Lab Manual. Communication System-I SE SEM I. GF s Godavari College of Engineering, Jalgaon. Dept. of Electronics & Telecommunication Engineering

Transcription

1 GF s Godavari College of Engineering, Jalgaon. Dept. of Electronics & Telecommunication Engineering Lab Manual Communication System-I SE SEM I Prepared by R.V.Patil(Asso.Prof.) 1

2 LAB COURSE OUTLINE Course Title Communication System-I Short Title & Course Code CS-I Course Description: In this laboratory course emphasis is on the understanding of need of modulation and demodulation and their uses. Laboratory Hours/Week No. Of Weeks Total Hours Semester Credits Total Semester Credits: 1 Prerequisite Course(s): Analog signal and its fundamentals. LAB COURSE CONTENT (Note: Minimum FOUR Experiments from each group.) Group A 1. Study of AM transmitter and calculate of modulation index of AM wave by envelope method. a. Sketch and recognize the resulting waveforms for a sinusoidal carrier being amplitude modulated by a single frequency audio signal. b. Draw and analyze graphs to show the resulting waveform, and frequency spectrum for a sinusoidal carrier amplitude modulated by an audio signal, to a given depth of modulation, m; c. Select and use the formula: To calculate the depth of modulation for given amplitude modulated RF signal. 2. Analyze and generate A.M. Demodulation signal by diode detector. a. Generate AM modulated wave form. b. Apply Modulated AM signal to demodulator. c. Observe clipping effect. d. Compare original modulating signal with demodulated output. 2

3 3. Study of FM and calculate of modulation index of FM wave. a. Generate FM waveform. b. Calculate Modulation Index. c. Compare over with A.M. modulation. 4. F.M. Demodulation (Phase discriminator/ratio detector method.) a. Generate FM modulated wave form. b. Apply Modulated FM signal to demodulator. c. Compare original output with demodulated output. d. Plot S-curve 5. To Construct and Verify Pre-emphasis and De-emphasis and Plot the Waveforms. a. Apply the sinusoidal signal as input signal to pre emphasis circuit. b. By increasing the input signal frequency observe the output voltage and calculate gain. c. Plot the graph between gain Vs frequency. d. Repeat same procedure for de-emphasis circuit. 6. Study of Amplitude limiter circuit. a. Apply sinusoidal signal. b. Find out limiting range of applied input signal. c. Draw the graph for same and discussed about result. Group B 1. Calculate gain for RF / IF stage with AGC and without AGC. a. Explain concept regarding with and without AGC. b. Calculate gain of RF/IF stages with AGC. c. Calculate gain of RF/IF stages without AGC 2. DSB-SC signal generation using balanced modulator. a. Verified Balance modulator. 3. Analyze voltage and waveform at various stages/points in A.M. radio receiver (i.e. Super-heterodyne Radio Receiver). a. Identify the different stages and write down the information about the individual stage. b. Observation may be any available information such as number, value, type or any other indication. c. Observed and draw waveform of various stages. d. Analyze signal each points. 3

4 4. PAM modulator & demodulator. a. Generate pulse amplitude modulated waveform. b. Observed waveform and made calculation. c. Detection of modulated waveform. d. Observed demodulated PAM waveform compute information. 5. PWM modulator & demodulator. a. Generate pulse width modulated waveform. b. Observed waveform and made calculation. c. Detection of modulated waveform. d. Observed demodulated PWM waveform compute information. 6. PPM modulator & demodulator. a. Generate pulse position modulated waveform. b. Observed waveform and made calculation. c. Detection of modulated waveform. d. Observed demodulated PPM waveform compute information. Reference Books: 1. G. Kennedy, B. Davis, Electronic Communication Systems, Tata McGraw Hill Edition, 4 th Edition, H. Taub, D. L. Schilling and G. Saha, Principles of Communication Systems, Tata McGraw Hill Edition, 3 rd Edition, S. Kundu, Analog and Digital Communication, Pearson, ISBN D. Roddy, J. Coolen, Electronic Communications, Pearson, 4th Edition, Guide lines for ICA: ICA shall be based on continuous evaluation of student performance throughout semester and practical assignment submitted by the student in the form of journal. Guide lines for ESE: ESE will be based on practical assignment submitted by the student in the form of journal. In ESE the student may be asked to perform any one practical out of 8. Evaluation will be based on paper work and performance in the practical. 4

5 Experiments list according to NMU Syllabus. INDEX Sr. No. Name of Experiment Page No. 1 Study of AM transmitter and calculate of modulation index of AM wave by envelope method. 2 Analyze and generate A.M. Demodulation signal by diode detector. 3 Study of FM and calculate of modulation index of FM wave. 4 F.M. Demodulation (Phase discriminator/ratio detector method.) 5 6 Calculate gain for RF / IF stage with AGC and without AGC. Analyze voltage and waveform at various stages/points in A.M. radio receiver (i.e. Super-heterodyne Radio Receiver). 7 PAM modulator & demodulator. 8 PWM modulator & demodulator. 9 PPM modulator & demodulator. 5

6 Evaluation Sheet Sr. No. Name of Experiment Date of Performance Date of Checking Grade/ Marks Remark 1 Study of AM transmitter and calculate of modulation index of AM wave by envelope method. 2 Analyze and generate A.M. Demodulation signal by diode detector. 3 Study of FM and calculate of modulation index of FM wave. 4 F.M.Demodulation. (Phase discriminator.) 5 6 Calculate gain for RF / IF stage with AGC and without AGC. Analyze voltage and waveform at various stages/points in A.M. radio receiver (i.e. Super-heterodyne Radio Receiver) 7 PAM modulator & demodulator. 8 PWM modulator & demodulator. 9 PPM modulator & demodulator. Total Marks Average Marks 6

7 Experiment No.-1 Date: Aim: - Study of AM transmitter and calculate of modulation index of AM wave by envelope method. Objective:- a. Sketch and recognize the resulting waveforms for a sinusoidal carrier being amplitude modulated by a single frequency audio signal. b. Draw and analyze graphs to show the resulting waveform, and frequency spectrum for a sinusoidal carrier amplitude modulated by an audio signal, to a given depth of modulation, m; c.select and use the formula: To calculate the depth of modulation for given amplitude modulated RF signal. Apparatus: - Expt. Kit, function generator, dual channel CRO, probes etc. Theory:- Radio transmitter and other electronics system would be impossible without modulation. It refers to low frequency signal controlling the amplitude, (Keeping frequency or phase Constant) of high frequency signal such type of modulation is called as amplitude modulation. The high frequency is called carrier & the low frequency signal is the modulating signal. Hundreds of carrier cycle normally occurs during one cycle of modulating signal. For this reason, an AM wave forms on an CRO look like the signal of figure (4.1) When low frequency signal controls the amplitude of high frequency signal we get Amplitude Modulation (AM). Let modulating voltage is given by v m = V m cosωmt Where ω m is the angular frequency of modulating signal. And V m is the amplitude of modulating signal. The carrier voltage is given by V c = V c cosω ct Where ωc is the angular frequency of modulating signal. The amplitude of carrier is no longer constant but varies with time as given below V(t) = Vc +ka.v m cosωmt Where ka.v m cosωmt is the change in carrier voltage is given by 7

8 V = V(t) cos ωct V= Vc[ 1+( ka.vm/ Vc) cosωm].cos ωct V= V c [1+m a cosωmt]. cosω ct Where m a is the modulation index or modulating factor is given by m a = k a. V m /V c 8

9 Percent Modulation:- Amplitude Modulated waveforms has maximum peak to peak value of 2V max and minimum peak to peak value of 2V min. The modulation coefficient is given by 2V max /2 2V min /2 m a = V max /2 + 2V min /2 V max V min = V max + V min V max V min % m = x 100 V max + V min The circuit provided on the front panel of kit generates AM waveforms as shown in figure. External AF (audio frequency) I/P signal called the modulating signal is applied at AF I/P points provided on 9

10 the kit.r 1 C 1 from a low pass filter, RFC & C 2 passes only modulating signal to the emitter of transistor Q by adjusting the port of the amplitude modulated wave can be control called modulation index control R 4. R 5 provides the base biasing voltage for the transistor. Capacitor C 3 by pass if AC signal appear at base of transistor. The carrier signal generated internally by collector of transistor & capacitor C 4 & C 5. The capacitor C 5 is used to impose the modulating signal on carrier by making a positive feedback between collector and emitter. The oscillations are generated resulting in amplitude modulated output. This amplitude modulated output to the AM demodulated section for getting original modulating signal. The modulation index can be varied by changing the amplitude of modulating AF input. Experimental Kit Circuit Diagram:- 10

11 Procedure:- 1] Study the circuit Amplitude modulation provided on the front panel of the kit. 2] Connect the CRO at the output to measure the carrier signal without applying modulating signal. Frequency, F c = Hz. Amplitude, V c = mv- 3] Apply the sine wave AF I/p at marked. 4] Adjust the amplitude of AF I/P and adjust potentiometer to get the amplitude o/p on CRO. 5] Vary the modulation index from signal generator. 6] For different amplitude of modulating voltage note down the modulation index by equation. V max V min m a = V max + V min 7] Vary input frequency and amplitude. Note the corresponding change in modulation index. 8] Draw all observes waveforms on graph paper. Observation Table:- A] Constant Frequency of AF input, fm= hz. Amplitude of Vmax Vmin Modulation Index Modulation Index AF input ( AM output) ( AM Output) M= ( Vmax- Vmin )/ M=Vm/Vc ( Vmax+ Vmin ) 11

12 B] Constant Amplitude of AF input, Vm= V Frequency of Vmax of AM Vmin of AM Modulation Index AF input Output Output (m) Result:- Conclusion:- Applications:- Oral Based Questions. Q:1 What is Amplitude Modulation? Q:2 What is the frequency range of AM? Q:3 What is modulation index and it s unit? Q:4 What is the B.W. of AM? Q:5 What is the value of IF for AM? 12

13 Experiment No.:-2 Date:- Aim: - Analyze and generate A.M. Demodulation signal by diode detector. Objective:- a. Generate AM modulated wave form. b.apply Modulated AM signal to demodulator. c.observe clipping effect. d. Compare original modulating signal with demodulated output. Apparatus:-Amplitude Demodulation kit, C.R.O. function generator, probes. Theory: AM broadcast signals use carrier frequencies between 540 and 1600kHz. In the studio, an audio signal modulates the carrier to produce an AM signal. A transmitting antenna of suitable length then radiated this AM signals into space. Miles away, a receiving antenna picks up the modulated RF signal. After being amplified, this signal is demodulated (the audio is recovered). The most common circuit in use is the envelope detector which consists of detector diode as shown in figure 1. In figure2, the peaks of the amplitude modulated (AM) I/p signals are detected to recover the upper envelope. For this reason the circuit is called an envelope detector if detector diode is connected as shown in figure(d=d1). During each carrier cycle the diode turns ON briefly and charges the capacitor to peak voltage of the particular carrier cycle between peaks, the capacitor discharges through resistor by making RC time constant much greater than the period of the carrier, we get only a slight discharge between cycles. In this way most of the carrier signal is removed. The output then looks like the upper envelope with a small ripple as shown in figure1.we can remove the small RF ripple that remains on the detected signal. After removing the carriers the circuit provides Demodulated 13

14 O/P which is same as the modulating signal. By changing capacitor C1 or C2 the ripple contains in O/P can change. Through C0 demodulated O/P without DC component (AGC voltages is obtained). Experimental Kit Circuit Diagram:- Procedure:- (1) Study the circuit provided on the front panel of the kit. (2) Connect CRO at O/P terminals. Apply AM I/P, observe & note rectified AM O/P without connecting capacitors. (3) Now connect capacitor C 1 in the circuit. Note the frequency & amplitude of demodulated O/P Vo. Compare this with modulating I/P. The frequency of demodulated & modulating signal will be same. (4) Repeat the above steps (3) for the capacitor C 2 in the circuit. 14

15 Observation Table:- Modulating Signal Demodulating Signal Capacitor 1 2 Result:- Amplitude Frequency Amplitude Frequency Attached Conclusion:- Oral based Questions:- Q:1 What is the use of LPF in AM Detector? Q:2 Which type of rectifier is used in AM detector? Q:3 What are the types of AM detector? Q:4 What are the drawbacks of simple diode detector? Q:5 Give the significance of AGC in practical diode detector? 15

16 Experiment No.-3 Date:- Aim:- Objective: Study of FM and calculate of modulation index of FM wave. Theory: a. Generate FM waveform. b.calculate Modulation Index. c.compare over with A.M. modulation. In the communication information like audio signal or speech writing signal is to be transmitted from one point to another. But if it is transmitted directly through antenna then transmitting antenna must be ¼ wavelengths long. To reduce the antenna size, the low frequency information (audio) signal is mixed with higher frequency. Such process is called as frequency modulation (FM). This higher frequency is called as carrier signal. At the receiver side, this signal is converted back to audio frequency called as demodulation process. Type of modulation: If the characteristics of carrier wave like amplitude, frequency or phase is change in accordance with the modulating signal (information signal) then such type of modulation is called as amplitude modulation (AM), Frequency modulation(fm) or phase modulation(pm) respectively. Frequency modulation: In this modulating signal e m is used to vary the carrier signal frequency f c. Let the change in the carrier frequency be km where k is constant known as frequency deviation constant, then the instantaneous carrier freq is given by F i = f c +km Where f c is the un-modulated carrier frequency In this the instantaneous frequency of the carrier is caused to vary by an amount proportional to the amplitude of the modulating signal. The amplitude is kept constant. More complex than AM this is because it involves minute changes in frequency. FM is more immune to effects of noise. FM and PM are almost similar. 16

17 Assume we have a carrier at a frequency of 100MHz, called the resting frequency. If a modulating signal is then applied, this will cause the carrier to shift (deviate) from its resting frequency by a certain amount. If the amplitude is increased then the amount of deviation also increases. The rate is proportional to frequency of the intelligence signal. If the signal is removed then the carrier frequency shifts back to its resting frequency. This shift in frequency compared with the amplitude of the modulating voltage is called the deviation ratio. The deviation ratio is also called the deviation constant and it defines how much the carrier frequency will change for a given input voltage level. The units are khz/v The following equation can be used to represent FM f f ( 1 kv cos t) c m Where f c is the unmodulated carrier frequency, k is proportionality constant and the last term is the modulating voltage It will be seen that the maximum deviation will occur when the cosine term is unity and hence we obtain f f c ( 1 kvm ) maximum deviation, kv f m c Figure.1 shows the FM waveform if modulating signal increase in positive direction towards positive peak, then carrier frequency at O/P (FM) decreases it becomes max at positive peak. If modulating signal decreases from positive peak towards negative half cycle it increases the carrier frequency at O/P. When modulating signal returns to 0, the carrier frequency zero at O/P. Therefore the modulating signal produces a frequency modulated waveform as shown in figure.1 It can be shown that the instantaneous value of the FM voltage is given by v Asin ct sin mt f m The modulation index for FM is defined as m f max frequency deviation modulating frequency f m 17

18 To solve for the frequency components of the FM requires the use of Bessel functions v A J J J J 4 { J o m f sin ct 1 m f [sin c m t sin c m t ] 2 m f [sin c 2 m t sin c 2 m t ] 3 m f [sin c 3 m t sin c 3 m t ] m [sin 4 t sin 4 t]...} f c m To evaluate the individual values of J is quite tedious and so tables are used. This is an approximate method used to predict the required bandwidth necessary for FM transmission BW 2 max fsmax About 98% of the total power is included in the approximation c m Figure.1 : FM waveform 18

19 Experimental Kit Circuit Diagram:- Observation Table:- Modulating I/P Signal F.M. O/P Signal Fm Vm Max.freq Deviation m f max frequency deviation modulating frequency f m 19

20 Procedure:- 1) Study the circuit diagram provided on the front panel of the kit. 2) Connect dual trace CRO at input and output side 3) Without applying modulating i/p signal observe and note Sinusoidal o/p on CRO (carrier signal) f c = Hz, V c =.V 4) Connect sine wave i/p of around 100Hz, 10Vp-p from signal generator to the point marked as AF i/p keep it s amplitude constant and vary the potentiometer R 1 to change amplitude of o/p signal. 5) Adjust the amplitude and frequency of modulating V fm = V 6) Now increase Time/div control of CRO. You will get deviated waveform. Note its difference which is called frequency deviation, 7) Find modulation index, m= /fm 8) By Changing position of R 1 amplitude of modulating signal will change the amount of frequency deviation, Note its minimum &maximum frequency deviation. 9) Keep pot R 1 at mid position change Freq of modulating signal, observed & note the change in frequency deviation it should remains const. Result:- Conclusion:- Application:- 20

21 Oral Based Questions Q.1: What is Frequency Modulation & Give frequency range for FM? Q.2: State the different types of FM modulators? Q.3: Specify the bandwidth requirement for FM? Q.4: Explain Modulation Index & Percent Modulation for FM? Q.5:What is Deviation ratio & What is the function of IC-8038? 21

22 Experiment No:-4 Date:- Aim:- F.M. Demodulation (Phase discriminator method.) Objectives:- a. Generate FM modulated wave form. b. Apply Modulated FM signal to demodulator. c. Compare original output with demodulated output. d. Plot S-curve Apparatus:- Experimental kit, CRO, function generator, patch chords. Theory:- Modulation is a fundamental requisite of communication systems. It is defined as the process by which some characteristics of signal called carrier is varied in accordance with instantaneous value of another signal called modulating signal. The signal containing information to be transmitted are referred to as modulating signals. The carrier Frequency is greater than the modulating frequency. The signal resulting from the process of modulation is called modulated signal. The process of extracting a modulating signal from a frequency modulated carrier is known as demodulation. Electronic circuits that perform the demodulation process are called FM detectors. The FM detector performs the extraction in two steps. 1) It converts the frequency modulated signal into a corresponding amplitude modulated signal by using frequency by using frequency depends on input frequency called frequency discriminator. 2) The original modulating signal is recovered from this AM signal by using a linear diode envelop detector. A simple R-L circuit can be used as discriminator, but this circuit has a poor sensitivity as compared to a tuned LC circuit. The various types of LC discriminators are there in FM. But they suffer from the threshold effect in presence of excessive noise. The threshold can be improved by pre-emphasis, and de-emphasis circuits. FM discriminator can be divided into two types. 1) Slope detectors & 2) Phase difference discriminators 22

23 In slop detectors the principle of operation depends on the slop of the frequency response characteristics of a frequency selective network. It is catagorized as 1) Simple slop detector 2) Balanced slop detector. The circuit consists of an inductively coupled double tuned circuit in which both primary and secondary is connected to top of the primary through a capacitor C. This capacitor performs following functions. 1) It blocks dc from primary to secondary. 2) It couples the signal frequency from primary to center trapping of the secondary. The primary (V3) thus appears across the inductance L. Almost entire V3 appears across L1 except a small drop across C may be kept negligible. The center trapping of the secondary has an equal and opposite voltage across each half winding. Therefore V1 & V2 are equal in magnitude but opposite in phase. The radio frequency voltages Va1 & Va2 applied to diodes D1 & D2 are given as Va1 = V3 + V1 Va1 = V3 V2 Voltages Va1 & Va2 depends on the phasors relation between V1,V2,V3. The diodes are so arranged that the output voltage Vo is equal to the arithmetic difference given by Vo = Vo1 Vo2 Therefore, the voltage Vo will instantaneous frequency in accordance with difference of (Vo1 Vo2). The detected O/P Vo can also be taken at any one of diode cathode w.r.t to ground. 23

24 Experimental Kit Circuit Diagram:- Procedure:- 1] Study the circuit provided on the front panel of the kit. 2] Apply the modulating i/p signal to frequency to modulation kit, of around 80Hz- 250Hz, and adjust its amplitude to get FM Output. 3] Apply the FM o/p signal from frequency modulation kit to the FM input of foster- seeley/phase shift discriminator kit. 4] Adjust the amplitude & frequency of modulating signal to get the FM demodulated output on the CRO. The o/p contains noise. 5] Observe the modulating signal applied to the FM kit on the channel of the CRO & the Demodulated o/p on the other channel. 6] Vary the i/p frequency and amplitude & observe the change in the demodulated o/p. 7] Also observe and note the FM signal converted into AM signal at Various test points. 8] Draw the modulating i/p, carrier, FM O/P & Demodulated o/p on graph paper. 24

25 Observations:- Conclusion:- 1) AF Modulating i/p :- Frequency = Amplitude = 2) Demodulated o/p :- Frequency = Amplitude = Oral Based Questions. Q:1 Why foster seeley discriminator called as phase shift discriminator? Q:2 Why discriminator must be preceded by limiter stage? Q:3 Is there any difference between discriminator & demodulator? Q:4 What are the advantages & disadvantages of discriminator? Q:5 Comment on frequency response of discriminator? 25

26 Experiment:-5 Date: Aim:- Calculate gain for IF stage without AGC. Objective:- a. Explain concept regarding with and without AGC. b. Calculate gain of IF stages without AGC c. Plot the graph of gain Vs frequency Apparatus:- Experimental kit, CRO, patch cords. Theory:- A radio receiver must perform a number of functions; the receiver must separate a desired radio signal from all other radio signals that may be picked up by the antenna and reject the undesired once. Next, the receiver must amplify the desired signal to a usable level. Finally, the receiver must receives the information from the radio carrier and pass it on to the user. IF amplifier is one of the critical part in the AM receiver. At this stage must of the gain and selectivity is obtained. It is a compromise between selectivity and stability, which is best, obtained at higher frequencies. One of the most common IF values is 455KHz. It is low enough to provide good selectivity and make high gain with minimum instability. The IF amplifier are tuned class A amplifier. Usually move than one stage of IF amplifier is used to get good sensitivity. The o/p of the IF amplifier appears across a tuned transformer ckt. For n stages of IF amplifier the total no. of IFT needs (n+1). When the i/p frequency is in the VHF range and beyond, a very high value of IF chosen. Most FM receiver operating in the 88 to 108 MHz band uses an IF of 10.7 MHz. In many communication receivers operating at higher frequencies, double conversion is used to solve the image and selectivity problems. In some high frequency receivers operating in the 3-30 MHz range, single conversion is used but a very high IF of 9Mhz is chosen as it effectively solve the image problems. In an IF amplifier the selectivity should not be too sharp. If the IF bandwidth is too narrow, it would cause side band cutting. This means that the higher modulating frequencies will be greatly reduced in amplitude thereby distorting the received signal. In order to widen the band width of the amplifier the over coupled tuned circuits are used between the two stage of the IF amplifier. The o/p voltage Vs frequency for such coupled circuits is 26

27 strictly dependent upon the amount of coupling or mutual inductance between primary and secondary windings. That is spacing between the winding s is determine how much of magnetic produced by primary will cut the turns of secondary. This affects not only the amplitude of o/p vtg, but also the bandwidth. Experimental Kit Circuit Diagram:- Procedure:- 1] Study the circuit provided on the front panel of the kit. 2] Switch ON the power supply connect the CRO at o/p side. 27

28 3] Apply AM i/p of 455KHz from AM generator to the i/p of ckt. Adjust the amplitude so that o/p should not be in saturation. 4] Keep i/p voltage constant (Vi), vary the AM i/p frequency from 300KHz-1650KHz & note the corresponding voltages at the o/p of first stage & second stage. Find out the gain Vo/Vin. This will provide max gain for IF 455KHz (approximate). 5] Connect o/p Vo to detector diode, AGC bias. o/p to AGC bias i/p. Observe & note demodulated o/p Vo which is modulating signal. 6] Plot the graph AM i/p frequency Vs gain Vo/Vi. Observation Table:- AM I/P Vi =.Constant. AM I/P IF Frequency Fi (KHz) O/P Voltage Vo Gain Vo/Vi 28

29 Result:- From the experiment it is concluded that the IF amplifier amplifies those frequencies for which it is tuned and rejects the unwanted frequencies. Also it is observed that the o/p voltage is maximum for the tuned IF frequencies. Oral Based Questions. Q:1 How IF is generated? Q:2 Give the different values of IF for different receivers? Q:3 What is the role of AGC in IF amplifier? Q:4 Draw & Comment on IF band pass characteristic of IF amplifier? Q:5 Draw & Comment on Ideal response of IF amplifier? 29

30 Experiment No. : 06(A) Date : Aim:- Analyze voltage and waveform at various stages/points in A.M. radio Objective: receiver (i.e. Super-heterodyne Radio Receiver). a. Identify the different stages and write down the information about the individual stage. b. Observation may be any available information such as number, value, type or any other indication. c. Observed and draw waveform of various stages. d. Analyze signal each points. Apparatus: Digital multimeter for voltage at different point, single trace CRO for observing waveforms, CRO probes. Theory: Transmission of radio signal Radio waves propagate with different modes in atmosphere. We very some of its property in with information so by catching those radio waves information can be extracted e.g in AM amplitude of radio carrier is varied in accordance with information and in FM, frequency of carrier is varied accordingly. Different stations are operating on different carrier frequencies. Therefore, they can be separately received. Signal reception: Antenna which emits radio energy into atmosphere can also absorb energy from atmosphere. So in the receiver we use antenna. Now this antenna is tuned to a particular frequency using variable capacitor across it called gang. So we can get one prominent ratio carrier at a time. Now this signal is very weak, it is amplified by tuned amplifiers and detected. Super heterodyning Principle: Super heterodyning means mixing. There is no. of different carrier Frequencies e.g 550 KHz to 1600KHz.By means of antenna we can stick to one carrier & extract 30

31 information out of it. But then forgoing amplifiers are required to work over a large frequency span, which is not possible. Over this span, characteristic of amplifying devices change and we will not get required fidelity. To avoid the difficulty all these amplifiers are made to work on single frequency called intermediate frequency [IF]. So better fidelity results. To do this antenna signal is mixed with local oscillator signal and resulting signal frequency is held fixed, equal to IF. To achieve this local oscillator frequency is also varied by gang so that antenna frequency and its frequency maintain fixed frequency difference, equal to IF. This IF has typical values like 455 KHz, 550 KHz etc. So each station is converted into IF using mixer and amplified signal is passed through detector and we get information output. In our kit we have given AM generator with variable carrier freq. stations. Modulating signal is variable. Since at a particular time modulating signal is of fixed frequency set by operator, we can observe different stages in receiver for our required signal. Here we connect our AM signal with patch cord to antenna, or even without actual connection signal can be observed but power is less, so to observe signal clearly we connect it with patch cord. Actually AM signal at the input of receiver required is less, but to observe it properly we have given strong AM signals and in receiver we have provided potential dividers i.e. deliberately we have reduced the gain of our receiver. Due to this we cannot hear weak commercial stations in our receiver. Procedure: 1] Connect mains cord of A.M to the 230/50Hz as supply. 2] Switch on the unit. 2] Tune the Radio receiver to any Station.(i.e.Jalgaon at 988KHz.) 3] Observe, Note the o/p(frequency, Amplitude) at the R.F. Amplifier Stage,Local oscillator stage, I.F. stage, Mixer stage, Detector stage, Audio Amplifier stage. 4] Draw the waveforms observed at different points on kit 31

32 Sensitivity test 1) Switch on the power supply. 2) Adjust Frequency of AM gen 625 KHz using carrier freq. adjust pots. Two pots are provided, one for coarse adjustment (left side) & other for fine adjustment (right side). 3) Adjust freq of signal gen. to 400 Hz, & amplitude 1Vpp & connect it to signal i/p of AM gen.keep AM signal o/p to maximum amplitude using amplitude adjust pot.& adjust AM signal such that we get 30% modulation index. For that purpose use DC bias pot of AM gen. block. 4) Adjust amplitude of AM signal 200mV p-p & connect it to I/P point of receiver. 5) At receiver to types of power amplifiers are used. a) IC LM380 based power amplifier b) Pushpull amplifier. Keep both toggle S/Ws to left most position.i.e LM380 power amplifier is used. 6) Observe oscillator freq. at a point provided at right hand lower corner of PCB. Vary gang capacitor to observe maximum & minimum freq. Note the direction. 7) Now adjust gang position such that we get audio o/p at o/p points of receiver. Keep audio pot at maximum position. 8) Now sensitivity of receiver is defined as i/p signal required getting 50 mw o/p powers, when loud speaker is replaced by equivalent resistance. We have used 8 ohm resistance. Corresponding to that we required 1.78 Vp-p at o/p. 9) Here very fine adjustment of gang knob & AM o/p knob is required. 10) Adjust AM o/p knob such that we get o/p 1.78 Vp-p. 32

33 11) Verify that station is properly tuned using gang knob.measure AM signal amplitude. 11) Now Adjust AM gen. Carrier freq. to 1MHz. & repeat the procedure as above. 12) Adjust AM gen. carrier freq to 1400 KHz & repeat the procedure as above. 13) Plot the Graph of AM i/p signal required for 50mW o/p power Vs AM station frequency. This is the sensitivity curve for given receiver. Selectivity Test: 1) As above adjust AM o/p (carrier freq = 1MHz, modulation index = 30%) 2) Connect AM o/p to i/p of receiver. 3) Adjust gang knob & AM strength such that we get audio o/p 1.78 Vp-p corresponding to 50 mw o/p power. 4) Measure carrier freq. using freq. counter. 5) Now slightly change the carrier freq. Using fine adjustment knob & measure o/p of receiver. 6) Take two to three readings for carrier freq. lower than 1MHz & two to three readings for carrier freq. higher than 1MHz. 7) Plot the graph of o/p Vs carrier freq. This is the selectivity curve of receiver. Fidelity Test: 1) Adjust AM o/p as in case of selectivity test. 2) Adjust gang knob & AM o/p strength such that we get o/p of receiver. 3) Now don t change final AM o/p strength. 4) Now vary audio freq from minimum to maximum & each time measure audio O/P. Tabulate the readings. Plot the graph of o/p Vs freq. of signal generator. This the fidelity curve. 5) Now Keep both toggle S/Ws to rightmost position i.e. pushpull power amplifier is used. 33

34 6) Repeat step 1 to 4. Plot fidelity curve for pushpull amplifier. Circuit Diagram: 5V 65K 2ND IFT 100K 3rd IFT [WHILE] 200K UF OA K 2ND IFT I/P 47 K 220K 1UF BF K 0.047UF BF K7 1K 330 BC UF ohm SPEAKA R 100UF POWER AMPLIFIER Result: Oral based Questions: 1. What is roll of RF amplifier in Radio Receiver? 2. What is roll of Local Oscillator in Radio Receiver? 3. What is roll of IF stage in Radio Receiver? 4. What is roll of Mixer stage in Radio Receiver? 5. What is roll of Audio amplifier in Radio Receiver? 34

35 Experiment No. : 06(B) Date : Aim:- Analyze voltage and waveform at various stages/points in F.M. radio Objective: receiverceiver). a. Identify the different stages and write down the information about the individual stage. b. Observation may be any available information such as number, value, type or any other indication. c. Observed and draw waveform of various stages. d. Analyze signal each points. Apparatus: Digital multimeter for voltage at different point, D.S.O.(200MHz) for observing waveforms, CRO probes, F.M receiver trainer kit,etc. Theory: There are two types of analog system is used, 1) F.M transmitter when function of carrier is charged in accordance with modulating signal. 2) A.M transmitter where amplitude ampl r of carrier is charge in accordance with the modulating signal. Where we will discuss about freq. modulating analog transmission system. The F.M receiver is superhetrodyne receiver & the dig. In panel shows, first how similar it is to the A.M receiver however the basic differences are as follows, 1] Generally much higher operating freq. in F.M. 2] Hence for timing & deemphasize is F.M. 3] Need for timing & deemphasize in F.M. 4] Totally different match of demodulator. 5] Different match of obtaining AGC. 35

36 Common circuit with an A.M receiver: A no. of section of the F.M receiver corresponds exactly to the use of others types of receiver. R.F amplifier: R.F amplifier is always used is on F.M amplifier receiver. The main purpose is to reduce noise fig. Control otherwise be a problem, because of large needs of an F.M. Frequency channels: The oscillator ckt takes of usual forms with the colpits & clap freq. Dominant being suited to VHF. Operation taking is normally such a non-problem in F.M broad cast receiver. This is because the tuning freq. Range is unity 12.5% much than in A.M broad casting. Intermediate freq. & R.F amplifier: Again the type & operation D.C not differ from their A.M counter parts. It is worth nothing however that if & switch required are higher than in A.M broad cast receiver operating in the 88 to 108 MHz. Amplitude limiting: In order to make the use of the advantages offered by a demodulator must be proceeded by amplifier amplitude changed in the signal fed to the F.M and demodulator. Basic F.M demodulator: The function of a freq. FM demodulator is to change the freq. Deviation of the incoming carrier into an A.M amplifier variation. About trainer: In this IM transmitter trainer a transistorized IM transmitter on 88 to 188 MHz band is provided to obtain GND. Range & clarity in receiver IC base function generator is provided as built in further to enable students to carry out the measurement with a transistorized. Procedure: 36

37 1] Connect mains cord of F.M transmitter to the 230/50Hz as supply switch off the unit. 2] Tune the Radio receiver to Jalgaon Station.i.e.106.4MHz. 3] Observe, Note & Draw the o/p (Frequency, Amplitude) at the R.F. Amplifier Stage, Local oscillator stage, I.F. stage, Mixer stage, Discriminator stage, Audio Amplifier stage. Conclusion: Signature with Date Subject In charge 37

38 Experiment No. : 07 Date : Aim:- Study of PAM and Demodulation Technique. Objective: a. Study of sampling, effect of variable sampling rate b. Observation of Flat top and Natural sampling. c. Reconstruction of original signal using filter d. Observed and draw waveform of various stages. Apparatus: Experimental kit, DSO or CRO, connecting wires, CRO probe. 38

39 Theory: According to sampling theorem, sampling rate should be rapid enough so that at least two samples are taken during the course of the period corresponding to the highest frequency spectral component. i.e. if our signal frequency is 1 khz then sampling frequency must be 2 khz minimum for proper reproduction of original signal from sampled signal. It is called as Nyquist criteria. In this Kit we have provided two different types of sampling 1] Natural Sampling. 2] Flat Top Sampling. 1] Natural Sampling: - In this case the sampled signal consists of sequence of pulses of varying amplitude whose tops are not flat but follow the waveform of I/p signal. 2] Flat Top Sampling: - A Flat topped pulse has constant amplitude established by the sample value of the signal at some point within the interval. We have arbitrarily sampled the signal at the beginning of the sampling pulse. To generate flat top sample the signal is held using a holding capacitor after sampling pulse. After holding time the capacitor is dumped using a dump pulse. So that between sampling pulse & dump pulse we get a fiat topped sample pulse. In this kit we have provided 1 khz sine wave & crystal clock to observe stable waveforms on normal oscilloscope. To verify sampling theorem you can vary freq of i/p signal from minimum to maximum & observe reconstructed o/p. OR IC 555 clock is given. By varying basic clock freq. sampling frequency can be varied & its effect on reconstructed signal can be observed. Also at receiver side effect of value of Holding capacitor (sample & hold ckt.) o reconstructed signal can be observed. Procedure:- 1. Switch on the power supply. 2. Check the oscillator frequency & Input frequency given to Binary counter given at Pin no Check 1KHZ sin frequency at Pin no Check different frequencies at Pin no 3 To 8 (320K,160K,80K,40K,20K,10K Respectively) 5. Check frequency at Pin no Through the selector switch to the internal position.so that internal frequency will become sampling freq. 7. Give 1KHZ sine signal as sine input at Pin no Check the sampled output / sampled & hold output at different sampling frequency. 9. Give the input to second order LPF& check the response of output. 10. Similarly used fourth order LPF& check the response of output. 39

40 Result:- Thus we verified PAM and demodulation. Oral Based Questions. 1. What are the types of Pulse modulation? 2. Explain advantages and applications of Pulse Modulation 3. Explain aliasing. 40

41 Experiment No. : 08 Date : Aim:- Study of PWM and Demodulation Technique. Objective: a. Study of timer IC 555. b. Study of monostable and astable multivibrator. c. Observed and draw waveform of various stages. Apparatus: Experimental kit, Bread bord, IC555,Registers, Signal Generator, Capacitor, DSO or CRO, connecting wires, CRO probe. Theory: If we apply variable voltage at control input (pin 5) of IC555 in astable mode then the width of output pulse will change accordingly. When we apply low frequency audio signal to the control input, internally the control voltage will change the reference voltage of comparator so charging time of capacitor changes every time and so width of o/p pulse also changes. So here as shown in figure the audio signal is applied at the control input through positive clamper circuit. The values of R A, R L and C are chosen to generate pulse train of around 64 KHz. The width of o/p pulse will change in accordance with the amplitude of audio signal as shown in figure. Ra = 3k,c = 0.02 micro farad, Rl = 1k 41

42 This pulse width modulator circuit need external pulse train input to trigger the the 555 monostable circuit. This trigger input sould be a square wave signal with a fixed frequency. The output of this pulse width modulator circuit will have same frequency with the trigger input, but with its pulse width proportional to the modulating input signal at pin 5. Pin No Function Name 1 Ground (0V) Ground 2 Voltage below 1/3 Vcc to trigger the pulse Trigger 3 Pulsating output Output 4 Active low; interrupts the timing interval at Output Reset 5 Provides access to the internal voltage divider; default 2/3 Control Voltage Vcc 6 The pulse ends when the voltage is greater than Control Threshold 7 Open collector output; to discharge the capacitor Discharge 8 Supply voltage; 5V (4.5V - 16 V) Vcc Procedure:- 1 Make the connection as per the circuit Diagram. 2 Connect 6 volt DC power supply to Vcc. 3 give the 1kHz,2v square wave signal to pin2 4 connect modulating i/p (sine wave with 500Hz, 1v)in between pin 1 and 5 5 Observe o/p at pin3. Result:- Oral Based Questions. 42

43 Experiment No. : 09 Date : Aim:- Study of PPM and Demodulation Technique. Objective: a. Study of timer IC 555. b. Study of monostable and astable multivibrator. c. Observed and draw waveform of various stages. Apparatus: Experimental kit, Bread bord, IC555,Registers, Signal Generator, Capacitor, DSO or CRO, connecting wires, CRO probe. Theory: PPM is also sometimes known as pulse-phase modulation The amplitude and width of the pulse is kept constant in PPM The position of each pulse, in relation to the position of a recurrent reference pulse, is varied by each instantaneous sampled value of the modulating wave It often used in optical communication, such as fiber optics, in which there is little multipath way interference It is used exclusively for transferring digital signals and cannot be used with analog systems It is used for transferring simple data and is not effective at transferring files. Pulse position modulation is similar to pulse width modulation, but the frequency is not constant. Like pulse width modulator circuit, pulse position modulator can be easily constructed using 555 IC. If you look at the output signal plot in the above timing chart, and compare it with our previous pulse width modulation, you can find the key difference: the pulse width modulator modulate the active factor (the duty cycle), t(on)/(t(on)+t(off)), without changing the total period of t(on)+t(off), while pulse position modulator modulate the t(on) and keep t(off) fixed. You can see that in the pulse-position-modulation, the frequency of the pulse is modulated as well as the duty cycle. PPM is simply PWM + monostable multi-vibrator. Means if we fed the PWM output to trigger input of IC555 in monostable mode then the output of monostable will be PPM wave. So to generate PPM we require one astable multi-vibrator to generate PWM signal and one monostable multi-vibrator to get desire PPM. The final circuit and the waveforms are as shown in the diagram. 43

44 Procedure: 1 Make the connection as per the circuit Diagram. 2 Connect 6 volt DC power supply to Vcc. 3 give the 1kHz,2v square wave signal to pin2 4 connect modulating i/p (sine wave with 500Hz, 1v)in between pin 1 and 5 5 Observe o/p at pin3 ie PWM pulses. 6 connect the o/p to pin2 of second timer IC. 7 Observe the o/p at pin 3 of second timer IC. Result: 44