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LABORATORY MANUAL COMMUNICATIONS LABORATORY EE 321 © Khosrow Rad Revised 2011 DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING CALIFORNIA STATE UNIVERSITY, LOS ANGELES Lab-Volt Systems, Inc 1 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) 3 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. 4 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. 5 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 6 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. Dr. Rad 7 EE321 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. Dr. Rad 8 EE321 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. Dr. Rad 9 EE321 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:
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