is the frequency of the station if the receiver uses 455kHz as the IF and fLO = fc + fIF?
3. What is the role of the mixer linking the RF and the IF stages of a Superheterodyne receiver?
4. Which stage of the receiver is responsible for most of the image frequency rejection?
5. How can intraband image frequencies be eliminated?
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Experiment 4 (Reception of AM Signals)
Part 1: Exercise 4-1 (The IF Stage Frequency Response) Part 2: Exercise 4-2 (The Envelope Detector)
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The actual setup and wiring diagram for parts 1 and 2 are shownin following
Connections and setup to investigate IF Filter (Part 1 of Experiment)
Setup to study demodulation (Part 2 of Experiment)
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AM Carrier after it passes through IF filter of AM/DSB Receiver
Equipment Required
AM/DSB/SSB FREQUENCY GENERATOR COUNTER
SPECTRUM FUNCTION ANALYZER GENERATOR A
FUNCTION OSCILLOSCOPE AM/DSB RECEIVER GENERATOR B
TRUE RMS POWER DUAL AUDIO VOLTTMETER/POWER SUPPLY AMPLIFIER METER
Brief description of the equipment: 7) Function generator A and B: These two function generator is used to generator message signals. 8) AM/DSB/SSB GENERATOR: It is used to for amplitude modulation and generator either double side band or single side band waveforms for transmission.
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9) TRUE RMS VOLTMETER/POWER METER: This equipment can be used to determine AC power. It has the option of display the signal level in terms of voltage, or display the signal strength in terms of dBM. 4) Power Supply: It is used to supply power to equipment 5) Dual Audio Amplifier: Use for amplifier signals from the receiver to a proper listening level. 6) AM/DSB Receiver: This equipment receives the amplitude modulated signals and demodulate it before sending it to audio amplifiers 7) Frequency Counter: This equipment can be used for monitoring frequency of signals. 10) Spectrum Analyzer: This equipment is used for displaying signals in frequency base for analysis. 9) Oscilloscope: Use to display waveforms in time domain.
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Exercise 4-1 (The IF Stage Frequency Response)
Step 1: Set up the equipment as follow:
AM/DSB/SSB FREQUENCY GENERATOR COUNTER
SPECTRUM FUNCTION ANALYZER GENERATOR A
FUNCTION OSCILLOSCOPE AM/DSB RECEIVER GENERATOR B
TRUE RMS POWER DUAL AUDIO VOLTTMETER/POWER SUPPLY AMPLIFIER METER
Set up the equipment in a manner where it will be easy to counter the appropriate modules together. We need to make sure all the level and gain control is set to the minimum to avoid any incidents.
Step 2: Set up the Spectrum Analyzer module to center at .5MHz and connect it to the IF OUTPUT of the AM/DSB Receiver.
Step 3: Place the AGC switch to the ( (inactive) position and select the SYNC DETECTOR on the AM/DSB Receiver. Also ask the instructor to activate Fault 2 (Flt 2).
Step 4: Set up the equipment to continuously monitor the frequency of the AM/DSB/SSB Generator. This can be accomplished by opening the top panel of the AM/DSB/SSB Generator and connect TP13 of the receiver to the frequency counter.
Step 5: Set the proper carrier level on the AM generator and connect it to the AUX IF INPUT of the AM/DSB Receiver. This can be done first set the CARRIER LEVEL on the AM generator tot the maximum level. Next, make sure it is at the LINEAR OVERMODULATION position, and Adjust the RF GAIN to the ½ turn cw. The last step is to connect the AM/DSB RF OUTPUT to the AUX IN INPUT of the AM/DSB Receiver. Dr. Rad 73 EE321
We should adjust he frequency carrier to 455kHz if we are going to measure the frequency response of the fixed-frequency.
Step 6: Adjust the RF TUNING control on the AM/DSB/SB Generator to 455kHz and observe the results on the Spectrum Analyzer.
Step 7: Place the 455kHz IF signal in the center of the spectrum analyzer and select proper span.
Step 8: Obtain the maximum output of the IF line. It can be accomplished by varying the RF TUNING control on the AM/DSB/SSB Generator until the maximum amplitude is obtained.
Step 9: Thrace out the filter response on the spectrum analyzer. It can be accomplished by varying the RF TUNING control on the AM/DSB/SSB Generator both sides of the 455kHz while the “max hold” is selected on the spectrum analyzer.
Step 10: Determine the 3-dB BW, 10-dB BW and 20-dB BW from the trace from step 9. Result: 3-dB BW = 455 kHz - 10-dB BW = 455 kHz - 20-dB BW = 455 kHz -
Step 11: Setup the TRUE RMS Voltmeter/Power Meter for measurement of the power of the IF Output. This can be accomplished by connecting the IF Output to the TRM RMS Voltmeter/Power Meter. Zero the voltmeter and select the dBm position.
Step 12: Center the filter to obtain the maximum power. This can be accomplished by varying the RF Tuning knob slightly left and right until the maximum power reading is obtained.
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Step 13 & Step 14: Fill out a table of data necessary to analyze the filter characteristics using the TRUM RMS Voltmeter/Power Meter, and sketch the frequency response curve corresponding to these values. Result: Frequency (kHz) 439 443 447 451 453 455 457 459 463 467 471
dBm Reading
Relative dB
Step 15: Determine the 3-dB, 10-dB and 20-dB bandwidth using the results from step 13 and step 14. Result:
3-dB BW =
10-dB BW =
20-dB BW =
Step 16: Compare the results of step 15 to step 10?
Step 17: Inject a signal at the AUX IF INPUT and connect the IF OUTPUT to the spectrum analyzer.
Step 18: Make sure the frequency is at the center of the filter.
Step 19: Send a 5kHz, 200mV signal from the Function Generator A to the AUDIO INPUT on the AM/DSB/SSB Generator.
Step 20: Center the modulated signal at the center of the spectrum analyzer and observe the signal.
Step 21: Injected signal and observe its response with respect to the change of frequency.
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Step 22: Use the fLSB and fUSB to determine the 3-dB bandwidth of the filter. This can be accomplished by adjusting the fLSB and fUSB to 3 dB lower than its maximum value. Result: fLSB = fUSB =
Step 23: Compare the result from Step 22 to the result from step 13&14.
Step 24: Compare the 3-dB bandwidth of the IF stage with the RF stage.
Review Question:
1. The typical bandwidth required to transmit a message signal in commercial AM is 10kHz. explain the influence, if an that this has on the bandwidth requirements of the IF stage. .
2. The IF stage bandwidth of a n AM receiver is measured and found to be 6kHz. What effect will this have on performance during reception of commercial AM stations? . 3. Which stage of a superheterodyne receiver is responsible for most of the receiver’s selectivity?
4. Why can the very large gain of the IF stage cause problems in a superheterodyne receiver?
5. During frequency-response measurements of the IF stage it is found that the LSB and USB are not attenuated equally. Can this be considered normal?
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Exercise 4-2 (The ENVELOPE DETECTOR)
The Envelope Detector
EXERCISE OBJECTIVE when you have completed this exercise, you will be able lo demonstrate how an AM signal is demodulated using the ENVelope detector found in the AM / DSB Beceiver.
Discussion
Any circuit whose output follows the envelope of an AM signal can serve as a deteclor, and be used to demodulate the RF wave. one of the most widely used and simplest detectors is the non-linear charging circuit formed by a diode in series with the parallel RC network shown in Figure 4-1. This kind of envelope detector is also known as a diode detector.
Figure 4-1. The Envelope Detector.
The circuit is designed to have a fast charge time and a slow discharge time, with the resistor controlling the discharge time constant. lf the AM signal shown in Figure 4-2 is applied to the input of this circuit, it will undergo half-wave rectification as illustrated.
Figure 4-2. Theoretical input and output signals of the envelope detector.
The operation of the envelope detector is as follows. On the positive half cycles of the input signal, the capacitor charges to the peak value of the input. Therefore, the voltage across BC will be equal to that of the input signal since the diode is forward- Dr. Rad 77 EE321
biased. When the input signal drops below this value, the diode turns off and the capacitor starts to slowly discharge through the resistor at a rate determined by the RC time constant. On the next positive half-cycle of the input signal, the diode is turned on and the capacitor again charges to the new value d6termined by the input signal. Figure 4-3 resumes this charge-discharge process.
Figure 4-3,The charge-discharge process of the capacitor. There is an optimum value for the discharge time constant RC. lf the time constant is too large, or too small, the output of the detector will not follow faithfully the envelope of the input signal. Figure 4-4shows the effects of having RC too large, or too small.
Figure 4-4 The effects of the discharge time constant.
The optimum value of RC is obtained when the time constant is equal to the maximum negative slope of the envelope. Because the envelope represents the message signal waveform, RC is a function of the modulating signal frequency and the modulation index. The following equation has been derived for the optimum value of the discharge time constant.
After detection, the output of the envelope detector is usually filtered with a low pass filter, in order to remove ripple and the unwanted harmonic content. often, a coupling capacitor is used to remove the dc level introduced by the carrier. One of the major drawbacks of the envelope detector is the fact that approximately 0-6 V potential difference must exist across the diode before it conducts. This means that a 0-6 V difference between the input signal voltage and the capacitor voltage is necessary befog the output begins following the input. This is more pronounced for weak input signals, and also when the modulation index is close to 100%. Figure 4-5 shows the two cases. The heavy line shows the demodulated signal waveform (before filtering) while the
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dashed line illustrates the envelope. The resulting audio will often be distorted and subject to fading because of this voltage loss, and weak stations may not be demodulated at all.
Figure 4-5. The effect of the diode's forward voltage drop.
More often than not, it is the negative envelope of the AM signal that is detected with the diode detector, and the diode in Figure 4-1 is simply reversed. The rea_ son for this is that detection of the negative- envelope provides the negative AGC voltage necessary to control the gain of the RF stage. lf the RF signal increases in strength, more negative feedback from the AGC circuit is sent to the RF stage to reduce its gain. In this way, the audio output level remains fairly constant in spite of variations in the strength of the RF signal.
Among the other types of detectors available is the PLL (phase-locked loop) Synchronous detector. The functional block diagram of the SYNC detector used in the AM / DSB Receiver is shown in Figure 4-6 below. This type of detector provides better detection of AM signals, and allows the RF TUNING to vary over a larger fre_ quency range before the station is lost. Coupled with the AGC circuit, it also allows greater variations in RF signal levels, and permits modulation levels approaching 100%.
Figure 4-6. Functional block diagram of the SYNC DETECTOR (shaded). Dr. Rad 79 EE321
Step 1: Set up the equipment as follow:
AM/DSB/SSB FREQUENCY GENERATOR COUNTER
AM/DSB FUNCTION RECEIVER GENERATOR A
FUNCTION OSCILLOSCOPE GENERATOR B
POWER DUAL AUDIO SUPPLY AMPLIFIER
Set up the equipment in a manner where it will be easy to counter the appropriate modules together. We need to make sure all the level and gain control is set to the minimum to avoid any incidents.
Step 2: Set up Function Generator A to send out a message signal of 1.0kHz and 200mV. If the modulation index (m) is equal to 1.0, the optimum value of the RC time constant for the 1.0kHz message signal is given by: RCoptimum=1/(m*2**fm)=
Step 3: Set up a carrier frequency of 950kHz. This can be accomplished by using the AM/DSB/SSSB Generator. Open the top panel and connect TP13 to the frequency counter to continuously monitor the frequency. Set the RF Gain to ¼ cw and make sure the CARIER LEVEL knob is pushed in and at the MAX position.
Step 4: Determine the LO frequency on the AM/DSB Receiver to receive the 950kHz signal. Result: fLO = fIF + fc
Step 5: Adjust the RF TUNING control on the AM/DSB Receiver to 1405kHz and connect the AM/DSB RF OUTPUT to the AM receiver. Select the ENV DETECTOR and place the AGC switch in the 1 (active) position.
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Step 6: Connect the IF Output of the AM/DSB Receiver to channel 1 of the oscilloscope and connect the Audio Output to channel 2. Set the time base control to .5ms/DIV and set the oscilloscope to trigger on the Audio signal.
Step 7: Set up the measurement of the oscilloscope. Both the channel should be set on the dc coupling mode with channel 1 at 1V/DIV and channel 2 at .5V/DIV. Set the reference level line for channel 2 on the 2nd graticule line of the oscilloscope and position the dc reference for channel 1 on the 5th graticule line.
Step 8: Verify the carrier frequency at 950kHz and describe the signals displayed on the oscilloscope.
Step 9: Approximate the modulation index. This can be accomplished by using one of the method described in experiment 2 for approximation. m =
Step 10: Observe what happen when the CARRIER LEVEL control on the AM/DSB/SSB Generator between MAX and MIN.
Step 11: Determine at what CARRIER LEVEL does the demodulated audio signal begin to be affected.
Step 12: Overmodulate the AM signal by reducing the CARRIER LEVEL to MIN and pull the knob out to the NONLINEAR OVERMODULATION position and observe what happen.
Step 13: Return the CARRIER LEVEL control to Max AND PUSH IT INTO the LINEAR OVERMODULATION position.
Step 14: Place the AGC switch on the receiver in the 0 (inactive) position and set the RF Gain of the AM generator to ½ cw.
Step 15: Observe what happens when the RF GAIN is slowly varied between 1/turn cw and MIN.
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Step 16: Observe what happens when the RF GAIN is varied with the AGC at 1(active position).
Step 17: Return the RF GAIN to the ¼ turn cw position and set the channel sensitivity control of the oscilloscope at 1 VOLT/DIV.
Step 18: Observe the difference on the demodulated signal between the SYNC DETECTOR AND THE ENV DETECTOR. This can be done by increase the amplitude of FUNCTION GENERATOR A until the modulation index is approximately 100%. Then switch between the SYNC DETECTOR AND THE ENV DETECTOR and observe what happens. . Step 19: Determine which detector performs better as the modulation index is approaching 100% before distortion occurs.
Review Question
1. Sketch the circuit for a simple diode detector.
2. Explain briefly how a diode detector operates. . 3. What effect does the .6V drop across the detector diode have on the demodulation IF signal.
4. Does an AGC circuit have any effect on the detection process? Explain.
5. Based on the results of this exercise, which detector coupled with an AGC circuit
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Experiment 5 (Single Sideband Modulation -SSB) Exercise 5-1 (Generating SSB signals by the Filter Method)
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Single Sideband Modulation – SSB
OBJECTIVE
When you have completed this unit, you will be capable of explaining and demonstrating SSB modulation using the AM / DSB / SSB Generator and the SSB Receiver.
DISCUSSION OF FUNDAMENTALS
The concept of SSB modulation can be represented as shown in Figure5 -1. The Spectrum of an SSB signal can be theoretically obtained in the manner suggested by first removing the carrier from the AM signal to produce the DSB spectrum shown in Figure 5-1 (b). Then, by removing one of the two sidebands from the DSB signal, one of the SSB spectra of Figure5 -1 (c) will be obtained. The advantages of SSB modulation are 1) there is no carrier presenting the spectrum of an SSB signal, and 2) only half the frequency bandwidth is required for communication since only one sideband is transmitted. Therefore SSB offers efficient power utilization and economic bandwidth use. These advantages are offset, however, by the fact that transmission and reception equipment is much more complex.
Figure 5-1. The concept of SSB modulation.
In practice it is simpler and easier too start with DSB, since this type of modulation is Obtained directly when message signal is combined with an RF Carrie through a Balanced mixer. To produce an SSB signal, all that remains to do is filter out on of the sidebands before the RF signal is transmitted. Figure5 -2 illustrates this theoretical process, which is known as the filter method of generating SSB signals
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Figure 5-2. SSB generation by the filter method.
The way in which this process is accomplished in the Analog communications Training System is shown in the function block diagram of Figure5 -3. The diagram is a modified version of the function representations shown on the front panel of the AM / DSB / SSB Generator.
Figure 5-3. Functional block diagram for generating an SSB signal.
The message signal is combined with the Beat Frequency Oscillator (BFO) signal, thus producing a DSB output centered at the BFO frequency. Sidebands election begins by adjusting the BFO frequency within its 450-46b kHz range. This causes the frequency contents of the message signal to be shifted relative to the passband of the fixed IF filter. Since the 455-kHz IF filter has a narrow 6-kHz bandwidth and sharp roll-off characteristics, the effect of frequency displacement (caused by varying the BFO frequency)is to "push" one of the sidebands outside the passband of the IF filter. once the desired sideband has been selected, the SSB signal is frequency-translated up to the carrier frequency determined by the Variable Frequency Oscillator (VFO). For the AM / DSB / SSB Generator, the SSB carrier frequency lies in the 8o-meter amateur band (3.7-4.0 MHz)
The final operations that are performed before transmission are RF filtering and amplification . The RF stage of the SSB section is designed to allow only the transmission of the difference frequency from the RF MIXER. This is necessary since the desired sideband selected at the lF stage has been reproduced by the second mixing operation. Recall that the combining of two signals through a balanced mixer always produces the sum and the difference frequencies at the output. Dr. Rad 85 EE321
Figure5 -4 illustrates this situation.
Figure 5-4. RF mixing reproduces the selected sideband
NEW TERMS AND WORDS
Beat Frequency Oscillator (BFO)- the narrow-range oscillator used to displace the spectrum of the message signals o as to position one of the sidebands outside the passband of the selective lF filter.
Sideband reversal- a phenomenon occurring in SSB modulation when the SSB receiver is adjusted to demodulate the opposite sideband relative to the transmitted sideband. For example, the SSB transmission is USB and the receiver is adjusted to demodulate the LSB (and vice versa).
SSB - a type of modulation in which the carrier is suppressed and only one of the two sidebands is transmitted, either the LSB or the USB.
variable Frequency oscillator (VFO) - the oscillator that determines the SSB carrier frequency
Analysis of Figure5 -5 shows that the whole operation of sidebands election consists essentially in moving the frequency contents of the messages signals so that only one of the sidebands is placed inside the passband of the fixed IF filter. When the BFO is tuned down to 452.5k Hz, F igure5 -5 (a), the LSB has been displaced to 450k Hz and outside the passband of the IF filter. At the same time the USB has been positioned in the center of the filter’s passband. The LSB has therefore been rejected(or greatly attenuated), and only the USB of the message signal remains for frequency translation up to the carrier frequency.
In Figure5 -5( b),t he BFO has been tuned to 455.0 kHz and because the maximum frequency of the messages signal is 2.5 kHz, both sidebands lie within the 6 kHz passband of the IF filter. If this signals is frequency-translated to the carrier frequency, the resulting RF output will be a DSB signal Dr. Rad 86 EE321
. LSB selection is illustrated in Figure5 -5 (c). T he BFO frequency has been tuned up t o 457.5k Hz. This shifts the USB to 460 kHz, and outside the passband of the IF filter. The LSB has been positioned in the center of the passband, thus completing the selection process. The selected LSB is then frequency-translated up to the carrier frequency and processed for transmission.
Figure5 -6 illustrates the selection process for SSB transmission of voice signals. The selection process is the same, and the only real difficulty arises when the sideband of the message signal are improperly positioned within the IF filter's passband. If the desired sideband is not positioned correctly, a part of the other sideband will be transmitted .This will result in a distorted and perhaps unintelligible output at the receiver when the RF signal is demodulated.
Figure 5-5. Sideband selection by mixing with a BFO signal.
EXERCISE OBJECTIVE
when you have completed this exercise, you will be able to explain and demonstrate the filter method of generating SSB signals using the AM / DSB / SSB Generator.
Discussion As stated earlier, SSB signals can be generated by filtering out one of the sidebands of a DSB signal. The following examples, illustrated in Figure 5-5, explain how this is done. The message signal is a 2.5-kHz sine wave and it will be combined with the BFO signal through the lF MIXER (see Figure 5-3). The BFO signal is a sine wave whose frequency can be adjusted anywhere between 450 and 460 kHz. The mixer output signal contains the sum and the difference frequencies of the two input signals, and the mixer's DSB output is filtered through the narrow-bandwidth IF filter before frequency translation u p to the carrier frequency. Three cases of BFO frequency adjustment (452.5, 455.0, and 457.5k Hz) are shown in Figure 5-5.
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Figure 5-5. Sideband selection by mixing with a BFO signal.
The lab components available for this and other experiments are shown in fig
AM/Noise Generator AM/DSB/SSB Generator AM/DSB Receiver Indirect FM/PM True RMS Voltmeter Dual Audio Amplifier Power Supply Frequency Counter Spectrum Analyzer
Audio Frequency Generator Oscilloscope
Lab-Volt modular communications lab components.
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Connections and setup to investigate SSB signal generation
A sideband and carrier of 455 KHz passing through filter
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Equipment Required
AM/DSB/SSB FREQUENCY GENERATOR COUNTER
SPECTRUM FUNCTION ANALYZER GENERATOR A
FUNCTION OSCILLOSCOPE AM/DSB RECEIVER GENERATOR B
TRUE RMS POWER DUAL AUDIO VOLTTMETER/POWER SUPPLY AMPLIFIER METER
Brief description of the equipment: 10) Function generator A and B: These two function generator is used to generator message signals. 11) AM/DSB/SSB GENERATOR: It is used to for amplitude modulation and generator either double side band or single side band waveforms for transmission. 12) TRUE RMS VOLTMETER/POWER METER: This equipment can be used to determine AC power. It has the option of display the signal level in terms of voltage, or display the signal strength in terms of dBM. 4) Power Supply: It is used to supply power to equipment 5) Dual Audio Amplifier: Use for amplifier signals from the receiver to a proper listening level. 6) AM/DSB Receiver: This equipment receives the amplitude modulated signals and demodulate it before sending it to audio amplifiers 7) Frequency Counter: This equipment can be used for monitoring frequency of signals. 11) Spectrum Analyzer: This equipment is used for displaying signals in frequency base for analysis. 9) Oscilloscope: Use to display waveforms in time domain.
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Exercise 5-1 (Generating SSB signals by the Filter Method)
Step 1: Set up the equipment as follow:
AM/DSB/SSB FREQUENCY GENERATOR COUNTER
SPECTRUM FUNCTION ANALYZER GENERATOR A
FUNCTION OSCILLOSCOPE GENERATOR B
POWER DUAL AUDIO SUPPLY AMPLIFIER
Set up the equipment in a manner where it will be easy to counter the appropriate modules together. We need to make sure all the level and gain control is set to the minimum to avoid any incidents.
Step 2: Generate a 2.5 kHz, 200mV sine wave from Function Generator A, and connect it to the AUDIO INPUT of the AM/DSB/SSB Generator.
Step 3: Setup the AM/DSB/SSB Generator. The CARRIER LEVEL should be at the minimum position and push in the LINEAR OVERMODULATION position. Set the SSB RF GAIN (amplifier A3) at ½ turner cw.
Step 4: Adjust the BFO Output to 452.5 kHz using the BFO TUNIGN knob. Result: BFO =
Step 5: Adjust the FVO TUNING control to measure a frequency of 4355 kHz at VFO OUTPUT (terminal 5). Result: fVFO =
Step 6: Prepare the Spectrum Analyzer ready for measurement. The center frequency should be center around ,5MHz. Dr. Rad 91 EE321
Step 7: Connect the SSB section to the input of the spectrum analyzer. Use the Tuning knob to place the spectrum analyzer of 455 kHz in the center of the screen.
Step 8: Find the best setting to place the display in the center of the spectrum analyzer.
Step 9: Sketch out the frequency spectrum .
fLSB = fUSB =
Step 10: Observe the frequency spectrum of the signal at the IF OUTPUT (terminal 4) and describe what happened.
Step 11: Observe and explain what happened as fBFO approaches 455 kHz.
Step 12: Observe what happens when the message signal frequency is increased to 3.5 kHz.
Step 13: Readjust the message signal to 2.5 kHz and tune the BFO to obtain 457.5 kHz. Observe what happens as fBFO is increased from 455 to 457.5 kHz. .
Step 14: Of this step is to vary fBFO between 453 and 457 kHz and observe what happens in the spectrum response as fBFO varies between 453 and 457 kHz.
Step 15: Set the spectrum analyzer up for the next measurement. Select the span to 1 MHz. Make sure SSB RF OUTPUT is at 455 kHz.
Step 16: Select the right setting to make sure the image is centered.
Step 17: Observe and describe the frequency spectrum using frequency span of 2 kHz/V.
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Step 18: Vary fBFO between 453 and 457 kHz and observe what happens.
Step 19: Conclude the effect caused by adjusting fBFO at 452.5 or 457.5 kHz.
Step 20: Determine the time waveform for an SSB signal modulated by a single-tone sine wave.
Step 21: Observe the waveform of the SSB signal at the SSB RF OUTPUT of the AM/DSB/SSB Generator when fBFO = 452.5 kHz. Compare the results with the predictions of step 20.
Time waveform from SSB Generator
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Review Question:
1. Sketch the frequency spectrum of an SSB signal (LSB and USB), and list the differences between an SB spectrum and those of an AM spectrum and a DSB spectrum.
2. What are the two principle advantages of SSB modulation over AM and DSB?
3. Describe briefly the filter method of generating SSB signals used in the Analog Communications Training System.
4. Observation of the spectrum at the output of an SSB generator shows that the spectrum is identical to the at of a DSB signal. Explain the problem with the generator.
5. The IF section of an SSB generator used for voice communications is tuned for operation at 455 kHz. The bandwidth of the IF stage is 2 kHz. What effect, if any, will the very narrow passband have on communications?
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Experiment 6: Fundamentals of Frequency Modulation Exercise 6-1 (FM Modulation Index) Exercise 6-2 (POWER DISTRIBUTION)
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EXERCISE OBJECTIVES
When you have completed this exercise, you will be able to establish the relationship between variations of the amplitude and frequency of the modulating signal and the sensitivity of the modulator, and the corresponding variations in the modulation index. You will be able to use these parameters to change the frequency deviation and the width of the spectrum of an FM signal.
DISCUSSION
The modulation index is just as important in frequency modulation as it is in amplitude modulation. However, it is not calculated in the same way in each case. Recall that the FM modulation index mf is equal to the ratio:
Therefore, any change in the frequency of the modulating signal will produce an opposite change in the modulation index for the same frequency deviation, as shown in Figure 5-1 (a).
If, for example, a carrier is frequency modulated by a 5 kHz signal, and the frequency deviation is 75 kHz, the modulation index equals 75/5 = 15.
If the frequency of the modulating signal is increased to 10 kHz, the modulation index will decrease to 7.5 (75/10).
If the frequency of the modulating signal remains constant and the frequency deviation is increased, the modulation index will increase, as shown in Figure 5-1 (b).
If the frequency deviation is changed from 75 kHz to 50 kHz, while the modulating signal frequency remains constant at 5 kHz, the modulation index will change from 15 to 10.
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Figure 6-1. Spectra of FM signals as a function of the modulation index mf.
The frequency deviation can be varied by changing the amplitude of the modulating signal or by using the DEVIATION control to vary the sensitivity of the FM modulator. The following equation shows the relationship between the modulation index, the amplitude and frequency of the modulating signal, and also the sensitivity of the modulator.
In this equation kfAm corresponds to the frequency deviation.
You will verify this relationship in the following exercise. The modulation index and the number of spectral lines will be varied using:
• the frequency and the amplitude of the modulating signal coming from the Dual FunctionGenerator
• the sensitivity of the modulator. This can be varied using the DEVIATION knob on the Direct FM Multiplex Generator.
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• As can be seen from the equation, mf is equal to the peak carrier frequency deviation (δ) caused by the amplitude of the modulating signal divided by the frequency of the message signal (Fm) so mf is a function of both the modulating signal amplitude and frequency. • Furthermore, mf can take on any value from 0 to infinity. Its range is not limited as it is for AM.
Figure 6.2 An un-modulated FM Carrier at 94.1 MHz
• Figure 6.3 A modulated FM Carrier 3. The lab components available for the FM experiments are shown in fig 6.3.
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FM/PM Receiver
Indirect FM/PM Generator True RMS Voltmeter
Power Supply
Spectrum Analyzer Dual Audio Amplifier
Frequency Counter Oscilloscope Audio Frequency Generator
Fig 6.4 Lab-Volt modular communications lab components.
The actual setup and wiring for part one is shown in figure 6.5 and part two in figure 6.6
Fig 6.5 Connections and setup to investigate FM modulation index
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Fig 6.6 Power to the sidebands setup (Part 2 of experiment 6)
Equipment Required
DIRECT FM TRUE RMS MULTIPLEX VOLTMETER GENERATOR POWER METER
FUNCTION FM/PM GENERATOR A RECEIVER
FUNCTION GENERATOR B SPECTRUM ANALYZER
POWER DUAL AUDIO OSCILLOSCOPE SUPPLY AMPLIFIER
Brief description of the equipment: 13) Function generator A and B: These two function generator is used to generator message signals. 14) Direct FM Multiplex Generator: Equipment use for transmitting FM signals 15) TRUE RMS VOLTMETER/POWER METER: This equipment can be used to determine AC power. It has the option of display the signal level in terms of voltage, or display the signal strength in terms of dBM. 4) Power Supply: It is used to supply power to equipment
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5) FM/PM RECEIVER: Receiver use for demodulating FM and PM signals 6) Frequency Counter: This equipment can be used for monitoring frequency of signals. 7) Spectrum Analyzer: This equipment is used for displaying signals in frequency base for analysis. 8) Oscilloscope: Use to display waveforms in time domain.
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Exercise 6-1 (FM Modulation Index)
Step 1: This step is to set up the equipment as follow:
DIRECT FM TRUE RMS MULTIPLEX VOLTMETER GENERATOR POWER METER
FUNCTION FM/PM GENERATOR A RECEIVER
FUNCTION GENERATOR B SPECTRUM ANALYZER
POWER DUAL AUDIO OSCILLOSCOPE SUPPLY AMPLIFIER
Set up the equipment in a manner where it will be easy to counter the appropriate modules together. We need to make sure all the level and gain control is set to the minimum to avoid any incidents.
Step 2: Set up all the test equipment for the experiment. The following is the setting for test equipment: 1. For Function Generator A, set the signal to sine wave, the frequency to 5 kHz, amplitude to 200mV. 2. On the TRUE RMS VOLTMETER/POWER METER, set the MODE to volt. 3. On the Direct FM Multiplex Generator, set the preemphasis to 0, Multiplix Signals to “all at 0 except L + R at 1”, Level to “Cal”, Deviation to “75 kHz (knob pushed – in ), and the RF GAIN to “50%” cw. 4. On the Spectrum Analyzer, set the frequency span to 1MHz and the frqeuncy range to 85-115 MHz.
Step 3: Connect the modulated signal and adjust the spectrum analyzer to appropriate span to observe the signal spectrum. It can be accomplished by connect the WBFM RF OUTPUT of the Direct FM Multiplex Generator to one of the WBFM RF INPUTS of the FM/PM Receiver and to the input of the Spectrum Analyzer. Use the Tuning control of the Spectrum Analyzer to move the carier line in the center of the spectrum analyzer, and decrease the Frequency Span to 10 kHz/V.
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Step 4: Adjust the carrier power level to +5 dBm.
Step 5: Tune the frequency of the FM/PM Receiver to the frequency of the Direct FM Multiplex Generator. This can be accomplished by tuning the frequency tuning control on the FM/PM Receiver to until the green TUNING LED lights up.
Step 6: Connect the message signal from Function Generator A to both the Input of the True RMS Voltmeter/Power Meter and the LEFT AUDIO INPUT of the Direct FM Multiplex Generator.
Step 7: Increase the amplitude of the message signal until the 2nd sideband pair of the FM spectrum is at a minimum. Result: The result of the spectrum is similar to the follow:
Figure 1
fm= 5KHz f = mf =
Step 8: Obtain the frequency deviation and calculate the modulation index. The frequency deviation can be obtained by set the DEVIATION push-button to WBFM on the FM/PM Receiver. The frequency deviation is displayed, and the modulation index can be calculated using Frequency deviation ∆f = frequency deviation /modulating signal frequency. Result: Frequency deviation ∆f = mf =
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Step 9: Decrease Frequency of the Function Generator until the 1st sideband pair pass to a minimum amplitude and to the maximum amplitude. The spectrum is show as follow:
Figure 2 fm= kHz f = kHz mf =
Compare this spectrum to figure 1.
Step 10: Determine the message frequency from function generator A. Result: fm =
Step 11: Read the frequency deviation from the receiver and calculate the modulation index using the relationship mf = ∆f/fm.
Step 12: Decrease in frequency of the modulating signal increases the modulation index.
Step 13: Readjust FREQUENCY A to obtain figure 1. Result: fm =
Step 14: Zero the TRUE RMS VOLTMETER/POWER METER and measure the modulating signal level. Result: Modulating signal level =
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Step 15: Increase the OUTPUT LEVEL A until we see the following frequency spectrum.
Figure 3
fm= f = mf =
Step 16: Measure the modulating signal level on the TRUM RMS VOLTER/POWER METER, and read the frequency deviation from the FM/PM Receiver. Result: Modulating signal level = ∆f = Observation: .
Step 17: Calculate the modulation index and compare figure 3 to figure 1 and figure 2.
Step 18: Readjust the amplitude for Function Generator A to obtain the frequency spectrum of figure 1.
Step 19: Set the sensitivity kf of the FM modulator to its minimum and observe what happen to the spectrum. This can be accomplished by turn the DEVIATION knob on the Direct FM Multiplex Generator completely counterclockwise, and pull it out.
Step 20: Increase the sensitivity of the FM modulator to maximum and observe what happen to the spectrum, the frequency deviation and modulation index.
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Review Questions:
1. An FM signal is modulated by a 10 kHz sinusoidal signal. What is the value of its modulation index if the frequency deviation is 10 kHz?
2. What is the relationship between the modulation index and the amplitude of the modulating signal?
3. What parameters can change the frequency devation?
4. An FM signal has a frequency deviation of 6 kHz when the modulating signal has an amplitude of 5 V, and a frequency of 1000 Hz. What will be the modulation index if the frequency of the modulating signal is doubled?
5. When the DEVIATION knob is adjusted, what parameter changes?
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Power Distribution
EXERCISE OBJECTIVES
When you have completed this exercise, you will be able to use the spectrum analyzer to find the power of each spectral component of an FM signal, and the total power of the signal.
DISCUSSION
In the preceding exercises, you saw that the spectrum of a signal frequency modulated by a sine wave theoretically contains an infinite number of spectral lines. These spectra lines are equally spaced on each side of the line corresponding to the carrier frequency fs. In practice, however, the number of significant spectral lines is limited. Once a certain point is reached, there is so little power left in the spectral component that it can be neglected.
In earlier exercises, you noticed that varying the modulation index caused corresponding variations in the relative amplitudes of the spectral lines. The respective amplitudes decreased the further the spectral line was from the carrier frequency line. The mathematical law for this decrease in amplitude as a function of the modulation index and the rank of each line ('lst, 2nd,3rd, etc.) is a complex one and refers to the Bessel functions. Table 1 is a practical tool built from the Bessel functions. lt gives the relative amplitude of each spectral line according to its rank, as a function of the modulation index. The relative amplitude is called the Bessel coefficient Jn (mf), where n is the rank of the spectral line and mf the modulation index.
For a given modulation index, each spectral component can be given a coefficient Jn(mf), for the n-th component. For example, if mf = 2, then Table 1 shows that the relative amplitude of the spectral line corresponding to the carrier (n = 0) is Jo(2) =0.22.The relative amplitudes of the other spectral lines, n = 1,2,3, 4, etc. depend on the Bessel coefficients J1(2) = 0.58, J2(2) = 0.35, J3(2) -=0.13, J4(2) = 0.03, etc. The coefficients listed in such a table go far enough to include at least 98% of the total power in a signal, which is enough for communications purposes.
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Table 1. Bsssel coefficient as a function of the modulation index.
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Note that in Table 1, the Bessel coefficient for the carrier equals zero when the modulation index is 2.4 [Jo(2.4) = ]1. This explains why the carrier disappears when the modulation index is 2.4. Figure 2.7 shows the relative amplitude of the other significant spectral lines when the modulation index is 2.4.
Figure2.7. Relative amplitude of the components when the modulation index is 2.4. The vertical scale of the Spectrum Analyzer is linear. The total power in a signal is proportional to the square of the amplitude of the unmodulated carrier. Therefore, it does not depend on the modulation index .only the spectral distribution depends on the modulation index. If there are many spectral lines, then less power is transmitted at the carrier frequency. This rule is not linear. Note: All though the index "c" corresponds to the carrier, its power will be called to correspond with the Bessel coefficient for the carrier, Js. since the power of a spectral component is proportional to the square of its amplitude, the Bessel coefficients can be used to calculate this power.
The tolal power Po in the carrier is proportional to the total power PT of the FM signal: 2 Po = Jo (mf) PT.
The power Pn contained in the n-th spectral component is given by: 2 PT= Jn (mf) PT since the spectrum is symmetrical around fC, this power must be multiplied by two. The total power contained in all the spectral components is equal to
PT = Po + 2P1 + 2P2 + 2 P 3 ...... e t c .
In this exercise, you will first find the total power of a frequency-modulated signal by using the line for the unmodulated carrier. Then you will increase the modulation index and see that more and more power goes into the spectral components.
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Exercise 6-2 (POWER DISTRIBUTION)
Step 1: This step is to set up the equipment as follow:
DIRECT FM TRUE RMS MULTIPLEX VOLTMETER GENERATOR POWER METER
FUNCTION FREQUENCY GENERATOR A COUNTER
FUNCTION GENERATOR B SPECTRUM ANALYZER
POWER DUAL AUDIO OSCILLOSCOPE SUPPLY AMPLIFIER
Set up the equipment in a manner where it will be easy to counter the appropriate modules together. We need to make sure all the level and gain control is set to the minimum to avoid any incidents.
Figure 2.7 . Relative amplitude of the components when the modulation index is 2.4 . The vertical scale of the Spectrum Analyzer is linear.
Step 2: Adjust the function generator A, Direct FM Multiplex Generator and the spectrum analyzer to appropriate settings for the experiment. 1. Function Generator A: f = 5 kHz A = 200 mV 2. Direct FM Multiplex Generator RF Gain: 50% cw Deviation: 75 kHz (knob push in)
Step 3: Calibrated the spectrum analyzer and connect it to the RF OUTPUT of the FM Multiplex Generator.
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Center the carrier frequency at center of the display. Adjust the RF Gain control to obtain -10dBm output level using the spectrum analyzer at 10 kHz/V.
Step 5 Connect the message signal to the AUX INPUT of the Direct FM Multiplex Generator. Slowly increase the level of the modulated signal until the spectral line of the carrier disappears. The modulation index is now 2.4. The spectrum look like the following:
Step 6: Slowly increase the signal level. Each time the amplitude of the center line goes through a maximum, record the value of the modulation index in the second column of Table 2. Use the table 1 to find the modulation index corresponding to each maximum. Continue to increase the signal level for three successive maxima. Result: MAXIMUM Mf 1 0 2 3 4 Table 2
Step 7: Determine how does the spectrum change when the modulation index goes from 0 to 10.5?
Step 8: Evaluate the power Pn of figure 3 of the first three spectral components nearest the carrier frequency and of the carrier.
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Figure 3 . FM Spectra for 3 successive maximum points of the carrier . EE321 112 Dr. Rad
FFigure 4 mW into 50 / dBm
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Result:
mf = 0
mf = 0
N 0 1 2 3
Pn(dBm)
Total Pn (mW) Po + ∑2Pn
2Pn(mW)
For figure 3a
mf = 3.5
n 0 1 2 3
Pn(dBm)
Total Pn (mW) Po + ∑2Pn
2Pn(mW) For figure 3b
mf = 7
n 0 1 2 3
Pn(dBm)
Total Pn (mW) Po + ∑2Pn
2Pn(mW)
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For figure 3c
mf = 10.5
n 0 1 2 3
Pn(dBm)
Total Pn (mW) Po + ∑2Pn
2Pn(mW)
Step 9: Observe the change in total power of the carrier and the spectral components when the value of the modulation index goes from 0 to 10.5?
Review Question:
1. If the modulation index is known, what other parameter must be known in order to determine the Bessel coefficient of a specific spectral component? .
2. A transmitter produces a signal with a total power of 100W. How much power is transmitted at the carrier frequency if the modulation index is 1.5?
3. What is the total power contained in the spectral components of an FM signal whose total power is 100W, if the power at the carrier frequency is 26W?
4. How does the spectral power distribution of a signal change when the modulation index increases?
5. How can the power be calculate from the n-th spectral component?
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Experiment 7: Generation of FM Signals)
Exercise 7-1 Direct Method of Generating FM Signals
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INTRODUCTION:
° Objectives:
The objective of this experiment is to investigate the fundamental factors involved in the generation of Frequency Modulation (FM) signals. We begin with the affect of the frequency then the amplitude of the message signal on the frequency deviation of the carrier and how often this deviation is occurring. It turns out that the FM carrier remains at a set frequency when the amplitude of the message signal is zero, and the carrier moves above the center frequency by an amount proportional to the positive amplitude of the massage signal, or drops below the center frequency by an amount proportional to the negative amplitude of the message signal. The rate of this back and forth, positive and negative deviations, is equal to the rate of change of the message signal from positive to negative or the frequency of the message signal.
° Basic Theory:
Generating an FM signal is similar to generating an AM signal in that it starts with an oscillator stage. This oscillator is tuned to the carrier frequency usually by a crystal for stability and displays a single peak on a spectrum analyzer. In order to send information using this carrier, the message signal’s amplitude will vary a reactive component in the carrier oscillator to slightly vary its value linearly up or down. This in turn will linearly move the frequency of the carrier up or down from the center by an amount directly proportional to the amplitude of the message signal. The rate at which this variance occurs depends on the frequency of the message signal. The best device for this task is a varactor. This is a reverse biased diode whose capacitive characteristics dominate and will change based on the fluctuation of the reverse bias imposed upon it. When this device is placed in the feedback portion of the oscillator it will determine the frequency that will get fed back, amplified, and presented at the output of the transmitter’s oscillator to be amplified and eventually transmitted over the air. This type of an oscillator is called a Voltage Controlled Oscillator or VCO. The mathematical representation of a transmitter’s instantaneous output was presented in the last experiment, and is shown here again with three variables that can affect the output. E varies during amplitude modulation, ω which represents 2πf, accounts for frequency when varied during Frequency Modulation, and varying the angle φ is a result of phase or angle modulation.
e(t) = EP sin(ωt + φ)
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Figure 7.1 An un-modulated FM Carrier at 94.1 MHz
4. The lab components available for the FM experiments are shown in fig 7.2.
FM/PM Receiver
Indirect FM/PM Generator True RMS Voltmeter
Power Supply
Spectrum Analyzer Dual Audio Amplifier
Frequency Counter
Oscilloscope Audio Frequency Generator
Figure 7.2 Modulated FM Carrier Equipment and its Connections
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The actual setup and wiring for this part is shown in figure 7.3.
Fig 7.3 Connections and setup to investigate FM Generation and Deviation
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Equipment Required
DIRECT FM OSCILLOSCOPE MULTIPLEX GENERATOR
FUNCTION FREQUENCY GENERATOR COUNTER A
FUNCTION SPECTRUM INDIRECT FM/PM GENERATOR ANALYZER GENERATOR B
POWER DUAL AUDIO FM/PM SUPPLY AMPLIFIER RECEIVER
Brief description of the equipment: 16) Function generator A and B: These two function generator is used to generator message signals. 17) Direct FM Multiplex Generator: Equipment use for transmitting FM signals 3) Power Supply: It is used to supply power to equipment 4) FM/PM RECEIVER: Receiver use for demodulating FM and PM signals 5) Frequency Counter: This equipment can be used for monitoring frequencies. 6) Spectrum Analyzer: This equipment is used for displaying signals in frequency base for analysis. 7) Oscilloscope: Use to display waveforms in time domain. 8) Indirect FM/PM Generator: Equipment use for generating FM/PM signals 9) FM/PM Receiver: Equipment use for detecting FM/PM signals
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Exercise 7 (Direct FM Generations)
Step 1: This step is to set up the equipment as follow:
DIRECT FM OSCILLOSCOPE MULTIPLEX GENERATOR
FUNCTION FM/PM GENERATOR RECEIVER A
FUNCTION SPECTRUM GENERATOR ANALYZER B
POWER DUAL AUDIO SUPPLY AMPLIFIER
After setting up the equipment, as required by the experiment, and shown below in the close up of fig. 7.4 we adjusted the levels of the message signal and RF signal to what was specified in steps 3-10 of the experiment and obtained the results shown in tables 7.1 & 7.2 below ( first at 75KHz then 5-100KHz deviation). A third table with all zero values (not shown) proves that there is no carrier deviation when the message is set at zero amplitude and is essentially absent.
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Fig 7.4 Close up of Deviation Readout when signal is tuned
Fig 7.5 Tuned signal (Green LED) and Deviation Reading (Red 7 Segment LED)
Set up the equipment in a manner where it will be easy to counter the appropriate modules together. We need to make sure all the level and gain control is set to the minimum to avoid any incidents.
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Set the following equipment to their proper settings to prepare for test measurements. Function Generator A: Frequency=6 kHz Output Amplitude=300mV peak Direct FM Multiplex Generator: Multiplex Signals Off Level Cal Deviation 75 kHz (knob pushed-in) RF Gain 50% cw
Step 3: Set up the message signal, Direct FM Multiplex Generator and the FM/PM Receiver for testing. This can be accomplished by the following steps: 1) Connect output from Function Generator A to the AUX INPUT o fth eDirect FM Multiplex Generator and to Channel 1 input of the oscilloscope. 2) Connect the WBFM RF OUTPUT of the Direct FM Multiplex Generator to the WBFM RF INPUT of the FM/PM Receiver. 3) Using the RF Tuning knob, tune the FM/PM Receiver to the frequency of the Direct FM Multiplex Generator. When the green LED comes on, it indicates the frequency is properly tuned. Step 4: Record the frequency deviation. This can be accomplished by observing the digital readout that shows the DEVIATION of the modulated signal. Select the WBFM mode and record the deviation.
Step 5: Vary the frequency of the message signal between 300 Hz and 15 kHz and observe the change in frequency deviation. .
Step 6: Observe the change in frequency deviation when the amplitude of the message signal is increased 10 times.
Step 7: Vary the amplitude of the message signal and record down the corresponding frequency deviation and record on table 1. Calculate the sensitivity kf using the relationship kf = ∆f(read)/Am and record it on table 1.
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Result: Am ∆f (read) Kf V peak kHz kHz/V 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.50 3.00 TOTAL AVERAGE SENSITIVITY Table 1
Step 8: Plot the curve of frequency deviation ∆f as a function of the amplitude Am of the message signal.
Step 9: Observe the frequency deviation when the DEVIATION knob is pulled-out. This can be accomplished by pull-out the DEVIATION knob, record the deviation for complete counterclockwise, and complete clockwise.
Step 10: Make a table similar to table 1, but this time with the DEVIATION knob pull-out. Result: Am ∆f (read) Kf V peak kHz kHz/V 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.50 3.00 TOTAL AVERAGE SENSITIVITY Table 2 EE321 124 Dr. Rad
Review Questions
1. What defines direct generation of an FM signal?
2. What is the basic principle behind the operation of a VCO? . 3. What does the varactor do in a direct FM generator?
4. In a variable frequency LC oscillator, the variable element is a varactor. The characteristic of this varactor is shown in Figure 5-6. Knowing that the oscillation frequency of an LC oscillator is equal to 1/(2√LC), what happens to this frequency if the voltage applied to the varactor. . 5. What is the frequency of the oscillator circuit in a direct FM generator if the level of the modulating signal is zero?
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Exercise 8 (Indirect Method of Generating FM Signals)
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to demonstrate the principles of indirect generation of FM signals. You will produce FM signals using the Armstrong modulator in the Indirect FM / PM Generator. Using the Frequency Counter and the Spectrum Analyzer, you will be able to analyze signals at various stages in the modulation process.
DISCUSSION
Direct generation of an FM signal consists of directly modulating the frequency of an oscillator. Indirect modulation uses a completely different approach. You saw that narrow band frequency modulation could be obtained by using a PM generator if the signal first passed through an integrator, as shown in Figure 8-1.
Figure 8-1. NBFM Generator.
The similarity of AM, NBFM, and PM spectra suggest a method to generate a narrow band angle-modulated signal. As shown in Figure 8-2, the PM modulator (Figure 8-2 (b)) is an AM modulator (Figure 8-2 (a)) with a phase shifting circuit added between the base oscillator and the summing amplifier.
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Figure 8-2- Principle of generation or AM (a) and PM (b) (Armstrong Modulator).
This method of generating an NBFM signal is called the Armstrong method. since the modulation index is limited (an NBFM or PM signal), this method of indirect modulation of a wideband signal involves increasing the carrier frequency and the frequency deviation using one or more frequency multipliers.
The frequency of the outgo signal is set by using a mixer and a fixed, very stable local oscillator. Figure 8-3 shows a block diagram of a wideband FM generator circuit (WBFM) using an Armstrong modulator.
Figure 8-3. Indirect generation or a WBFM signal using an Armstrong modulator.
Together, the mixer following the multiplier, and the bandpass filter are used to lower the signal frequency. At the mixer output, the frequency of the signal is equal to N times the base oscillator frequency, plus or minus the local oscillator frequency (NfBO ± fLO). For example, if N = 332, the base oscillator frequency equals 32.23 KHz, and if the local oscillator frequency equals 17 .48 MHz, the frequencies in the signal will be: EE321 127 Dr. Rad
(332 x 0.03223) + 17.48 = 28.18 MHz and (332 x 0.003223) - 17.48 = 6.78 MHz
The filter eliminates the sum, so the frequency of the output signal is equal to N times the base oscillator frequency minus the local oscillator frequency (NfBO ± fLO) . In our particular case, the frequency of the remaining signal is 6.78 MHz. All the spectral components are translated up to this range of frequencies.
The mixer, the local oscillator, and the filter make up the frequency conversion stage. In broadcast transmitters, this enables the carrier frequency to be precisely set within the allocated frequency band (88-108 MHz).
Since the local oscillator frequency is fixed at 17.48 MHz, it can be produced by a quartz crystal, and is very stable. Frequency stability is one advantage of indirect FM generation as compared to direct FM generation. On the other hand, indirect generator circuits are much more complex.
The Indirect FM / PM Generator which you will use is slightly different than the basic circuit. The first multiplier (f x 332) is followed by a mixer. A low pass fitter is ahead of the second multiplier (l x 15), Figure 8-4 shows the block diagram of the Indirect FM / PM Generator.
Figure 8-10. Block diagram of the Indirect FM / PM Generator used.
.
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Step 1: This step is to set up the equipment as follow:
INDIRECT OSCILLOSCOPE FM/PM GENERATOR
FUNCTION FREQUENCY GENERATOR COUNTER A
FUNCTION SPECTRUM GENERATOR ANALYZER B
POWER DUAL AUDIO SUPPLY AMPLIFIER
Set up the equipment in a manner where it will be easy to counter the appropriate modules together. We need to make sure all the level and gain control is set to the minimum to avoid any incidents.
Step 2: Determine the frequency multiplication ratio between TP6 and the NBFM Signal. This can be accomplished by measure the frequency at TP 6 from the FM/PM GENERATOR using the frequency counter. Record it down and measure the frequency of the PM/NBFM RF OUT. Record it down and look at the ratio between the two frequencies. .
Step 3: Measure the frequency at TP20 and TP15 and determine their ratio. This can be accomplished by measuring the frequency at TP20 and TP15 using a frequency counter. Use the results to determine the frequency difference between TP20 and signal at the PM/NBFM RF OUTPUT.
Step 4: Measure the frequency of the WBFM RF OUTPUT and determine the multiplication factor of the 2nd frequency multiplier.
.
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Step 5: Set up the Spectrum Analyzer for measurement.
Step 6: Connect the WBFM RF OUTPUT of the indirect FM/PM Generator to the INPUT of the Spectrum Analyzer. Set the span to 10 kHz/V while keeping the carrier line in the center of the screen. Adjust the RF Gain control on the Indirect FM/PM Generator to obtain a height of 5 division for the carrier frequency line and connect the modulation signal to the Audio Input.
Step 7: Make sure the modulation signal is 5 kHz. Adjust the amplitude of the modulation signal until we obtain modulation index of 5.5 to obtain similar spectrum response as follow: Result:
Step 8: Connect the PM/NBFM RF OUTPUT of the Indirect FM/PM Generator to the INPUT of the Spectrum Analyzer. This can be accomplished by initially setting the span to 1 MHz, adjust the carrier line to the center of the spectrum analyzer. Slowly adjust the span down to 10kHz/V while maintaining the carrier line at the center of the spectrum analyzer. Result:
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Step 9: Determine the difference in dB between the carrier and the first spectra component and use the result to determine the modulation index.
Step 10: Display the frequency characteristics of the output of the mixer at the PM/WBFM RF OUTPUT. This can be accomplished by centering the center frequency of the spectrum analyzer at 6.78 MHz and frequency span of 10kHz/V. Result:
Review Questions:
1. Why do we say that the Armstrong method produces a WBFM signal indirectly? 2. What is the purpose of the filter after the mixer? . 3. Why is very good frequency stability possible with indirect frequency modulation? . 4. Why is frequency multiplication used?
5. What is the main purpose of the mixer
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MATLAB
After becoming familiar with some of the basic features of MATLAB , we can readily simulate some basic analog communications problems similar to those in the actual laboratory experiments.
In the first project, you learned how to define time and data vectors, how to define variables (for example H, w, t, Fs, etc), data ploting in time and frequency, and experimented with filtering signals, thus you will have to refer to what was learned in the first project for the completion of this assignment.
In the previous assignment you were given step by step instructions just to familiarize you with the program. This time, it is assumed that you already know how to deal with the basics. Remember that the help command will get you out of trouble most of the time (example: help filter). Also, the lookfor command will search for specific words (example: lookfor filter).
Amplitude Modulation:
PART 1
We would like to amplitude-modulate a 10 Hz information sinusoid, with a carrier frequency of 1000 Hz. Using the knowledge from the previous assignment, do the following:
A. Specify the samplig frequency Fs=10000 Hz
NOTE: We choose the sampling frequency high enough to avoid aliasing, and have enough data to analyze in our time domain.
B. Create a NORMALIZED time vector t with 10000 samples, that goes according with the already specified sampling frequency t = [1:Fs]/Fs; C. Specify the carrier frequency Fc=1000; D. Specify the information signal frequency: f =10; E. Create your information signal: y=sin(2*pi*f*t);
F. MATLAB contains a command called modulate, that allows you to modulate using a wide number of techniques. Type help modulate, and find out how to create another vector that will contain the amplitude modulated signal. Example:
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m=modulate( *,*,*,*,*)
NOTE: You are supposed to fill the remainder of the command. There are 5 variables you are supposed to specify, but neglect the last (OPT command), just concentrate on the first 4. Also, note that the METHOD specification will have to be between quotes
example: m=modulate(*, *, *, 'am');
G. After you have created the vector for the am waveform, plot the information signal vs. time and the modulated signal vs time. PRINT THE PLOTS
H. Repeat steps f and g for the following:
i. Amplitude modulation double sideband transmitted carrier ii. Amplitude modulation single-sideband
I. After plotting all of the waveforms, compare them visually, and EXPLAIN in your own words why the waveforms are different. Which of the 3 modulation techniques is the most suitable for demodulation with an envelope detector, and what is the percentage of modulation based on the graph you obtained?
J. Assuming that the name of your vector for amplitude modulation double sideband transmitted carrier is mod, plot the power spectral density of the information signal and the modulated signal in two adjacent plots, and compare them and EXPLAIN why they are different and what we expect to see.
subplot(211) psd(y); subplot(212) psd(mod)
K. To find out how to demodulate an am signal, type
help demod
and recover the information signal that was originally modulated. Use the 'am' or the 'amdsb-tc' modulated signals obtained in steps F or H. Plot the demodulated signal, print it and compare it to the information signal.
PART 2
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MATLAB has stored in memory several variables that are useful in tutorials. The way that those variables are recalled to the workspace ib by typing:
load filename
where filename is the name of the file of interest. Clean the workspace by typing clear all. This will remove all the variables previously created so that there is no confussion. Load the file called gong. This returns a data vector y which are sampled of the sound of a gong, and a variable Fs that specifies the sampling frequency. (NOTE: to see what are the variables in your workspace, you can always type the command whos)
1. Plot the gong sound vs time. (Note: you do not have to create a time vector, simply type plot(y)) 2. If your computer is equiped with speakers, type the following:
sound(y,Fs)
You should hear the sampled signal representing the sound of the gong !!!!
3. Plot and print the power spectral density of the sound vector. Remember that the frequency axis is normalized, therefore a frequency of 1 represents half the sampling frequency or Fs/2.
4. Amplitude Modulate the sound with a frequency of 2000 Hz. Try playing the modulated signal on the speakers just for fun with the sound command.
5. Obtain and print the power spectral density of the modulated signal.
6. On the above plot, identify the carrier frequency, and compare the shape of this frequency plot to that of the psd of the un-modulated sound. EXPLAIN ! (Hint: there should be symmetry about some point!)
7. Demodulate the AM signal, and play the resulting sound with the appropriate sampling frequency. Just for fun, play the sound with different sampling frequencies and try to figure out why it sounds different.
If you want to experiment with other sounds, try the following files:
train, laughter, chirp, splat, handel
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Frequency Modulation:
A. Follow steps A through E in the previous exercise. We will be using the same sampling, carrier, and information frequencies to be able to compare the time waveforms of AM and FM signals.
B. In addition to the information signal y, create two more vectors x and z that represent a sawtooth wave and a square wave of the same frequency and amplitude
x=sawtooth(2*pi*f*t); z=square(2*pi*f*t);
C. Type help modulate, and find out how to FREQUENCY modulate your three signals (x, y and z). Once again, do not worry about the (OPT) variable, just specify the first four variables.
D. Plot the sine, sawtooth and square signals vs. time and their corresponding FM signals, and explain what is their relationship. To observe the waveforms better, plot only 2000 samples of each. An example of this is shown below:
plot(t(1:2000),y1(1:2000))
note: t(1:2000) means the first 2000 samples of the time vector.
E. Listen to the modulated and un-modulated signals on the speaker!
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