2666 1530 analog communications_lammanual (1)

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2666 1530 analog communications_lammanual (1)

  1. 1. MANDSAUR INSTITUTE OF TECHNOLOGY ANALOG COMMUNICATIONS LAB MANUAL EC-405 DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERINGAnalog Communications, EC-405 Page 1
  2. 2. MANDSAUR INSTITUTE OF TECHNOLOGY EC 405 ANALOG COMMUNICATIONS LAB 1. i) To Measure carrier with audio signal and to observe waveform on CRO. ii) To Measure the modulation index of the amplitude modulated waveform. 2. To recover original baseband signal from amplitude modulated waveform. 3. i) To modulate a sinusoidal carrier with an audio signal and observe Frequency modulated waveform on CRO. ii) To measure the modulation index of the Frequency-Modulated waveform. 4. To recover original baseband signal from frequency modulated waveform. 5. To study AM Transmitter. 6. To study AM Receiver. 7. To modulate a sinusoidal carrier with an audio signal & observe phase modulated waveform on CRO. 8. To recover original baseband signal from phase modulated wave.Analog Communications, EC-405 Page 2
  3. 3. MANDSAUR INSTITUTE OF TECHNOLOGYExpt. No: 1 AMPLITUDE MODULATIONAim: - 1) To Measure carrier with audio signal and to observe waveform on CRO. 2) To Measure the modulation index of the amplitude modulated waveform.Equipment Required: - Audio frequency Generator, Amplitude Modulation and DemodulationTrainer Kit, CRO, Cords.Waveforms:Fig.1 Wave forms a) Carrier signal b) Modulating signal c) AM signalTheory: - Amplitude modulation: “Amplitude modulation is a process in which the amplitude ofhigh frequency carrier is varied in accordance with the amplitude of a low frequency modulatingsignal.”In amplitude modulation, the information signal varies the amplitude of the carrier sinewave. In other words, the instantaneous value of the carrier amplitude changes in accordance withAnalog Communications, EC-405 Page 3
  4. 4. MANDSAUR INSTITUTE OF TECHNOLOGYthe amplitude of the modulating signal. Fig shows a single frequency sine wave modulating a highfrequency carrier signal. Note that the carrier frequency remains constant during the modulationprocess but its amplitude varies in accordance with the modulating signal. An increase in themodulating signal amplitude causes the amplitude of the carrier to increase. An increase ordecrease in the amplitude of the modulating signal causes a corresponding increase or decrease inBoth the positive and negative peaks of the carrier amplitude.Let the signal potential designated as em = Em cos(ωmt)And let the unmodulated carrier be written as ec = Ec cos(ωct)The carrier frequency (ωc) is much larger than the signal frequency (ωm).The resulting modulated wave is Eam = (Ec+Emcos(ωmt))cos(ωct) Eam = Ec(1+ m cos(ωmt))cos(ωct)The factor m is known as modulation index m = Em/EcFor the practical we can use m = [(Vmax - Vmin)/(Vmax + Vmin)]/100Modulation index:- In order for proper AM to occur, the modulating signal voltage must be lessthan carrier voltage. Therefore, the relationship between the amplitudes of the modulating signaland the carrier is important. This relationship is expressed in terms of a ratio known as themodulation index, m. Modulation index is the ratio of the modulating signal voltage to the carrierVoltage. Modulation index, m= Em / Ec, orThe modulation index should be a number between 0 and 1. If the amplitude of the modulatingvoltage is higher than the carrier voltage, m will be greater than 1. This will cause severe distortionof the modulated wave form. This condition is called over modulation. When m=1 the condition iscalled full modulation. Whenever the modulation index is multiplied by 100, the degree ofmodulation is expressed as a percentage. In this case modulation index is called percentagemodulation. %m= (Em/Ec) x100Analog Communications, EC-405 Page 4
  5. 5. MANDSAUR INSTITUTE OF TECHNOLOGYProcedure: -1. Switch ON the mains supply.2. Connect carrier wave to the socket marked “CARRIER IN” and 1KHz audio signal from audio frequency generator to the socket marked “Audio In”.3. Slowly increase the level of audio signal input and keep observing the amplitude-modulated waveform on CRO.4. For a particular level of audio signal measure Vmax and Vmin.5. Calculate % of modulation Ma = [(Vmax - Vmin)/(Vmax + Vmin)]/100Observation Table: -Result: - Thus, we have measured the modulation index on CRO and calculated the percentage ofModulation.Analog Communications, EC-405 Page 5
  6. 6. MANDSAUR INSTITUTE OF TECHNOLOGYExpt. No: 2 AMPLITUDE DEMODULATIONAim: - To recover original baseband signal from amplitude modulated waveform.Equipment Required: - Amplitude Modulation and Demodulation Trainer Kit, Audio frequencygenerator CRO & CRO Cords.Theory: – Fig 2.1 shows AM demodulator (detector) circuit. A demodulator is a circuit that acceptsa modulated signal and recovers the original modulating information. A demodulator circuit is thekey circuit in the radio receiver. Fig (2.1) AM Detector (Envelope detector)Diode detector:- The simplest and most widely used amplitude demodulator is the diode detectorshown in fig (2.1). The AM signal is applied to the rectifier circuit consisting of diode, capacitorsand resistors. The diode conducts when the negative half cycles of the AM signals occur. Duringthe positive half cycles, the diode is reverse biased and no current flows through it. To recover the original modulating signal filter is connected after the diode. The filter isdesigned such that capacitors have very low impedance at the carrier frequency. At the frequencyof the modulating signal, they have much higher impedance. The result is that capacitors effectivelyshort or filter out the carrier, thereby leaving the original modulating signal. The fig 2.2b shows thedemodulated signal.Analog Communications, EC-405 Page 6
  7. 7. MANDSAUR INSTITUTE OF TECHNOLOGY Fig (2.2) Wave forms a) AM signal b) Demodulated signal Fig (2.3) Amplitude DemodulationAnalog Communications, EC-405 Page 7
  8. 8. MANDSAUR INSTITUTE OF TECHNOLOGYProcedure:-1. Connect the output of AM modulator to the input of envelope detector.2. Connect the CRO to the output of envelope detector.3. Observe the demodulated wave form, measure the frequency of this waveform and compare itwith the original modulating signal.4. Sketch AM wave and demodulated waveform.Result:-Analog Communications, EC-405 Page 8
  9. 9. MANDSAUR INSTITUTE OF TECHNOLOGYExpt. No: 3 FREQUENCY MODULATIONAim: - 1) To modulate a sinusoidal carrier with an audio signal and observe Frequency Modulated Waveform on CRO. 2) To measure the modulation index of the Frequency-Modulated waveform.Equipment Required: – Frequency Modulation Kit, Audio generator, Cathode Ray OscilloscopeCRO Cords, Patch Cords.Block diagram: Fig (3.1) Block diagram of FM modulatorWaveforms:Fig (3.2) Wave forms a) Carrier signal b) Modulating signal c) FM signalAnalog Communications, EC-405 Page 9
  10. 10. MANDSAUR INSTITUTE OF TECHNOLOGYTheory: –Frequency modulation: “Frequency modulation is a process in which the frequency of ahigh frequency carrier is varied in accordance with the amplitude of a low frequency modulatingsignal.” In FM, the carrier amplitude remains constant, while the carrier frequency is changed by themodulating signal. As the amplitude of the information signal varies, the carrier frequency shifts inproportion. As the modulating signal amplitude increases, the carrier frequency increases. If theamplitude of the modulating signal decreases, the carrier frequency decreases. The reverserelationship can also be implemented. A decreasing modulating signal will increase the carrierfrequency above its center value; whereas an increase in modulating signal will decrease the carrierfrequency below its center value. As the modulating signal amplitude varies, the carrier frequencyvaries above and below its normal center frequency with no modulation. The amount of change incarrier frequency produced by the modulating signal is known as the frequency deviation.Maximum frequency deviation occurs at the maximum amplitude of the modulating signal. The frequency of the modulating signal determines how many times per second the carrierfrequency deviates above and below its nominal center frequency 100 times per second. This iscalled the frequency deviation rate. An FM signal is illustrated in fig (3.2c). With no modulating signal applied, the carrierfrequency is a constant amplitude sine wave at its normal constant center frequency. Themodulating information signal [Fig (3.2a)] is a low frequency sine wave. As the sine wave goespositive, the frequency of the carrier increases proportionately. The highest frequency occurs at thepeak amplitude of the modulating signal. As the modulating signal amplitude decreases, the carrierfrequency decreases. When the modulating signal is at zero amplitude, the carrier will be at itscenter frequency point. When the modulating signal goes negative, the carrier frequency will decrease. The carrierfrequency will continue to decrease until the peak of the negative half cycle of the modulating sinewave is reached. Then, as the modulating signal increases towards zero, the frequency will againincrease.Modulation Index:- Modulation Index is the ratio of the maximum deviation frequency to thefrequency of modulation. In other words it is the ratio of the spread in frequency spectrum to thefrequency that was used to modulate the carrier.Modulation index mf for FM is defined as: mf = max frequency deviation/ Modulating frequency = δ/ fmAnalog Communications, EC-405 Page 10
  11. 11. MANDSAUR INSTITUTE OF TECHNOLOGYProcedure: - 1. Feed modulating audio signal to the sockets marked „MODULATING SIGNAL IN‟ and observe output on CRO. Vary the amplitude and frequency of audio signal and see their effects. 2. Disconnect the audio signal and measure frequency of carrier waveform with frequency counter connected between sockets marked „FM OUT‟. Let it be fc. 3. Now again feed audio signal to the „MODULATING SIGNAL IN‟ sockets and observe the FM wave on CRO. Note down the maximum frequency of a part of the FM waveform on CRO. Let it be fmax. Calculate frequency deviation δ = (fmax - fc) 4. Now connect frequency counter to the sockets marked „MODULATING SIGNAL OUT‟ and note down the frequency of audio signal. Let it be fm. 5. Calculate the modulation index mf = (max frequency deviation)(modulating frequency) = δ fmObservation Table :-S. No. Modulating frequency fm Frequency Deviation δ Modulation Index mfResult: - We have successfully done experiment.Analog Communications, EC-405 Page 11
  12. 12. MANDSAUR INSTITUTE OF TECHNOLOGYExpt. No: 4 FM DEMODULATORAim: - To recover original baseband signal from frequency modulated waveform.Equipment Required:- Frequency Modulation and Demodulation Kit, Patch Cords, Cathode RayOscilloscope, CRO Cords.Block diagram :- Fig (4.1) Block diagram of FM demodulatorWaveforms:-Fig (4.2) Wave forms a) Modulating signal (CH1 of CRO) b) FM signal c) Demodulated signal (CH2 of CRO)Theory:- Fig (4.1) shows the block diagram of FM demodulator. Demodulation is the process ofrecovering the low frequency modulating signal. Here in FM demodulator the frequency modulatedAnalog Communications, EC-405 Page 12
  13. 13. MANDSAUR INSTITUTE OF TECHNOLOGYsignal is inputted. The output of the demodulator is the original low frequency modulating signal.Fig (4.2) shows original modulating signal, FM signal and demodulated signal. There are three general categories of FM demodulator circuits, which are, “phase-lockedloop demodulator,” “slope detection/FM discriminator” and “quadrature detector.” Although slopedetection is rarely used in practice because of its nonlinearity, it is relatively simple to implement.Hence, we consider slope detection to illustrate basic concepts in this experiment. Fig.(4.3) Circuit of slope detector The basic idea is to set the center frequency of a tuned circuit such that the FM carrier signalfalls on the slope of the resonance curve. This way, any increase in frequency of the FM signal willresult in a larger signal amplitude at the output of the tuned circuit, and any decrease will result in asmaller amplitude. Consequently, the output of the tuned circuit becomes an amplitude-modulatedsignal (in addition to FM modulation). At this point, an envelope detector such as the one used forAM detection can be employed to detect the information-bearing signal. As can be seen from thefigure above, frequency-to-amplitude conversion will in general be nonlinear. However, frequencydeviation of FM can be kept small enough to approximate linear transfer characteristics. Aconceptual circuit for the slope detector is shown below. We will, however, use an active transistorcircuit for actual implementation.Procedure: -1. Connect the trainer kit to the mains supply and switch ON.2. Observe the modulating signal at „modulator output‟ by varying „frequency control‟ &„amplitude control‟ knob.3. Observe the un-modulated carrier signal at „FM out‟.4. Connect the „modulator O/P‟ to the „modulating I/P‟.5. Connect „FM out‟ to „FM in‟ of demodulator.6. Connect CH1 of CRO to the „modulating I/P‟ & CH2 to the „demodulator O/P‟ as shown in theblock diagram. Observe the demodulated wave form and compare it with the modulating signal.Result: - We have successfully recovered the original baseband signal from FM wave.Analog Communications, EC-405 Page 13
  14. 14. MANDSAUR INSTITUTE OF TECHNOLOGYExpt. No: 5 AM TRANSMITTERSAim: - To study AM Transmitters.Theory: - A transmitter performs the modulation process and raises the power level of a modulatedsignal to the desired extent for effective radiation. The AM transmitters are divided into twocategories: Low Power Level Modulation:- Fig.(5.1) shows low level modulated AM transmitter block diagram. In this block diagram, observe that a linear class B power amplifier is used after class C modulator amplifier. The linear class B power amplifier performs the major power amplification and feeds the amplified AM signal to antenna, In this block diagram, the modulator amplifier performs modulation at relatively low power levels. Hence this is called low level modulated AM transmitter. The modulated AM signal is amplified by class B power amplifier to avoid distortion in the output. Fig.(5.1) Low level modulated AM transmitter block diagram High Power Level Modulation:- Fig.(5.2) High level modulated AM transmitter block diagramAnalog Communications, EC-405 Page 14
  15. 15. MANDSAUR INSTITUTE OF TECHNOLOGY Fig.(5.2) shows the block diagram of AM transmitter. The crystal oscillator generates carrier frequency. The buffer amplifiers and driver amplifiers amplify the power level of the carrier to required value. The amplified carrier is given to class C modulator amplifier. The modulating signal and given to audio amplifier. It is further amplified by audio power amplifier at a level suitable for modulation. The class C modulator amplifier modulates the carrier input according to modulating audio signal. The output of the class C modulating amplifier is AM and it is given to antenna through some antenna matching network. The antenna matching network is generally tuned LC circuit in collector circuit of modulator amplifier. In this AM transmitter, the modulator amplifier operates at high power levels and delivers power directly to the antenna. This is called High level modulated AM transmitter.Result: – We have successfully studied AM transmitters.Analog Communications, EC-405 Page 15
  16. 16. MANDSAUR INSTITUTE OF TECHNOLOGYExpt. No: 6 AM RECEIVERSAim: - To study AM Receivers.Theory: - The TRF (Tuned Radio Frequency) Receiver and Superheterodyne Receiver are the twomain configurations of the receivers, they have real practical or commercial significance. Most ofthe present day receivers use superheterodyne configuration. But the TRF receivers are simple andeasy to understand. Tuned Radio Frequency Receiver:- It consists of two or three stages of RF amplifiers, detector, audio amplifier and power amplifier. The RF amplifier stages placed between the antenna and detector are used to increase the strength of the received signal before it is applied to the detector. These RF amplifiers are tuned to fix frequency, amplify the desired band of frequencies. Therefore, they provide amplification for selected band of frequencies and rejection for all others. As selection and amplification process is carried out in two or three stages and each stage must amplify the same band of frequencies, the ganged tuning is provided. Fig.(6.1) Block diagram of TRF receiver The amplified signal is then demodulated using detector to recover the modulating signal. The recovered signal is amplified is further by the audio amplifier followed by power amplifier which provide significant gain to operate a loud speaker. Superheterodyne Receiver :- To solve basic problems of TRF receivers, in this receivers, first all the incoming RF frequencies are converted to a fixed lower frequency called intermediate frequency (IF). Then this fixed intermediate frequency is amplified and detected to reproduce the originalAnalog Communications, EC-405 Page 16
  17. 17. MANDSAUR INSTITUTE OF TECHNOLOGY information. Since the characteristics of the IF amplifier are independent of the frequency to which the receiver is tuned, the selectivity and sensitivity of super heterodyne receiver are fairly uniform throughout its tuning range. Fig.(6.2) Block diagram of superheterodyne receiver Mixer circuit is used to produce the frequency translation of the incoming signal down to the IF. The incoming signals are mixed with the local oscillator frequency signal in such a way that a constant frequency difference is maintained between the local oscillator and incoming signal. This is achieved by using ganged tuning capacitors. The RF amplifier provides some initial gain and selectivity. The output of the RF amplifies is applied to the input of the mixer. The mixer also receives an input from the local oscillator. The output of the mixer circuit is difference frequency (f0-fs) commonly known as IF (Intermediate frequency). This signal is amplified by one or more IF amplifier stages, and most of the receiver gain is obtained in this IF stages. The highly amplified IF signal is applied to the detector circuit to recover the original modulating information. Finally the output of detector circuit is fed to audio and power amplifier which provides a sufficient gain to operate a speaker. Another important circuit in the superheterodyne receiver are AGC and AFC circuit. AGC is used to maintain a constant output voltage level over a wide range of RF input signal levels. Result: – We have successfully studied AM Receivers.Analog Communications, EC-405 Page 17
  18. 18. MANDSAUR INSTITUTE OF TECHNOLOGYExpt. No: 7 PHASE MODULATIONAim: - To modulate a sinusoidal carrier with an audio signal & observe phase modulated waveformon CRO.Equipment Required: - 1. Phase Modulated Kit 2. Audio generator 3. Cathode Ray Oscilloscope 4. CRO Cords. 5. Patch CordsTheory: - In this type of angle modulation, The phase angle ψ(t) is varied linearly with amodulation signal f(t) about an un-modulated phase angle ωct. That is to say, the instantaneousvalue of a phase angle ψ(t) is equal to the phase of the phase of an un-modulated carrier (ωct) plus atime varying component proportional to f(t) mathematically, ψi(t) = ωct +Kpf(t) ---------------- (1)Note that θ0 is time independent and hence, has been ignored.The proportionally constant kp, is known as phase sensitivity of the modulator, expressed inradians/volts. The carrier wave after phase modulation has the phase angle given by eq. (1) and isrepresented as Φpm = A cos ψi(t) = A cos[ωct + Kpf(t)]Modulation Index – In AM the degree of modulation is measured as a percentage from 0% to 100%or as a modulation factor from 0 to 1. In angle modulation the degree of modulation is measured bythe modulation index. The equation for modulation index is m = fd/fmWhere,Analog Communications, EC-405 Page 18
  19. 19. MANDSAUR INSTITUTE OF TECHNOLOGY fd = The frequency deviation fm = The modulating frequency Fig. (7.1) PM modulationProcedure: – 1. Switch ON the instruments using ON/OFF switch. 2. Connect the oscilloscope channel „A‟ across modulated output and observe the carrier wave as in fig. Ic 2206 used as a carrier wave generator modulator. 3. Connect the circuit as shown in circuit diagram.Analog Communications, EC-405 Page 19
  20. 20. MANDSAUR INSTITUTE OF TECHNOLOGY 4. Connect oscilloscope channel „A‟ across AF signal output & channel „B‟ across modulated output. Now set the frequency of AF signal at 10 KHz and increase the AF amplitude. Now observe the phase modulation corresponding to the AF signal.Result: – We have successfully done the experiment.Analog Communications, EC-405 Page 20
  21. 21. MANDSAUR INSTITUTE OF TECHNOLOGYExpt. No: 8 PHASE DEMODULATIONAim: - To recover original baseband signal from phase modulated wave.Equipment Required: - 1. Phase Modulated Kit 2. Cathode Ray Oscilloscope 3. CRO Cords. 4. Patch CordsTheory:- In this type of angle modulation, The phase angle ψ(t) is varied linearly with a modulationsignal f(t) about an un-modulated phase angle ωct. That is to say, the instantaneous value of a phaseangle ψ(t) is equal to the phase of an un-modulated carrier (ωct) plus a time varying componentproportional to f(t) mathematically, ψi(t) = ωct +Kpf(t) ---------------- (1)Note that θ0 is time independent and hence, has been ignored.The proportionally constant kp, is known as phase sensitivity of the modulator, expressed inradian/volts. The carrier wave after phase modulation has the phase angle given by eq. (1) and isrepresented as Φpm = A cos ψi(t) = A cos[ωct + Kpf(t)]Modulation Index – In AM the degree of modulation is measured as a percentage from 0% to100% or as a modulation factor from 0 to 1. In angle modulation the degree of modulation ismeasured by the modulation index. The equation for modulation index is m=Where,Analog Communications, EC-405 Page 21
  22. 22. MANDSAUR INSTITUTE OF TECHNOLOGY fd = The frequency deviation fm = The modulating frequency Fig.(8.1) PM DemodulationAnalog Communications, EC-405 Page 22
  23. 23. MANDSAUR INSTITUTE OF TECHNOLOGYProcedure: – 1. Connect the circuit as shown in fig. 2. Connect the oscilloscope channel „A‟ across AF signal output and set the frequency between 1 KHz to 3 KHz. Now connect channel „B‟ across Demodulated output. Observe the wave shape. If modulated output is not proper, then adjust potentiometer given on the band and get proper Demodulator output.Result: – We have successfully recovered the original baseband signal from phase modulation.Analog Communications, EC-405 Page 23
  24. 24. MANDSAUR INSTITUTE OF TECHNOLOGYAnalog Communications, EC-405 Page 24

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