Operational Diagrams of Transmitters & Receivers

Professor Lance Breger and Professor Kenneth Markowitz TABLE OF CONTENTS

Page

Preface ...... 4 - 5

Introduction...... 6. - 7

Operation of Adder- Transmitter ...... 9

Operation of AM Transmitter in General...... 10.

Operation of AM Transmitter Showing AM Modulator in Detail. . . . . 11

Operation of TRF AM Receiver...... 12.

Operation of Superheterodyne AM Receiver ...... 13

Operation of PM Transmitter...... 14.

Operation of FM Transmitter in General...... 15

Operation of FM Transmitter with Indirect Modulation...... 16.

Operation of FM Transmitter with Direct Modulation...... 17

Operation of FM Receiver...... 18 - 19

Operation of PM Receiver...... 20.

Conclusion...... 21.

Copyright 2004 Lance Breger 3 PREFACE ● Students: M. B. Suranga Perera, Nikolay Ostrovskiy and Johnny Lam. They produced the professional The purpose of these diagrams is to graphically explain the overall operation of AM, PM, and graphics in these vector art diagrams and created the pages. FM communications systems using very little mathematics. This explanation is accomplished by tracing a simple sinusoidal signal through all stages of each system. Although students who are "mathematically To pay these students, the following persons or organizations at NYCCT generously offered financial challenged" will find these diagrams very helpful, most students who are beginning the study of electrical advice or funds: communications systems can benefit from these same diagrams. More advanced courses can also use ● Former Dean Phyllis Sperling of the School of Technology & Design these diagrams as a basis on which to organize and present abstract mathematics. ● Former Dean Annette Schaefer of the School of Arts and Sciences The unique features of these diagrams are the following: ●  ● presenting the signal in both the time domain and domain together at each stage of the com- Professor Joseph Rosen, head of the Freshman Year Program at NYCCT and current acting Dean of the School of Liberal Arts and Sciences munication process ● using a color code to show the distribution of information in the signals in both the time domain and ● Ms. Jewel Escobar, Executive Director of NYCCT Foundation frequency domain simultaneously. ● Federal Work Study Program administered by the Department of Financial Aid at NYCCT. These diagrams were produced by translating the statements and logic from the following two textbooks into simple graphics: ● The Department of Architectural Technology deserves thanks for allowing almost all of the computer Modern Electronic Communication, 6th edition, drawings to be done on their computers using Adobe Illustrator. by Gary M. Miller, Prentice Hall, Upper Saddle River, NJ, 1999. Electronic Communication Techniques, 4th edition, ● Dean Robin Bargar by Paul H. Young, Prentice Hall, Upper Saddle River, NJ, 1999. ● Professor Joel Mason, Chair of the Department of Advertising Design and Graphic Arts (ADGA), for allow- Graphing calculators were used to plot equations, and some of the particularly difficult steps were ing the intermediate and final color prints to be produced on the ADGA Print Laboratory's DocuColor 2060 checked by simulation using Multisim or by TIMS (Telecommunications Instructional Modeling Systems) press under a grant by Xerox.

The history of preparing this booklet is a long one. Before beginning the arduous work of produc- Your comments will be used to produce an improved second edition of this booklet. We, the authors, would ing these diagrams, we inspected about 70 standard textbooks on electrical communications to determine greatly appreciate being informed of any mistakes or of other comments about this booklet by emailing me: whether we could save ourselves a lot of effort by simply using their diagrams; but none of those books [email protected] contained the above simultaneous diagrams. Some of the basic ideas underlying these diagrams were pre- sented by us in 2001 at a conference of FIE (Frontiers In Education) in Reno, Nevada. Completion of this booklet took about three more years of devoted labor, research, and collaboration.

During the preparation of this booklet, many others at the New York City College of Technology (NYCCT) aided us. Helpful technical suggestions were offered by several of our colleagues:

● Professor Aron Goykadosh

● Professor John Fehling

● Professor Misza Kalechman

● Professor Mohammed Kouar

● Professor Mohammad Razani.

● Professor Lloyd Carr, who initiated and directed the vector file production, page imposition for printed versions and the final pdf files. Professor Carr also helped to edit the final text.

4 5 INTRODUCTION A color code has been used to draw the various signals in these diagrams mainly to help students trace the flow of information, which is vital to understanding the operation of a communications system. Of To help the reader use this set of diagrams of AM, PM, and FM efficiently, two sets of comments course, the real signals or waves are invisible, i.e., colorless, to our eyes; but the colors arbitrarily assigned have been added to the diagrams to elucidate them. The first set of comments applies generally to all the to the signals in these diagrams show their content of information. diagrams and is included in this introduction. The second set of comments applies specifically to individual diagrams, and each of these comments is inserted in the main text adjacent to the diagram that it explains. ● Blue signals in the time domain or in the frequency domain indicate the pure information signal.

GENERAL COMMENTS ● Red signals indicate the pure . The main objective of these diagrams is to help students understand the operation of transmitters and receivers of various types by showing how the signal changes as it propagates through each stage in a series ● Purple signals show that the information signal and the carrier wave have been mixed, just as blue and red of cascaded stages, whose electrical operating characteristics are also usually shown in the diagrams. For colors mix to form the color purple. simplicity the information signal is assumed to be a sinusoid. The signal itself is shown in two adjacent forms: ● Green indicates the operating characteristic in graphs of various devices, like and filters. • in time domain, i.e., how the signal changes in time as it would be observed on an oscilloscope inserted at a particular ● Black signals indicate . point in the system A somewhat different color code has been used for the arrows showing signal propagation in the left-most • in frequency domain, column. As above, red arrows mean the carrier wave; and blue arrows mean the information signal. However, i.e., how the signal changes in sinusoidal composition as it would be observed on a spectrum analyzer the mixed state of carrier plus information is indicated by the split arrow, half being red and half being blue. inserted into the system at the same point as the above oscilloscope. [The diagrams on the FM receiver and the PM receiver violate this rule slightly by showing noise separately.] In symbols and equations used these diagrams, the red carrier wave is symbolized by "c;" and the blue information signal is symbolized by "m" (not "i" as is used by many texts) since "m" stands for the modulating Being able to relate these two views of the signal is a major lesson in communications, since beginners usu- signal, i.e., the information. ally think only in the time domain; but experts in communications usually think in the more abstract frequency domain. In addition, the mathematical equations for important signals in the time domain have also been The scale of the vertical voltage axis in the time domain differs from voltage scale in the frequency inserted to help the student correlate this information with the two graphic forms of the signal. domain because otherwise the size of the bars in the frequency domain would have been too small to have been read easily. To remind the student that these scales are different, an arrow has been drawn and labeled The general structure of each diagram is as follows. In the left-most column, the various cascaded as "amplitude" in the earliest sinusoid in the time domain in each diagram. ["Amplitude" in all these drawings stages are arranged vertically from top to bottom. Arrows represent the signal flow between the stages. To means the amplitude of a sinusoid, not of any other function.] This arrow then reminds the reader of how big the right of each arrow and at the same level are the two graphic representations of the signal itself, i.e., first the amplitude in the frequency domain really is in the time domain. in the time domain and then in the frequency domain. Thus, the three columns of the diagram are arranged from left to right from most concrete to most abstract. In addition, the operating characteristics of some devices, e.g., amplifiers and filters, are also shown. These characteristics are shown in the domain where In general, the wave forms and operating characteristics shown in these diagrams are mere "car- their operation is most easily represented. For example, the operation of filters is most easily described in toons," or idealizations, of reality, not exact pictures. This approximation is appropriate because exact repre- the frequency domain; while the operation of a limiter in an FM receiver is most easily described in the time sentations would be difficult to understand. For example, in the case of FM as seen in the frequency domain, domain. if the various sidebands were drawn to scale, the sidebands could not easily be distinguished from the carrier in the size of diagrams drawn here. (continued on page 7) Finally, the amount of detail shown varies from diagram to diagram. For example, some diagrams show the operating characteristics of ideal amplifiers; others do not. In each case, our aim was to show as much detail as possible without cluttering the diagram.

6 7 OPERATION OF ADDER-ANTENNA TRANSMITTER This diagram shows a transmitter that does not work to emphasize the reason that real transmitters do work. That is, this trans- mitter leaves the information, which is shown in blue, at low so that it can not be radiated; the radiating antenna only broadcasts high frequencies. This implies that only the high-frequency carrier wave is broadcast, which is useless. The aim of a communications system is to broadcast information, not to broadcast the pure carrier wave!

fm fc Amplitude (Volts)

Voltage (Volts) Em Ec Time (Sec.)

Low Frequency High Frequency Information Carrier fm fc Frequency (Hz)

Voltage (Volts) Amplitude (Volts)

Em Ec Time (Sec.)

Combined Signal fm fc Frequency (Hz) (fm << fc ) Antenna Gain 1

Antenna removes low frequencies 0 Frequency (Hz)

Amplitude (Volts)

Voltage (Volts) Ec Time (Sec.)

High Frequency Carrier fm fc Frequency (Hz)

8 Copyright © 2004 Lance Breger 9 OPERATION OF AM TRANSMITTER IN GENERAL OPERATION OF AM TRANSMITTER SHOWING AM MODULATOR IN DETAIL This transmitter works, while the previous one fails, because the AM modulator raises the information, shown in blue, to high This diagram differs from the previous one only in that it shows the details of the AM modulator. The operation of the modula- frequencies so that it can be broadcast along with the carrier by the high-pass antenna. The AM modulator automatically raises tor consists of two stages. First, the information signal and carrier are passed through a nonlinear device, e.g., a or the frequency of the information when encoding information by changing the amplitude of the carrier sinusoid. diode, to generate the required upper and lower sidebands along with unnecessary sinusoids of many other frequencies. [The diagram mentions an "ideal" nonlinear device; "ideal" means that most of the unnecessary harmonics have been omitted for clarity.] Second, a high-frequency bandpass filter removes the unnecessary sinusoids and passes only those in the required AM-modulated carrier. fm fc Amplitude (Volts) Voltage (Volts) Em Ec Amplitude Time fm fc (Sec.) Amplitude (Volts) Voltage (Volts) Em Ec Amplitude Low Frequency High Frequency Time fm fc Frequency (Sec.) Information Carrier (Hz)

Low Frequency High Frequency fm fc Frequency AM Information (blue) Carrier (red) (Hz) MODULATOR Amplitude (Volts) AM MODULATOR Voltage (Volts) IDEAL NON-LINEAR E DEVICE c Voltage (Volts) Time Amplitude (Volts) Ec (Sec.) Em/2 Em/2 Em/2 Em/2 Time Combined Signal (Sec.) dc fm (fc-fm)(fc fc+fm) Frequency (Hz) fm (fc-fm)(fc fc+fm)Frequency (Hz) Gain of Filter 1 (fm << fc ) BAND-PASS Filter selects carrier and sidebands. FILTER 0 Gain Frequency (Hz) K>1 (Volts) Voltage Amplitude Input (Volts) AMPLIFIER Amplifier gain = constant K>1. Ec Time Frequency (Sec.) Em/2 Em/2 (Hz) Voltage (Volts) Amplitude (Volts) Combined Signal fm (fc-fm)(fc fc+fm)Frequency (Hz) (f << f ) m c Amplifier Gain K>1 Time AMPLIFIER Amplifier gain = constant K>1. (Sec.) Frequency Voltage (Volts) (Hz) Amplitude (Volts)

(fc-fm)(f fc+fm) Combined Signal fm c Frequency Time (Hz) (Sec.) Antenna Gain

ANTENNA Antenna passes only high frequencies. Combined Signal fm (fc-fm)(fc fc+fm)Frequency 0 (Hz) Frequency Antenna Gain (Hz) 1 Voltage (Volts) ANTENNA Antenna passes only high frequencies. Amplitude (Volts) 0 Frequency Voltage (Volts) (Hz) Amplitude (Volts) Time (Sec.) Time (Sec.)

Combined Signal Combined Signal f (f -f )(f +f ) fm (fc-fm)(fc fc+fm)Frequency m c m fc c m Frequency (Hz) (Hz) Copyright © 2004 Lance Breger

10 Copyright © 2004 Lance Breger Copyright © 2004 Lance Breger 11 OPERATION OF TRF AM RECEIVER OPERATION OF SUPERHETERODYNE AM RECEIVER The main point of interest here is the structure of the AM . The basic structure of the AM detector and of the AM modulator The structure of this receiver and that of the TRF receiver are very similar. The main difference is the frequency range in which are the same: a nonlinear device followed by a bandpass filter. In the detector, the nonlinear device reproduces the original signal most of the signal amplification is done. The TRF receiver amplifies the signal at the same high radio frequencies at which it is sinusoid from the AM-modulated carrier and also produces many unnecessary sinusoids of other frequencies; the filter removes received initially by the antenna. Unfortunately, amplification at these high frequencies is inefficient, i.e., the amplifier gains are these unnecessary sinusoids and passes the original information signal. The main difference between the AM modulator and low. To correct this error, the does most of its signal amplification at a lower "intermediate frequency" the AM detector is the frequency band passed by the filter: the modulator passes a band at high radio frequencies; the detector band for greater efficiency. Since this intermediate frequency band is the fixed passband of the filter in the IF amplifier, each passes a band at low audio frequencies. incoming signal must be lowered to this fixed intermediate frequency band by using a frequency converter, which consists of a mixer (which is a nonlinear device) connected to an oscillator of variable frequency. This is called the "." By changing the oscillator frequency appropriately, any station's signal can be lowered to the given intermediate frequency range for efficient amplification. ANTENNA Amplitude (Volts)

Voltage (Volts) ANTENNA Voltage (Volts) Amplitude (Volts) Time Time (Sec.) (Sec.) (fc-fm) fc (fc+fm) Frequency (Hz)

(fc-fm)(fc fc+fm)Frequency RF AMP. & RF AMP. & (Hz) bandpass filter Amplitude (Volts) f LO Voltage bandpass filter Amplitude (Volts) (Volts) Time Voltage (Volts) LO (Sec.)

(fc-fm) f (fc+fm) Frequency c (Hz) Time (Sec.) NON-LINEAR Voltage (Volts) Amplitude (Volts) DEVICE (mixer) (fm < fi < fc < fLO) f LO Time (Sec.) (fc-fm)(fc fc+fm)Frequency DETECTOR (Hz) (fi-fm) fi (fi+fm) (fc-fm) fc (fc+fm)Frequency (Hz) IDEAL Gain 1 Amplitude (Volts) IF amplifier NON-LINEAR Filter selects band around fi DEVICE Voltage (Volts) FILTER the intermediate frequency. 0 Frequency (Hz) Voltage (Volts) Amplitude (Volts) Time Time (Sec.) (Sec.) (fi-fm) fi (fi+fm) Frequency (Hz) (f -f )(f +f ) IF AMPLIFIER dc fm c m fc c m Frequency Voltage Amplitude (Volts) (Hz) PER SE (Volts) Gain Time 1 BAND-PASS (Sec.) Filter selects information signal. FILTER Frequency 0 (fi-fm) fi (fi+fm) DETECTOR (Hz) Frequency IDEAL Amplitude (Volts) (Hz) NON-LINEAR Voltage (Volts) Voltage (Volts) Amplitude (Volts) DEVICE

Amplitude Time Time (Sec.) (Sec.) dc fm (fi-fm) fi (fi+fm) Frequency (Hz) Gain BAND-PASS 1 fm Frequency Filter selects information signal AUDIO (Hz) FILTER 0 Frequency AMPLIFIER (Hz) Voltage (Volts) Amplitude (Volts) Voltage (Volts) Amplitude (Volts) Amplitude Time (Sec.) fm Frequency (Hz) AUDIO Time AMPLIFIER (Sec.) Voltage (Volts) Amplitude (Volts)

Amplitude Time fm Frequency (Sec.) (Hz) SPEAKER fm Frequency (Hz) SPEAKER

Copyright © 2004 Lance Breger Copyright 2004 Lance Breger 12 Copyright © 2004 Lance Breger Copyright © 2004 Lance Breger 13 OPERATION OF PM TRANSMITTER OPERATION OF FM TRANSMITTER IN GENERAL Both the FM transmitter and PM transmitter have similar structures. The main difference between them is the kind of modulator FM transmitters differ from PM transmitters in yet another way: before the information signal is applied to the modulator, only used. FM modulators encode information by changing the frequency of the carrier wave. PM modulators encode information by in the FM transmitter is the signal passed through a preemphasis stage. Preemphasis distorts the signal by amplifying high fre- changing the phase of the carrier wave. quencies more than low frequencies, i.e., the high frequencies are "emphasized" before modulation. The purpose of this signal in the FM transmitter is to facilitate later on in the FM receiver the detection of these high frequencies of the signal in the presence of much high-frequency noise generated in the FM demodulator in the receiver.

fm fc Amplitude (Volts) fm

Voltage (Volts) Amplitude (Volts) Voltage (Volts) Em Ec Amplitude Time Amplitude Time (Sec.) Low Frequency (Sec.) fm Frequency Information (Hz)

Gain Low Frequency High Frequency Preemphasis amplifies high frequencies fm fc Frequency PREEMPHASIS to overcome high-frequency noise Information Carrier (Hz) in FM receiver 1 Frequency (Hz) PM Amplitude (Volts) Voltage (Volts) MODULATOR Em Time (Volts) (Sec.) Voltage (Volts) Amplitude EcJ0(m) Low Frequency Frequency Information fm E J (m) E J (m) (Hz) Time c 1 c 1 fc Amplitude (Volts) (Sec.) EcJ2(m) EcJ2(m) Voltage (Volts) ...... Em Ec Combined Signal (fc-2fm)((fc-fm) fc (fc+fm) fc+2fm) Time fm Frequency (Sec.) (Hz) Low Frequency High Frequency Frequency Information Carrier fm fc (fm << fc ) Amplifier Gain (Hz) FM K>1 AMPLIFIER Amplifier gain = constant K>1. MODULATOR Amplitude (Volts) Voltage (Volts) Frequency (Hz) Time EcJ0(m) Voltage (Volts) Amplitude (Volts) (Sec.) EcJ1(m) EcJ1(m) Combined Signal ...EcJ2(m) EcJ2...(m)

fm (fc-2fm)((fc-fm) fc (fc+fm) fc+2fm)Frequency Time (fm << fc ) (Hz) (Sec.) AMPLIFIER ...... Amplitude (Volts) Voltage (Volts) (f -2f )((fc-fm) f (f +f ) f +2f ) Combined Signal fm c m c c m c m Frequency (Hz) Antenna Gain Time (Sec.) 1 ...... Combined Signal Frequency ANTENNA fm (fc-2fm)((fc-fm) fc (fc+fm) fc+2fm) Antenna passes only high frequencies. (Hz) 0 Antenna Gain Frequency 1 ANTENNA Antenna passes only high frequencies. Voltage (Volts) Amplitude (Volts) (Hz) 0 Frequency Voltage (Volts) (Hz) Amplitude (Volts) Time (Sec.) Time (Sec.) ...... Combined Signal Combined Signal (fc-2fm)((fc-fm) fc (fc+fm) fc+2fm) ...... f Frequency Frequency m f (fc-2fm)((fc-fm) fc (fc+fm) fc+2fm) (Hz) m (Hz)

14 Copyright © 2004 Lance Breger Copyright © 2004 Lance Breger 15 OPERATION OF FM TRANSMITTER WITH INDIRECT MODULATION OPERATION OF FM TRANSMITTER WITH DIRECT MODULATION There are two general types of FM modulators. In the first type, i.e., those with indirect modulation, the carrier wave is pro- In the second type of FM modulation, i.e., direct modulation, the oscillator producing the carrier wave, is actually incorporated duced by a very stable oscillator located outside the modulator itself, as is also done in PM modulation. The indirect FM modu- into the FM modulator itself. Otherwise, FM transmitters with direct modulation and those with indirect modulation are essentially lator is just a PM modulator preceded by a special stage that integrates the information signal; this integration is carried out by the same. passing the signal through an amplifier whose gain is inversely proportional to frequency.

fm fm Amplitude (Volts) Amplitude (Volts) Voltage (Volts) Voltage (Volts)

Amplitude Time (Sec.) Amplitude Time (Sec.) Low Frequency Information Low Frequency fm Frequency Information (Hz) fm Frequency Gain (Hz) Preemphasis amplifies high frequencies PREEMPHASIS to overcome high-frequency noise Gain in FM receiver. 1 Preemphasis amplifies high frequencies Frequency PREEMPHASIS to overcome high-frequency noise Amplitude Voltage (Volts) (Volts) (Hz) in FM receiver. 1 Frequency Time (Hz) (Sec.) Voltage (Volts) Amplitude (Volts) Em

Low Frequency Information fm Frequency (Hz) Time FM Modulator Gain (Sec.) Integrator multiplies amplitude by 1/f Low Frequency INTEGRATOR 0 1 Information and phase-shifts sinusoid by -90 . f fm Frequency Frequency (Hz) (Hz) FM f Amplitude (Volts) c Voltage (Volts) MODULATOR

0 E E -90 m c Amplitude Time Voltage (Volts) (Sec.) (Volts) E J (m) Low Frequency High Frequency Time c 0 Frequency Information Carrier fm fc (Sec.) E J (m) E J (m) (Hz) c 1 c 1 EcJ2(m) EcJ2(m) PM Combined Signal ...... MODULATOR (fc-2fm)((fc-fm) fc (fc+fm) fc+2fm) Frequency fm (f << f ) (Hz) Voltage (Volts) Amplitude (Volts) m c

EcJ0(m) Time Amplifier Gain (Sec.) EcJ1(m) EcJ1(m) EcJ2(m) EcJ2(m) AMPLIFIER K>1 Combined Signal ...... Amplifier gain = constant K>1. Frequency fm (fc-2fm)((fc-fm) fc (fc+fm) fc+2fm) (fm << fc ) (Hz) Frequency Amplifier Gain (Hz) Voltage (Volts) K>1 Amplitude (Volts) AMPLIFIER Amplifier gain = constant K>1.

Frequency Voltage (Volts) Amplitude (Volts) (Hz) Time (Sec.) ...... Time Frequency (Sec.) Combined Signal fm (fc-2fm)((fc-fm) fc (fc+fm) fc+2fm) (Hz) ...... Antenna Gain Combined Signal f (fc-2fm)((fc-fm) fc (fc+fm) fc+2fm)Frequency m (Hz) 1 Antenna Gain ANTENNA Antenna passes only high frequencies. 1 ANTENNA Antenna passes only high frequencies. 0 Frequency 0 Voltage (Volts) (Hz) Frequency Amplitude (Volts) Voltage (Volts) Amplitude (Volts) (Hz)

Time Time (Sec.) (Sec.) ...... Combined Signal Frequency Combined Signal Frequency fm (fc-2fm)((fc-fm) fc (fc+fm) fc+2fm) fm (fc-2fm)((fc-fm) fc (fc+fm) fc+2fm) (Hz) (Hz)

16 Copyright © 2004 Lance Breger Copyright © 2004 Lance Breger 17 OPERATION OF FM RECEIVER ANTENNA Amplitude (Volts) Voltage (Volts) external noise

The FM receiver is very similar to the superheterodyne AM receiver. The main difference is that the FM Time (Sec.) receiver uses an FM demodulator; while the superheterodyne AM receiver uses an AM demodulator. external noise ...... fc Frequency RF AMP. & (Hz) In addition, the FM receiver has two stages for noise control that the AM receiver lacks. First, just bandpass filter Voltage (Volts) external noise Amplitude (Volts)

ahead of the demodulator, there is a limiter to clip off and thereby remove external noise. [This cannot be fLO LO Time (Sec.) done in AM because clipping the noise would also remove the information, that is encoded in the amplitude external noise ...... Combined Signal of the carrier wave.] Second, after demodulation in the FM receiver, there is a deemphasis stage, that is Frequency NON-LINEAR (Hz) involved in combating internal noise. The deemphasis stage corrects the earlier distortion of the signal in DEVICE (mixer) Amplitude (Volts)

the preemphasis stage of the FM transmitter, where high-frequency sinusoids are amplified more than low- f Mixer and local oscillator copy LO frequency ones. Fm signal and noise to lower intermediate frequency range...... external noise...... (f < f < f < f ) m i c LO (fi-2fm)((fi-fm)fi (fi+fm) fi+2fm) (fc-2fm)((fc-fm) fc (fc+fm) fc+2fm) Frequency In the FM transmitter, the purpose of distorting the signal is to facilitate the FM receiver's detecting (Hz) IF amplifier Gain 1 the signal's high frequencies in the presence of high-frequency noise. This noise is generated inside the Filter selects band around f FILTER i 0 FM demodulator in the receiver. Basically the deemphasis stage in the receiver is also a nonuniform ampli- the intermediate frequency. Frequency (Hz) Voltage (Volts) Amplitude (Volts) fier which corrects the initial distortion in the transmitter by emphasizing the low frequencies over the high external noise frequencies (i.e., deemphasizes the high frequencies) which were preferentially amplified (i.e., emphasized) Time (Sec.) ...... external noise by the preemphsis stage in the FM transmitter. Combined Signal (fi-2fm)((fi-fm) fi (fi+fm) fi+2fm) Frequency IF AMPLIFIER (Hz) PER SE In contrast to the AM receivers, the diagram of the FM receiver also shows a noise signal although Amplitude (Volts) Voltage (Volts) external noise noise is really present in both types of receivers. However, noise is only shown in the FM case because only Time FM receivers have special stages to combat noise, i.e., the limiter for external noise and the deemphasis (Sec.) ...... external noise Combined Signal stage for internal noise. AM receivers have no special stages to combat noise. (fi-2fm)((fi-fm) fi (fi+fm) fi+2fm) Frequency Vout (Hz)

0 Limiter produces constant output voltage 45 Vin To simplify the description of the operation of the FM receiver in the presence of noise, we consider LIMITER for input voltage above a critical value. that the external noise is white noise (i.e., noise consisting of sinusoids of all frequencies and all with the Amplitude (Volts) Voltage (Volts) same amplitude) generated by an impulse and that this noise can be treated separately from the modulated external noise FM signal. This latter simplification is justifiable when all of the modulated FM signal passes through a filter Time (Sec.) but only the portion of the noise in the passband of the filter passes...... external noise Combined Signal (fi-2fm)((fi-fm) fi (fi+fm) fi+2fm) Frequency (Hz) DEMODULATOR FM demodulator introduces Demodulator noise power Note that in contrast to all preceding diagrams, in the diagram of this FM receiver and of the following PM (detector, increasing noise power discriminator) at higher frequencies. receiver, the graphs of the time domain and of the frequency domain are not exactly the same as the output of an actu- Frequency (Hz) al oscilloscope or of an actual system analyzer. This is because only in these diagrams noise is not combined with Voltage (Volts) Amplitude (Volts) Amplitude Time the FM or PM signals to produce the total signal that the real instruments display in (Sec.) different forms. demodulator noise fm Frequency Gain (Hz) De-emphasis circuit attenuates DE-EMPHASIS higher frequencies to balance them with lower frequencies. Frequency (Hz) Voltage (Volts) Amplitude (Volts)

Amplitude Time (Sec.) demodulator noise

fm Frequency (Hz) AUDIO AMPLIFIER

Voltage (Volts) Amplitude (Volts)

Time (Sec.) demodulator noise

fm Frequency (Hz) SPEAKER

18 Copyright © 2004 Lance Breger 19 OPERATION OF PM RECEIVER CONCLUSION The PM receiver and the FM receiver are basically the same except that the PM receiver has no deemphasis stage, since there is no need for one. This is because in the PM transmitter there is no preemphasis stage to initially distort the signal. Therefore, It is hopeful that the above comments will help the reader to better understand and use the associ- there is no need in the PM receiver for a deemphasis stage to correct this non-existent distortion. ated diagrams. These relevant graphics can in a simple way give a student a global view of the complex subject of radio transmitters and receivers.

ANTENNA The communications circuits that are diagrammed here can also be simulated on a computer, e.g., Voltage (Volts) Amplitude (Volts) external noise by using Multisim. However, our diagrams have the following advantages over simulation alone: Time (Sec.) ... external... noise...... 1) global view: (fc-2fm)((fc-fm) fc (fc+fm) fc+2fm) Frequency RF AMP. & (Hz) The form of the signal can be seen simultaneously at all stages in the cascade instead of only at a few Voltage (Volts) external noise Amplitude (Volts) bandpass filter stages at time as in the simulations. Thus, the overall operation of the system is much easier to grasp Time fLO LO (Sec.) in these diagrams than by simulation alone. external noise Combined Signal ...... (fc-2fm)((fc-fm) fc (fc+fm) fc+2fm) Frequency NON-LINEAR (Hz) 2) operational characteristics: DEVICE (mixer) Amplitude (Volts) The diagrams show the operational characteristic of the filters, amplifiers, and other parts of the circuit. Mixer and local oscillator copy fLO Simulation does not. Fm signal and noise to lower intermediate frequency range...... external noise...... (fm < fi < fc < fLO) (fi-2fm)((fi-fm) fi (fi+fm) fi+2fm) (fc-2fm)((fc-fm) fc (fc+fm) fc+2fm) Frequency 3) distribution of information: (Hz)

IF amplifier Gain 1 By using a color code, these diagrams show where in the signal the information lies; simulation does Filter selects band around f FILTER i the intermediate frequency. 0 not. Knowing the location of the information is critical to understanding the operation of the communica- Frequency (Hz) tion system Voltage (Volts) Amplitude (Volts) external noise Time (Sec.) ...... external noise Hence, these diagrams support and enhance the use of simulation by providing a more comprehen- Combined Signal (fi-2fm)((fi-fm) fi (fi+fm) fi+2fm) Frequency (Hz) sive view, operational characteristics of devices, and color coding to locate the information. IF AMPLIFIER PER SE Amplitude (Volts) Voltage (Volts) external noise

Time (Sec.) ...... external noise Combined Signal (fi-2fm)((fi-fm) fi (fi+fm) fi+2fm) Frequency Vout (Hz) Limiter produces constant output voltage 0 LIMITER 45 Vin for input voltage above a critical value.

Amplitude (Volts) Voltage (Volts) external noise

Time (Sec.) ...... external noise Combined Signal (fi-2fm)((fi-fm) fi (fi+fm) fi+2fm) Frequency (Hz) DEMODULATOR (detector, discriminator) Amplitude (Volts) Voltage (Volts)

Amplitude Time (Sec.) demodulator noise

fm Frequency (Hz) AUDIO AMPLIFIER

Voltage (Volts) Amplitude (Volts)

Time (Sec.) demodulator noise

fm Frequency (Hz) SPEAKER

20 Copyright © 2004 Lance Breger 21 The purpose of these diagrams is to graphically explain the overall operation of AM, PM, and FM communications systems using very little mathematics.

Copyright 2004 Lance Breger