Communication Systems Laboratory Manual

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1 Communication Systems Laboratory Manual Prof R M Ranade NAME ROLL NO. BRANCH SECTION Department of ECE & EI Engineering

2 Published by: Department of ECE & EI Engineering # LAB MANUAL IS FOR INTERNAL CIRCULATION ONLY First Edition: Dec 2006 Copyright Reserved No part of this manual may be reproduced, used, stored without prior permission.

3 PREFACE There was a long felt need by faculty and students for a comprehensive lab manual on Communication systems. This encouraged the author to come up with an illustrative write up of the experiments. The manual is written on the basis of teach yourself pattern and it is expected that students with clear understanding of the manual should be able to perform the experiments with out any difficulty. Initial write up deals with the basic aim and general information about experiments. General disciplines, safety guidelines and report writing and are also discussed. Standard symbols that are commonly and most frequently used in communication Systems are listed and expected that student would adhere to it while preparing the lab report etc. Ten selected experiments are presented in the best suitable fashion and are expected to be student friendly. At the end of each experiment some sample question and space for writing the answer is provided to test the basic understanding of the experiment. The manual aims the practical performed for the communications systems subject of a typical University. Teacher s copy of the experimental results and answer for the questions are available as sample guidelines. We hope that manual would be useful to students of all the branches and authors request the readers to kindly forward their suggestions/constructive criticism for further improvement of the manual.

4 Table of contents S. No Content Page No. 1. General Information 2 2. Technical Information 5 3. Details of Experiment 3.1 Experiment 1: To study the Amplitude modulation and demodulation. 3.2 Experiment 2: To study the Frequency modulation and demodulation. 3.3 Experiment 3: To study the Pulse amplitude modulation and demodulation 3.4 Experiment 4: To study the Pulse Width modulation and demodulation. 3.5 Experiment 5: To study the Pulse Position modulation and demodulation. 3.6 Experiment 6: To study the FSK modulation and demodulation. 3.7 Experiment 7: To study the PSK modulation and demodulation. 3.8 Experiment 8: To study the Time Division Multiplexing and Demultiplexing. 3.9 Experiment 9: To study the circuit of the Pulse code modulation and demodulation Experiment 10: To study the circuit of the Delta modulation and demodulation Value Added Experiments 3.11 Experiment 11: To study & design Optic Digital Link Experiment 12: To study Rise Time & Fall Time Distortions Experiment 13: To study Propagation Delay

5 General Information Laboratory experiments are integral part of the course. It is said that You see you forget; you read you remember; you do you understand. That makes the practical as an integral part of learning process. Some of the purpose of conducting experiments can be enumerated as below: - To get familiarized with the basic circuits, measuring instruments and kits. Circuit drawing, making connection, using instruments, collecting, processing and interpreting the data. Understanding basic modulation techniques and various basic of circuits needed for modulation. Reporting and analyzing the observations. Verify theory with experimental results and understanding physical concepts. Observing safety and developing group-working culture. To make laboratory experiments safe and effective, each student must obey the following rules. a) Safety High voltage equipment in the power lab can seriously injure or kill you instantly. A Faculty must check your circuit connection and wiring before you may apply power. Dress: Boys: Loose clothes not allowed, Shirt should be tucked properly, Shoes with the rubber sole, No slippers. Girls: Skirts with large flares not permitted, Shoes with the rubber sole, No slippers. Make sure that all the power sources are off when you are connecting the circuit. Avoid short circuits by connecting the circuit tight. It may cause destruction and fire. Keep safe distance from supply source and rotating machine. For changing any connection power supply must be switched off. Follow the instructions given by faculty or course instructor. Failure to obey safety rules may result in being expelled from the Lab. b) Attendance All students are required to attend and contribute adequately while performing experiments in the group. In order to pass the course, each group of students must write a satisfactory report for each lab experiments. Faculty will take attendance. Failure to be present for an experiment will result in losing entire mark for the corresponding lab. However, genuine cases may be considered for repeat Lab. 2

6 Students must not attend a different lab group/section from the one assigned at the beginning of the class (unless otherwise approved by the instructor). If a student misses a lab session due to illness and/or family crisis and can provide a legitimate proof, he/she may be able to make-it-up with a different lab section. In such case the student should contact instructor as soon as possible. c) Preparation and Performance Before coming to the laboratory, each student must read and review appropriate pre allocated Lab experiment. Faculty might check your preparedness and understanding of the experiment and failure to satisfactory reply may de-bar you from conducting the experiments. Record your observations and calculations in the Lab Manual. Do not hesitate to clear any of your doubts concerning the experiments. Leave the work place clean after you have finished with your experiments. Disassemble the circuit and put all the wires and equipment back at its original place. d) General working discipline in the Lab Students are advised to strictly follow the instructions given below while working in the electrical Lab: - Attendance in the laboratory is mandatory. For any absence, students have to fill up the format available in the lab to indicate reasons for absence. Students will not be allowed after 10 minutes from the scheduled time. Leaving the Lab early NOT PERMITTED. Students should bring their Lab manual and are understood to have gone through the manual thoroughly. Any confusion may be clarified from the faculty before starting the experiments. Traces of the waveforms should be taken on the trace paper and signatures of the faculty should invariably be obtained thereon. These traces should be pasted in the space provided for this purpose. Before switching on the supply, the students MUST get the connections CHECKED & must follow proper instructions. While handling the Oscilloscopes due care should be taken and in case of doubts help of the Lab Assistant and the faculty should be taken. Students should communicate with their lab group mates at low pitch so that adjoining groups should not get disturbed. Students are to remain within their allotted experimental area. Be alert all the time. e) Lab Reports Each student is required to write reports for the experiments conducted. 3

7 Reports are due one week after the completion of the experiment. All reports must be neatly written. The Lab report must contain the following: 1. Duly completed Title page. 2. Each report to include connection diagrams, graphs if applicable, equations, calculations, notes, etc. Only standard symbols should be used to draw the diagrams. 3. Calculations & Comparisons with appropriate equations and comments where required. 4. Summary section can be included explaining what you have learned doing the experiments 5. Questions given at the end of each experiment have to be answered by student in the space provide in the Lab Manual. 6. The students should maintain and preserve the lab manual properly and deposit the same with the concerned lab in-charge at the end of the semester. Individual comments/notes must be written for the further improvement of the lab manual. Sufficient space is provided at the end of the manual. 4

8 Technical Information Need for Modulation Modulation means to vary or change. A typical telephone conversation electrical signal can not be transmitted because it will need a very large antenna as minimum length of an antenna to effectively transmit waves is of the order of one tenth of the wavelength which is very large at frequencies of the order of 4 K Hz. Even if we are able to transmit this signal it will not go to large distances as attenuation caused by the ground is very high more over only one such transmission can be carried out in one local area as otherwise the channels will interfere being on same frequency. To overcome this problem we first take a signal, say a telephone conversation, and then impress it on a constant radio wave called a carrier. Once done the voice signal varies or modulates this radio wave. The two go together over the air. A voice frequency in the audible or audio range, what we can hear, thus modulates or varies a constant frequency in the radio range, which we can't hear. By doing this we have used a much higher frequency thereby reducing the size of the antenna substantially and also we can use different carrier waves for different telephone conversations so that there is no interference among these. Different modulation techniques, such as A.M., F.M., P.C.M. and so on, represent different ways to shape or form electromagnetic radio waves. There are many reasons to modulate a signal in a particular way. Amplitude modulation, like that still used by Citizens' Band radios, produces a simple, robust wave that doesn't use much spectrum or radio bandwidth. It's plagued by noise though and requires high transmitting power. Frequency modulation, such as analog cell phones use, provides better sound but it needs more bandwidth to achieve that quality and is technically more complex to produce. And then there are modulation types just for transmitting digital information. GSM and IS- 136 use these schemes. Amplitude Modulation Amplitude modulation means a carrier wave is modulated in proportion to the strength of a signal. The carrier rises and falls instantaneously with each high and low of the conversation. See the diagram below. See how the voice current produces an immediate and equivalent change in the carrier. 5 Amplitude Modulated Wave

9 Low frequency commercial broadcast stations in the "A.M band" use amplitude modulation. Most C.B. or citizens band radios use it too. It's a simple, robust method to form a radio wave but it suffers from static and high battery power requirements, reasons enough that few personal communications devices use it. Frequency Modulation Frequency modulation confuses many people but it shouldn't. FM is not limited to the FM band. It is not frequency dependent, that is, it can be used at high or low frequencies. That's because it is a modulation technique, a way to shape a radio wave, not a service by itself. The word frequency in FM relates, instead, to the rate at which this method varies a carrier wave, not to any particular radio frequency it is used on. 6

10 An FM signal quality is low distortion, little static, good voice quality and immunity from electrical and atmospheric interference. It's why television audio and analog cellular use it. FM also exhibits a capture effect, whereby the receiver seizes on the strongest signal and rejects any others. That's unlike A.M, with signals fading in and out. What's more, F.M. needs far less power to transmit a signal the same distance than A.M. See the difference in the waveform on the right in the diagram below? You don't have the modulated carrier varying in amplitude, as with A.M., but in the number of cycles or rate it is varying. To start with we have an unvarying carrier wave as we do with A.M. But in F.M. the carrier wave is engineered to deliver a uniform output signal. When we impress upon the carrier an audio signal, such as a 440-hertz dial tone, things begin to happen. Frequency modulation varies the carrier at a rate of 440 cycles per second, matching the original signal. This differs dramatically from A.M., where a wildly swinging sine wave would be produced instead. In F.M. a quick change in audio frequency results in a quick rate change to the carrier. Frequency shift keying, an F.M. variation Conventional cellular makes much use of frequency shift keying modulation to send signaling and control messages. It's old technology, in fact, the earliest modems were built with this technique, but FSK works well for what it does. To explain its title, FSK means sending data by slightly shifting frequencies. Simple. Keying, by the way, simply means forming or creating a signal. When you "key up" a microphone you create a signal. You turn on. Frequency shift keying uses the existing carrier wave, say, MHz. The data rides 8kHz above and below that frequency. It's just like the earliest modems. 0s and 1s. 0s go on one frequency and 1s go on another. They alternate back and forth in rapid succession. FSK gives you only two states to send information. There's a low limit, then, how much and how quickly you can send information. There is a more efficient way. Phase Modulation Three ways exists to modulate a signal: by amplitude, frequency or phase. And although there are dozens of modulation techniques, under the most confusing names possible, all of them will fit into one of these categories. We've looked at amplitude modulation, which changes the carrier wave by signal strength, and frequency modulation, which converts the originating signal into cycles. Now we look at phase modulation, which changes the angle of the carrier wave. Phase modulation is strictly for digital working and is closely related to F.M. Phase in fact enjoys the same capture effect as F.M. A digital signal means an ongoing stream of bits, 0s and 1s, on and off pulses of electrical energy as shown below: - How does P.M. represent those on and offs? It does so by playing with angles. A continuous wave produced to transmit analog or digital information. The many phases or 7

11 angles of a sine way give rise to different ways of sending information. Following diagram shows use of 180 phase shift in the carrier frequency for sending 1 and no shift for sending 0, known popularly as Binary Phase Shift Keying (BPSK). Quadrature phase shift keying This scheme, used by most high-speed modems, allows quicker data transfer than FSK. It gives at least Binary four states PSK to Carrier send information. (Note the 180 There's phase a shifts good at chance bit edges) you've heard this type as your modem makes a dial up connection. IS-136 uses this technology to enable its digital control channel, allowing PCS like services for conventional cellular. GSM uses a variation called Gaussian Minimum Shift Keying. Quadrature phase shift keying changes a sine wave's normal pattern. It shifts or alters a wave's natural fall to rest or 0 degrees. By forcing changes in a sine wave you can convey information. You don't stop or abbreviate the sine wave; you change its shape or angle of attack. See the diagram below. 8

12 As an example, 90 degrees, 0 degrees, 180 degrees, and 270 degrees might be represented by binary digits 00,01,10, and 11 respectively. You arrange a circuit so that at each point you wish to transmit a bit you force a shift in the sine wave. The receiver expects these shifts and decodes them in the proper sequence. Again, we are putting digital information on a carrier wave. We are shaping a carrier wave to do this, to carry more pulses more efficiently. That's why, confusing though it may be to visualize, we have the make and break, up and down pattern of digital, carried on the smooth, up and down shape of an analog looking wave. Wireless services use amplitude, frequency, and phase modulation to send both analog or digital radio signals. But what converts an analog signal to digital in the first place? An encoding scheme. Pulse amplitude modulation first measures or samples the strength of an analog signal. Pulse code modulation encodes these plots into binary words, namely 0s and 1s. These binary digits are represented by on and off pulses of electrical energy. A digital signal thus produced usually modulates the current carrying the signal within a landline. Modulation and pulses, therefore, get digital messages going. Once completed, the resulting digital signal can be sent over the air with another modulation technique for doing just that. We'll now go over some digital basics and then see in detail how pulse modulation works. Basic Digital Principles: Turning speech into digital Sampling: This first step in digitizing is called pulse amplitude modulation or PAM. Amplitude refers to a signal's strength, the relative rise and fall that PAM takes measurements of. These levels, ranging from 0 to 256, are plotted against time. To have a coordinate like those below you must have two magnitudes. The signal strength and the time it occurred. Once you have those you have a plot that can be put into binary. After PAM takes its measurements, each sample gets converted to a Quantized level which finally gets converted to an 8 bit binary code word. Let's say one piece of conversation, a fraction of a second's worth, actually, hits a strength level of 175. It's now put into binary, transmitted by turning on or off an electrical current. The bits , for example, represent 175. Voltage turned on or off. Since this second step encodes the previous information, it is called pulse code modulation or PCM. That's what the code in PCM stands for. 9

13 Putting the measured strength or amplitude into 8 bit code words is also called quantization. A name for both steps is called voice coding. And every code word generated is time stamped so it can be put back together in the order it was made. Pulse code modulation needs 64,000 bits (64kbs) every second to represent speech. A normal landline digital phone call after sampling takes up 64,000 bits. And how better techniques for wireless exist, which reduce bandwidth to 7,500 bits. That's efficient. Similarly, differential quadrature phase shift keying is more efficient than FSK, with at least four possible states to carry information in every wave. Continuous wave produced to transmit analog or digital information. The many phases or angles of a sine permit different ways to modulate Three methods of digital signal modulation. A digital signal, representing the binary digits 0 and 1 by a series of on and off amplitudes, is impressed onto an analog carrier wave of constant amplitude and frequency. In amplitude-shift keying (ASK), the modulated wave represents the series of bits by shifting abruptly between high and low amplitude." In frequency-shift keying (FSK), the bit stream is represented by shifts between two frequencies." In phase-shift keying (PSK), amplitude and frequency remain constant; the bit stream is represented by shifts in the phase of the modulated signal." Index 10

14 Experiment No. Date of Completion Signatures Experiment 1: To study the Amplitude modulation and demodulation Experiment 2: To study the Frequency modulation and demodulation Experiment 3: To study the Pulse amplitude modulation and demodulation Experiment 4: To study the Pulse Width modulation and demodulation Experiment 5: To study the Pulse Position modulation and demodulation Experiment 6: To study the FSK modulation and demodulation Experiment 7: To study the PSK modulation and demodulation Experiment 8: To study the Time Division Multiplexing and De-multiplexing Experiment 9: To study the circuit of the Pulse code modulation and demodulation Experiment 10: To study the circuit of the Delta modulation and demodulation Value Added Experiments Experiment 11: To study & design Optic Digital Link Experiment 12: To study Rise Time & Fall Time Distortions Experiment 13: To study Propagation Delay Communication Lab/ Experiment No.-1 11

15 Experiment No.-1 Date of Performance Bench No. Faculty s Signature Date Objective To study the Amplitude modulation and demodulation Equipment Required AM kit, CRO, function generator, connecting wires. Theory Amplitude modulation is defined as a system in which amplitude of the carrier signal is made proportional to the instantaneous amplitude of the modulating voltage. The modulating signal is a low frequency signal as compared to the high frequency carrier. Let the e t = A sin w t and carrier signal and modulating signal be represented as m( ) m ( m ) ec () t Ac sin( wct ) E () t = ( A + A cos( w t) ) cos( w t) AM =. The amplitude-modulated signal is represented by c m m c A m A = Ac 1 + cos( wmt ) cos( wct ) A Where m is defined to be modulation c Ac ( Vmax Vmin ) index. From the modulated wave modulation index m = ( Vmax + Vmin ) Brief Description of the kit The amplitude modulation kit has blocks of modulating signal, carrier signal, modulator and demodulator. If modulating signal generator block is not given on the kit then it is applied from the external source i.e. function generator. If carrier signal generator block is not given on the kit it means it is inbuilt within the modulator. We can vary the frequency and amplitude by varying the knobs. Modulating Signal Carrier Wave Modulated Wave 12 Time

16 Procedure 1. Switch on the kit. 2. Observe the modulating signal on channel one of CRO and observe the carrier signal on the channel two of the CRO. 3. Connect the modulating signal and carrier signal to the modulator and observe the waveform on the CRO. 4. Measure Am and A c & calculate modulation index. 5. Apply the amplitude-modulated waveform to the demodulator input. 6. Observe the waveform at the output of the demodulator. 7. Adjust the potentiometers provided (if any) in demodulator unit until you get the demodulator output corresponding to the modulating signal. Precautions: 1. Ensure that test set up is properly earthed and common ground is provided to both the boars. 2. Do not switch on the test equipment unless all wires are properly connected and checked for correct connections. 3. Use proper connection wires. 4. Do not touch/ fiddle with the components mounted on the kit. 5. Do not overload test equipment. 6. Do not exceed the set limits of the test equipment. 7. There should not be any loose connections. 8. Switch off the kit as soon as the experiment is over. 13

17 LAB REPORT TO COMPLETED ON NEXT TURN Observations Made Traces taken Ideal waveforms expected as taught in theory: 14

18 Explanation for the difference in the actual and practical waveforms if any: Conclusion Questions 15

19 Q. 1 What is Amplitude modulation? Ans. Q.2 Explain over modulation. Ans. Q.3 Write down the techniques of AM in detail. Ans. 16

20 Q.4 Draw and explain modulated waveform Ans. Q5. What is modulation index? Ans. Draw the neat block diagram of the Experiment 17

21 Viva-voice and Remarks by the Faculty Faculty s Signature Date Checked Grade Communication Lab/ Experiment No.-2 18

22 Experiment No.-2 Date of Performance Bench No. Faculty s Signature Date Objective To study the Frequency modulation and demodulation Equipment Required FM kit, CRO, function generator, connecting wires. Theory Frequency modulation is defined as a system in which frequency of the carrier signal is made proportional to the instantaneous amplitude of the modulating voltage. The modulating signal is a low frequency signal as compared to the high frequency carrier. Let the carrier signal and modulating signal be represented as em () t = Am sin( wmt ) and ec () t = Ac sin( wct ). The frequency-modulated signal is represented by f E () = + ( w t) FM t Ac cos 2π fct sin m fm f Where is defined to be modulation index = β Brief Description of the kit f m The frequency modulation kit has blocks of modulating signal, carrier signal, modulator and demodulator. If modulating signal generator block is not given on the kit then it is applied from the external source i.e. function generator. If carrier signal generator block is not given on the kit it means it is inbuilt within the modulator. We can vary the frequency and amplitude by varying the knobs (if provided). Block Diagram Frequency Modulation Modulating signal Generator Carrier Demodulation Generator 19 Modulator Frequency Modulated output

23 Frequency Modulated Input Signal Demodulator Demodulated Carrier Wave Modulating Signal Modulated Wave Procedure 1. Switch on the kit. 2. Observe the modulating signal on channel one of CRO and observe the carrier signal on the channel two of the CRO. 3. Connect the modulating signal and carrier signal to the modulator and observe the waveform on the CRO. 20

24 4. Measure f and f m & calculate modulation index. 5. Apply the frequency-modulated waveform to the demodulator input. 6. Observe the waveform at the output of the demodulator. 7. Adjust the potentiometers provided (if any) in demodulator unit until you get the demodulator output corresponding to the modulating signal. Precautions: 1. Ensure that test set up is properly earthed and common ground is provided to both the boars. 2. Do not switch on the test equipment unless all wires are properly connected and checked for correct connections. 3. Use proper connection wires. 4. Do not touch/ fiddle with the components mounted on the kit. 5. Do not overload test equipment. 6. Do not exceed the set limits of the test equipment. 7. There should not be any loose connections. 8. Switch off the kit as soon as the experiment is over. LAB REPORT TO COMPLETED ON NEXT TURN 21

25 Observations Made Traces taken Ideal waveforms expected as taught in theory: 22

26 Explanation for the difference in the actual and practical waveforms if any: Conclusion Questions 23

27 Q. 1 What is Frequency modulation? Ans. Q.2 Explain difference between Amplitude and Frequency modulation. Ans. Q.3 Write down the techniques of FM generation. Ans. 24

28 Q.4 Draw and explain frequency modulated waveform. Ans. Q5 Write the equation of a frequency modulated wave and explain modulation index. Ans Draw the neat block diagram as per experiment in Kit 25

29 Viva-voice and Remarks by the Faculty Faculty s Signature Date Checked Grade 26

30 Communication Lab/ Experiment No.-3 Experiment No.-3 Date of Performance Bench No. Faculty s Signature Date Objective To study Pulse Amplitude Modulation and Demodulation. Equipment Required PAM kit, CRO, function generator, connecting wires. Theory Modulation may be of two types: MODULATION CONTINUOUS WAVE PULSE AMPLITUDE ANGLE PULSE PULSE PULSE AMPLITUDE TIME CODE FREQUENCY PHASE POSITION In direct pulse modulation techniques, we have 1. Pulse Amplitude Modulation. 2. Pulse width modulation and 3. Pulse position modulation. Pulse Amplitude Modulation PULSE DURATION PULSE Pulse amplitude modulation may be used to transmit analog amplitude information in discrete time periods or sample, which is regular in time i.e. the amplitude of a pulse, is directly proportional to the amplitude of the analog signal at that instant when the pulse occurs. During the rest of the period the amplitude is zero. 27

31 Here again the minimum rate at which the sampling pulses should occur depends on Nyquist rate, which is least 2fm where fm is the maximum frequency component spectrum in of input signal m (t). Otherwise reproduction or reconstruction of the analog signal is not possible. A capacitor is provided at of the output of the sampling switch which when it is returned to ground will act as Sample and Hold circuit if desired. The rate of sampling should confirm to the Nyquist rate, which implies that the sampling rate should have a frequency of not less than 2fm where fm is the maximum frequency content in the input analog signal. Otherwise & what is called as a Fold over distortion or Aliasing distortion will occur and filtering of the sampling frequency component becomes impossible. If m (t) is the analog signal in which analog signal fm is the maximum frequency constant and is being sampled by the pulses S (t) whose period is T and pulse width t. Then the resulting signal is m (t) x S (t) will have a spectrum of frequencies much beyond fm. Pulse amplitude Modulation Carrier Expanded in time domain Modulating Signal Pulse Amplitude Demodulation The O/P waveform of the modulated PAM connected to the input of the demodulator 28

32 PAM and connects CRO to the demodulated O/P terminal. Hence we get AF signal i.e. information or sending signal to the demodulated O/P of PAM demodulator. (The demodulator consists of a low pass filter with a cut off frequency at fm.) Block Diagram PAM Modulation AF signal Generator PAM Modulator PAM Clock Generator Demodulation AF signal Generator Clock Generator PAM Modulator PAM Demodulator Procedure 1. Switch on the experimental kit. 2. Observe AF signal generator O/P on a dual trace CRO. 3. Its frequency is various from 30Hz to 15KHz in following three steps. 4. Short the two sockets provided in AF signal generator section and vary frequency adjust pot from its minimum position to maximum position. 5. Remove the short between those two terminals provided in AF signal generator section. 6. Now change the position of SPDT switch provided in AF signal generator section. 29

33 7. So above steps covers frequency range from 30Hz to 15KHz.Amplitude varies from 0 to ±2.5Vp-p. 8. Now make connections as per circuit diagram shown in fig1. for analog signal sampling. 9. Connect of signal generator at to the analog signal input terminal of the analog signal sampler. 10. Connect pulse generator O/P to the sampling pulse input terminals. 11. Short C1 to ground. 12. Observe the O/P of the analog signal sampler O/P on CRO. 13. During demodulation, make the connections as per the circuit diagram shown in fig-2. Precautions 1. Test setup should be properly earthed. 2. Do not switch on the equipment unless all the test wires are properly connected. 3. Use proper connections. 4. Do not overload test equipments. 5. Do not exceed the limit for test equipment for any measurement. LAB REPORT TO COMPLETED ON NEXT TURN 30

34 Observations Made Traces Taken Ideal waveforms expected as taught in theory: 31

35 Explanation for the difference in the actual and practical waveforms if any: Conclusion Questions 32

36 Q. 1 What is the modulator output? Ans. Q.2 What is operating voltage of 555 IC? Ans. Q.3 What is the IC used for AF signal? Ans. Q.4 PAM is which type of modulation? Ans. Q.5 What is frequency for clock generator? What is the IC used for clock generator? 33

37 Ans. Q.6 Draw and explain waveform of PAM modulated output? Ans. Draw the block diagram as per Kit 34

38 Viva Voice and Remarks by Faculty Faculty s Signature Date Checked Grade Communication Lab/ Experiment No.-4 35

39 Experiment No.-4 Date of Performance Bench No. Faculty s Signature Date Objective To study Pulse Width Modulation and Demodulation. Equipment Required PAM kit, CRO, function generator, connecting wires. Theory Modulation may be of two types MODULATION CONTINUOUS WAVE PULSE AMPLITUDE ANGLE PULSE PULSE PULSE AMPLITUDE TIME CODE FREQUENCY PHASE PULSE POSITION PULSE DURATION Pulse modulation may be used to transmit analog information, such as continuous speech or data. It is a system, in which continuous waveforms are sampled at regular intervals. Information regarding the signal is transmitted only at sampling times, together with any synchronizing pulse that may be required. In direct pulse modulation techniques, we have: - 1. Pulse Amplitude Modulation. 2. Pulse width modulation and 3. Pulse position modulation. Pulse Width Modulation 36

40 In pulse width modulation, we have fixed amplitude and starting time of each pulse, but the width of each pulse is made proportional to the amplitude of the modulating signal at that instant. One monolithic multivibrator can generate pulse Width Modulation. 555IC is connected in monostable mode. Initially, the sampling clock is given to pin-2 of the 555IC (which is connected in monostable mode) and the modulating signal is given to the pin-5 o the same 555IC. Now, if we observe the output at pin 3, we get pulse width modulated signal. The width of each pulse is varied, as per the amplitude of the modulating signal, which is applied at pin-5 of the 555IC. Pulse Width Demodulation The demodulation of the pulse width modulation is quite a simple process. Pulse Width modulation is fed to an integrating (RC) circuit (Low Pass Filter) from which the modulating signal emerges, whose amplitude at any time is proportional to the pulse width modulation at that time. The TL084 is a quadruple operational amplifier fabricated on signal monolithic chip. It is specified over a temperature range from 40 to 85 degree Celsius. 37

41 Block Diagram PWM Modulation AF signal Generator PWM Modulator PWM output Clock Generator PWM Demodulation AF signal Generator Clock Generator PWM Modulator PWM Demodulator AF O/P Procedure 1. Switch on the experimental kit. 2. Observe the clock generator output and the modulating signal outputs. 3. Connect clock generator output to the clock in put point of PWM modulator and observe the same clock on channel of a dual trace CRO. 4. Trigger the CRO with respect with to CH1. 5. Apply a variable DC voltage of 8 to 12 volts from any external regulated power supply. 6. Observe the PWM output on CH2. 38

42 7. If we observe the PWM output, its width varies according to the modulating voltage. 8. A variable amplitude modulating signal is given to observe how the PWM looks like. PRECAUTIONS 1. Test setup should be properly earthed. 2. Do not switch on the equipment unless all the test wires are properly connected. 3. Use proper connections. 4. Do not overload test equipments. 5. Do not exceed the limit for test equipment for any measurement. 39

43 LAB REPORT TO COMPLETED ON NEXT TURN Observations Made Traces Taken 40

44 Ideal waveforms expected as taught in theory: Explanation for the difference in the actual and practical waveforms if any: Conclusion 41

45 Questions Q. 1 What is the modulator output? Ans. Q.2 What is operating mode of 555 IC? Ans. Q.3 How is the frequency of carrier signal changed? Ans. 42

46 Q4 What is the IC used for PWM modulator? Ans Q5 PPM is which type of modulation? Ans 43

47 Draw the block diagram as per kit Viva-voice and Remarks by the Faculty Faculty s Signature Date Checked Grade 44

48 Communication Lab/ Experiment No.-5 Experiment No.-5 Date of Performance Bench No. Faculty s Signature Date Objective To study the Pulse Position modulation and demodulation Equipment Required PPM Kit, CRO, Function Generator, Connecting Wires Theory Pulse modulation may be used to transmit analog information, such as continuous speech or data. It is a system in which continuous waveforms are sampled at regular intervals. Information regarding the signal is transmitted only at the sampling times, together with any synchronizing pulses that may be required. At the receiving end, the original waveforms may be reconstructed from the information regarding the samples, if these are taken frequently enough. The information about the signal is not supplied continuously, as in amplitude modulation and frequency modulation. The resulting receiver output can have negligible distortion. Pulse Position Modulation and Demodulation Pulse modulation may be subdivided broadly into two categories Analog and Digital. In the former, the indication of the sample amplitude may be infinitely variable, while in the latter a code which indicates the sample amplitude to the nearest predetermined level is sent. Here pulse position modulation is an analog communication type. In pulse position modulation, we have fixed amplitude of each pulse, but the position of each pulse is made proportional to the amplitude of the modulating signal at that instant. The demodulation of the pulse position modulation is quite a simple process. Pulse position modulation is fed to an integrating (RC) circuit (low pass filter) from which a modulating signal emerges whose amplitude at any time is proportional to the pulse position modulation at that time. 45

49 BLOCK DIAGRAM: PULSE POSITION MODULATION AF SIGNAL GENERATOR CLOCK GENERATOR PWM MODULATOR PPM MODULATOR PPM O/P PULSE POSITION DEMODULATION paappppppp AF SIGNAL GENERATOR CLOCK GENERATOR PWM MODULATOR PPM MODULATOR PPM AF O/ DEMODULATION 46

50 Procedure 1) Switch on the experimental kit. 2) Observe the clock generator output and modulating signal outputs. 3) Connect clock generator output to the clock input point of PPM modulator and observe the same clock on channel of a dual trace CRO. 4) Trigger the CRO with respect to CH1. 5) Apply a variable dc voltage of 8 to 12 volts from any external regulated power supply. 6) Observe the PPM output on CH2. 7) By varying the modulating voltage, PPM output clock position changed but its width maintain constant. 8) If we observe the PPM output, its width varies according to the modulating voltage. 9) A variable amplitude-modulating signal is given to observe how the PWM & PPM signals are varying for AC modulating voltages. 10) In this case we have to trigger the CRO with respect to modulating voltage. PRECAUTIONS 1) Test setup should be properly earthed. 2) Do not switch on the test equipment unless all the test wires are properly connected. 3) Use proper connectors. 4) Do not overload test equipments. 5) Do not exceed the limit for test equipment for any measurement. 47

51 LAB REPORT TO COMPLETED ON NEXT TURN Observations Made Traces taken 48

52 Draw ideal waveforms expected as taught in theory: Explanation for the difference in the actual and practical waveforms if any: Conclusion 49

53 QUESTIONS: Q1 What is modulated output? Ans Q2 What is the IC used for PPM demodulator? Ans Q3 What is the frequency of clock generator? Ans 50

54 Q4 Draw the waveform of PPM modulated output? Ans Q5 What is the difference between PWM & PPM? Ans 51

55 Draw the block diagram as per Kit Viva-voice and Remarks by the Faculty Faculty s Signature Date Checked Grade 52

56 Communication Lab/ Experiment No.-6 Experiment No.-6 Date of Performance Bench No. Faculty s Signature Date Objective To study the FSK modulation and demodulation Equipment Required FSK kit, CRO, function generator, connecting wires. Theory FSK (Frequency Shift Keying) is one of the digital modulation techniques where the carrier frequency is shifted in accordance with the input data. Two sets of frequencies are taken and one of the frequencies is transmitted, selection of which is done by logic 0 or 1 at the data stream. For demodulation the incoming modulated signal is put through a filter, which is tuned to one of the incoming frequency so that it allows one frequency and stops the other frequency. The passed frequency is detected by using an envelope detector and then given to a comparator, which gives the original data. Brief Description of the kit In FSK kit basically a 555 IC is connected in astable multivibrator mode, it generates a clock pulse of frequency determined by the values of R and C parameters. This clock is given to divide by 16 counter (IC74163), which generates divided by 2, 4, 8 and 16 outputs of the input clock. In this system divided by 2 and 8 outputs are taken as two carrier frequencies, which are given to a FSK modulator. Depending on the level of modulating data either divide by two or divide by eight frequencies is passed through the FSK modulator. For demodulation the FSK modulator output is given to high Q tuned filter, which is tuned to one of the frequencies transmitted. This filter stops one frequency and allows other frequency. The filter is constructed with an operational amplifier TL084 IC. The passed frequency is given to a full wave rectifier output of which is given to an envelope detector. This envelope detector acts as a peak detector during pulse period. The output of the envelope detector is given to a comparator, which removes the granular noise arising due to envelope detection, and gives back the original data stream applied at the input of the FSK modulator. 53

57 0 1 0 Block Diagram FSK Modulation Data Generator Carrier Generator f 4f Modulator FSK output Demodulation High Q Filter Modulated Input Envelope Detector Comparator O/p 54

58 Procedure 1. Connect any one data output of the 7490-decade counter to the data input of the FSK modulator. Connect two frequencies to input of the modulator. 2. Switch on the kit. 3. Observe the data input on channel one of CRO and observe the output of the modulator on the channel two of the CRO. 4. Trigger CRO with respect to channel one and take the trace of the output. 5. Repeat this process for other data streams available. 6. Connect the FSK modulated output to the input of FSK demodulator. 7. Observe the waveform at the output of the demodulator. 8. Adjust the potentiometers provided in demodulator unit until you get the demodulator output corresponding to the data input. Precautions: 1. Ensure that test set up is properly earthed and common ground is provided to both the boars. 2. Do not switch on the test equipment unless all wires are properly connected and checked for correct connections. 3. Use proper connection wires. 4. Do not touch/ fiddle with the components mounted on the kit. 5. Do not overload test equipment. 6. Do not exceed the set limits of the test equipment. 7. There should not be any loose connections. 8. Switch off the kit as soon as the experiment is over. 55

59 LAB REPORT TO COMPLETED ON NEXT TURN Observations Made Traces taken 56

60 Ideal waveforms expected as taught in theory: Explanation for the difference in the actual and practical waveforms if any: Conclusion 57

61 Questions Q. 1 Comment on the bandwidth requirement for FSK modulation and compare it with ASK modulation. Ans. Q.2 Differentiate between analog and digital modulation. Ans. Q.3 Draw the typical waveform at the output of envelope detector and explain granular noise. Ans. 58

62 Q.4 Explain the FSK modulator used in the kit and with help of clear diagram. Ans. Q.5 What is frequency synthesizer? Draw typical out puts at various pins of IC Ans. Q.6 What is a PLL? Explain in brief with help of a suitable diagram. Do you think there is a requirement of PLL in this experiment? Ans. Q.7 What kind of Modulator has been used in the kit? Explain it s functioning in brief? Ans. 59

63 Draw the block diagram as per kit Viva-voice and Remarks by the Faculty Faculty s Signature Date Checked Grade 60

64 Communication Lab/ Experiment No.-7 Experiment No.-7 Date of Performance Bench No. Faculty s Signature Date Objective To study the PSK modulation and demodulation Equipment Required PSK kit, CRO, function generator, connecting wires. Theory PSK (Frequency Shift Keying) is one of the digital modulation techniques where the carrier phase is shifted in accordance with the input data. The carrier phase is shifted by +90 for a mark (1) and -90 for a space (0). Alternatively for a space (0) a carrier without a phase shift can be transmitted and for a mark (1) phase shift of 180 can be given. For demodulation reverse process is done to get the original data back. Brief Description of the kit In the kit for PSK modulation and demodulation IC-8038 is a basic waveform generator, which generates sine wave, triangular wave and square wave forms. The sine wave generated by thisic-8038 is used as a carrier signal to the system. The square wave generated by IC-8038 is at ±12 volts level. This is converted to a +5 volts signal with the help of a transistor and diode. This square wave is used as a clock input to a decade counteric-7490, which generates the modulating data output (i.e. the modulating bit stream). IC 4051 is n two input analog multiplexer. At the two inputs of this multiplexer the carrier wave and 180 phase shifted carrier are applied. Modulating data input is applied to its control input. Depending on the level of the control signal, carrier signal applied with or without phase shift is steered to the output. An operational amplifier using IC-741 creates the 180 phase shift to the carrier signal. During the demodulation, the PSK signal is converted into a +5 volt square wave signal using a transistor and is applied to one input of an EX-OR gate. To the second input of the gate, carrier signal is applied after conversion into a + 5 volts signal. So the EX-OR gate output is equivalent to the original modulating data signal. 61

65 Block Diagram PSK Modulation Data Generator PSK output Carrier Generator Modulator Demodulation Demodulator Original Data Modulated input 62 A Typical Phase Modulated Wave

66 Procedure Switch on the kit 1. Apply the carrier signal to the input of the modulator. 2. Apply the modulating data signal to the modulator input and observe this signal on one channel of the CRO. 3. Observe the output of the PSK modulator on the channel 2 of the CRO. 4. Apply the PSK output and the carrier to the demodulator input. 5. Observe the demodulator output and compare it with the modulating data signal applied to the modulator input. Precautions: 1. Ensure that test set up is properly earthed and common ground is provided to both the boars. 2. Do not switch on the test equipment unless all wires are properly connected and checked for correct connections. 3. Use proper connection wires. 4. Do not touch/ fiddle with the components mounted on the kit. 5. Do not overload test equipment. 6. Do not exceed the set limits of the test equipment. 7. There should not be any loose connections. 8. Switch off the kit as soon as the experiment is over. 63

67 LAB REPORT TO COMPLETED ON NEXT TURN Observations Made Traces Taken 64

68 Ideal waveforms expected as taught in theory: Explanation for the difference in the actual and practical waveforms if any: Conclusion 65

69 Questions Q. 1 Comment on the bandwidth requirement for PSK modulation and compare it with ASK modulation. Ans. Q.2 Explain the working of the modulator with help of a suitable diagram. Ans. Q.3 Draw and explain the waveforms at the output of PSK modulator. Assume Input data to be Ans. 66

70 Q.4 What is the operating voltage of modulator? Ans. Q.5 Comment on the performance of PSK versus ASK modulation in a noisy channel. Ans. Q.6 Explain working of Demodulator in this experiment and draw the waveforms at input and output. Ans. Q.7 Explain the function of decade counter. What would happen if the data were not derived from the carrier frequency? Ans. 67

71 Draw the block diagram as per Kit Viva-voice and Remarks by the Faculty Faculty s Signature Date Checked Grade 68

72 Communication Lab/ Experiment No.-8 Experiment No.-8 Date of Performance Bench No. Faculty s Signature Date Objective To study the Time Division Multiplexing and De-multiplexing Equipment Required TDM kit, CRO, BNC leads, connecting wires. Theory Multiplexing is the process of sending a number of separate and independent signals together over the same channel, simultaneously without any interference among them. Time Division Multiplexing (TDM) is a method of interleaving the signals in time domain where samples of different signals are transmitted in predefined time slots, which are earmarked for each signal. To satisfy the sampling theorem, the sampling rate should be greater than or equal to twice of the maximum frequency content of the signal to be transmitted. Brief Description of the kit The kit is built over an 8 to 1 multiplexer IC For clock generation IC 555 is used in astable mode which gives a clock frequency of 10 KHz. Control signals for the multiplexer are generated IC 74163, which gives outputs of A0, A1 and A2. Data is generated through IC 7490, which gives outputs D1 to D8. For demultiplexing process, IC demultiplexer is used. The same control lines A0, A1 and A2 are used making the control signals for multiplexer and demultiplexer to be the same. This way the multiplexer and demultiplexer both remain in synchronization. The selection of channels is done as per following logic: A2 A1 A0 Channel Selected L L L L L L L L8 69

73 Block Diagram 7490 Data Generator 1 To 8 Demux D1 D2 D3 D4 D5 D6 D7 D8 8 To 1 MULTIPLEXER L1 L2 L3 L4 L5 L6 L7 L8 A0 A1 A Address Generator 70

74 Procedure 1. Connect the circuit as shown in the block diagram. 2. Switch on the experiment kit. 3. Observe the MSB bit of the address generator on one of the channels of a dual trace CRO and trigger the CRO w.r.t. the same channel. 4. Observe the output of the 8-to-1 multiplexer on the second channel of the CRO. 5. Apply a low (ground) signal to the 8 multiplexer inputs one by one and observer how time is divided among the channels w.r.t. the address generator. Precautions: 1. Ensure that test set up is properly earthed and common ground is provided to both the boars. 2. Do not switch on the test equipment unless all wires are properly connected and checked for correct connections. 3. Use proper connection wires. 4. Do not touch/ fiddle with the components mounted on the kit. 5. Do not overload test equipment. 6. Do not exceed the set limits of the test equipment. 7. There should not be any loose connections. 8. Switch off the kit as soon as the experiment is over. 71

75 LAB REPORT TO COMPLETED ON NEXT TURN Observations Made Traces taken 72

76 Draw ideal waveforms expected as taught in theory: Explanation for the difference in the actual and practical waveforms if any: Conclusion 73

77 Questions Q.1 What are the advantages of multiplexing? Ans. Q.2 What is the need for multiplexing? Ans. Q.3 What do you mean by synchronization? Ans. 74

78 Q.4 In a practical system how synchronization will be received as the same address generator cannot be used in two different locations. Ans. Q.5 Compare FDM and TDM. Ans. Q.6 Describe Nyquist sampling rate, what would happen if the sampling rate were less than double of the highest frequency? Ans. Q.7 What is T-1 system of multiplexing? Describe it in brief. Ans. 75

79 Draw the block diagram as per Kit Viva-voice and Remarks by the Faculty Faculty s Signature Date Checked Grade 76

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