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AC Lab Manual

Department of ECE

ANALOG COMMUNICATIONS

LAB MANUAL FOR B.TECH-ECE

III-I SEMESTER

VAAGESWARI COLLEGE OF ENGINEERING

Beside L.M.D Police station, Ramakrishna colony

KARIMNAGAR - 505481.


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LIST OF EXPERIMENTS

1. Amplitude Modulation &Demodulation2. Frequency Modulation & Demodulation3. Double Side-Band System4. Pre-Emphasis & De-Emphasis5. Phase Locked Loop6. Single Side Band System7. Frequency Synthesizer8.

Pulse Amplitude Modulation & Demodulation

9. Pulse Position Modulator and Demodulator10.Pulse Width Modulator & Demodulator11.Sampling Theorem Verification12.Time Division Multiplexing & Demultiplexing13.Automatic Gain Control Characteristics

AMPLITUDE MODULATION &DEMODULATION

AIM:To study the function of Amplitude Modulation & Demodulation (under modulation,

perfect modulation & over modulation) and also to calculate the modulation index.


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APPARATUS:

1. Amplitude Modulation & Demodulation trainer kit.2. C.R.O (20MHz)3. Function generator (1MHz).4. Connecting cords & probes.

CIRCUIT DIAGRAM:

Modulator:

Demodulator:

EXPERIMENTAL PROCEDURE:

1. As the circuit is already wired you just have to trace the circuit according to the circuitdiagram given above.


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2. Connect the trainer to the mains and switch on the power supply.3. Measure the output voltages of the regulated power supply circuit i.e. +12v and

_12v.

4. Observe outputs of RF and AF signal generator using CRO, note that RF voltage isapproximately 300mv pp of 1 MHz frequency and AF voltage is 10vpp of 2KHz

frequency.

Modulator:

5. Now connect RF and AF signals to the respective inputs of modulator.

6. Initially set both the signals at zero level.

7. Connect one of the input of oscilloscope to modulator output and other input to AF

signal.

8. Adjust RF signal amplitude with the help of potentiometer so the t output of the

modulator is 300mv pp by keeping AF signal at zero level.

9. Now vary the amplitude of af signal and observe the amplitude modulated wave at

output, note the percentage of modulation for different values of AF signal.

%modulation= ( BA / B + A ) X 100

10.Observe 100% amplitude modulation and over modulation by varyingamplitude of the AF signal.

Demodulator:

1. Now connect the modulator output to the demodulator input.2. Observe demodulated signal at output of demodulator at approximately

50% modulation using oscilloscope.3. Compare it with the original AF signal ( Note : only wave shape,

amplitude will be attenuated, phase may change. )

4. Find the detected signal is same as the AF signal applied. Thus noinformation is lost in the process of modulation.

5. If you want to observe AM wave at different frequencies then connectAF signal from external signal generator to the modulator and observe

amplitude modulated wave at different frequencies.


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EXPECTED WAVEFORMS:-


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RESULT:

FREQUENCY MODULATION & DEMODULATION

AIM: To study the functioning of frequency modulation & demodulation and to calculate

the modulation index.

APPARATUS:

1. Frequency modulation & demodulation trainer kit.

2. C.R.O (20MHz)


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3. Function generator (1MHz).

4. Connecting chords & probes.

CIRCUIT DIAGRAM:

Modulator:

Demodulator:


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1. Switch on the power supply of the kit (without making any connections).2. Measure the frequency of the carrier signal at the FM output terminal with input

terminals open and plot the same on graph.

3. Connect the circuit as per the given circuit diagram.4. Apply the modulating signal of 500HZ with 1Vp-p.5. Trace the modulated wave on the C.R.O & plot the same on graph.6. Find the modulation index by measuring minimum and maximum frequency

deviations from the carrier frequency using the CRO.

frequencysignalmodulating

deviationFrequencymaximum

f

SMr

7. Repeat the steps 5& 6 by changing the amplitude and /or frequency of themodulating Signal.


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8. For demodulation apply the modulated signal as an input to demodulator circuitand compare the demodulated signal with the input modulating signal & also draw

the same on the graph.

EXPECTED WAVEFORMS:


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NOTE: Note down all the input and output wave forms of the signals applied and obtained

respectively.

RESULT:

DOUBLE SIDE-BAND SYSTEM


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AIM:To study the following of the Balanced Modulator as a

1. Frequency Doublers2. DSB-SC Generator.

APPARATUS:

1. DSB modulator and demodulator trainer kit

2. C.R.O (20MHz)

3. Connecting cords and probes

4. Function generator (1MHz)

CIRCUIT DIAGRAM:

Modulator:

Demodulator:


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Frequency Doubler:

1. Connect the circuit as per the given circuit diagram.2. Switch on the power to the trainer kit.3. Apply a 5 KHz signal to both RF and AF inputs of 0.1VP-P.4. Measure the output signal frequency and amplitude by connecting the output to

CRO.

5. Repeat the steps 3 and 4 by changing the applied input signal frequency to100KHZ and 500 KHz. And note down the output signals

NOTE: Amplitude decreases with increase in the applied input frequency.

Generation of DSB-SC:

1. For the same circuit apply the modulating signal(AF) frequency in between1Khz to 5Khz having 0.4 VP-P and a carrier signal(RF) of 100KHz having 0.1

vpp

2. Adjust the RF carrier null potentiometer to observe a DSB-SCwaveform at the output terminal on CRO and plot the same.

3. Repeat the above process by varying the amplitude and frequencyof AF but RF maintained constant.

DSB-SC

SIGNAL PRODUCT

MODULATOR

OSCILLATOR

MESSAGE

SIGNAL

CARRIER

SIGNAL

LOW PASS

FILTER


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NOTE: Note down all the waveforms for the applied inputs and their respective

outputs.

EXPECTED WAVEFORMS:


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Note: In frequency doubling If the input time period is T after frequency doubling

the time period should be halfed.i.e,T/2.

RESULT:


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PRE-EMPHASIS & DE-EMPHASIS

AIM: To study the functioning of Pre-Emphasis and De-Emphasis circuits.

APPARATUS:

1. Pre-emphasis & De-emphasis trainer kits.

2. C.R.O (20MHz)

3. Function generator (1MHz).

4. Patch chords and Probes.

CIRCUIT DIAGRAM:


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Pre-Emphasis

1. Connect the circuit as per the circuit diagram2. Apply a sine wave to the input terminals of 2 VP-P (Vi)3. By varying the input frequency with fixed amplitude, note down the output

amplitude (VO) with respect to the input frequency.

4. Calculate the gain using the formulaGain = 20 log (VO/ VI) db

Where, VO = output voltage in volts.

VI = Input voltage in volts.

And plot the frequency response.

De-Emphasis

1. Connect the circuit as per circuit diagram.2. Repeat steps 2, 3 & 4 of Pre-Emphasis to de-emphasis also.

EXPECTED WAVEFORMS:

Gain Gain

(-db) (-db)

Frequency (Hz-kHz) Frequency (Hz-kHz)

Fig: Pre-emphasis Fig: De-emphasis


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TABULAR COLUMN:

VI=2v

S.No Input Frequency

(50Hz to 20KHz)

Output voltage (Vo)

(volts)

GAIN

20 log (VO/ VI) db

RESULT:


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PHASE LOCKED LOOP

AIM: To study the characteristics of PLL.

APPARATUS:

1. PLL Trainer Kit2. C R O (20MHz)3. Digital Multimeter

BLOCK DIAGRAM:


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Rtvalue (pot resistance in

Ohms)

Theoretical value

(frequency in KHz)

Practical value

(frequency KHz)

Lock range:

5. Calculate the lock range of the circuit for a 5KHz free running frequency andrecord in table 1.2.

6. Connect pins 4,5 with the help of springs and adjust potentiometer to get a freerunning frequency of 5KHz . Connect square wave generator output to the input

of PLL circuit. Provide a 5KHz square signal of 1 Vppapproximately (make this

input frequency as close to the Vccfrequency as possible).

7. Observe the input & Output of the PLL.8. Observe the input and output frequencies while slowly increasing the frequency of

the square wave at the input. For some range output and input are equal (This is

known as lock Range and PLL is said to be in lock with the input signal). Record

the frequency at which the PLL breaks lock. (Output frequency of the PLL will

be around VCO frequency and in oscilloscope you will see a jittery waveform

when it breaks lock instead of clean square wave). This frequency is called as

upper end of the lock range and records this as F2.

9. Beginning at 5KHz, slowly decrease the frequency of the input and determine thefrequency at which the PLL breaks lock on the low end record it as F1

10.Find the lock range from F2 F1 and compare with the theoretical values fromstep5.


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Theoretical Value

(frequency in KHz)

Practical Value

(frequency in KHz)

Capture range:

11.Calculate the capture range of the circuit for a 5KHz free-running frequency(consider filter capacitor (CC) is 0.1F).

12.With the oscilloscope and counter still on pin 4, slowly increase the inputfrequency from minimum (say 1KHz), Record frequency (as F3) at which the

input and output frequencies of the PLL are equal, this is known as lower end of

the capture range.

13.Now keep input frequency at maximum possible (say 10KHz) and slowly reduceand record the frequency (as F4) at which the input and output frequencies of PLL

are equal. This is known as upper end of the capture range.

14.Find capture range from F4F3and compare it with the theoretical value (fromstep 11)

15.Repeat the steps from 11 to 14 with CCvalue 0.2F

Filter Capacitor (CC) Theoretical value Practical value

0.1F

0.2F


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SINGLE SIDE BAND SYSTEM

AIM: To generate SSB using phase method and demodulation of SSB signal using

Synchronous detector.

APPARATUS:

1. SSB modulator and demodulator trainer kit

2. C.R.O (20MHz)

3. Function Generator (1MHz).

CIRCUIT DIAGRAM:

Modulator:

Demodulator:

SSB-SCSIGNAL PRODUCT

MODULATOR

OSCILLATOR

MESSAGESIGNAL

CARRIER

SIGNAL

LOW PASS

FILTER


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SSB Modulation:

1. Connect the circuit as per the given circuit diagram.

2. Switch on the kit and measure the output of regulated power supplies positive and

Negative voltages.

3. Observe the outputs of RF generators using CRO .Where one output is 00phase the

Another is 900phase shifted(or) is a sine wave and shifted w.r.t other (or) is a cosine

wave.

4. Adjust the RF output frequency as 100KHz and amplitude as 0.2 Vp-p

(Potentiometers are provided to vary the output amplitude & frequency).5. Observe the two outputs of AF generator using CRO.

6. Select the required frequency (2kHz, 4kHz, 6kHz) form the switch positions for

A.F.

7. Adjust the gain of the oscillator by varying the AGC potentiometer and keep the

Amplitude of 10Vp-p.

8. Measure and record the above seen signals & their frequencies on CRO.

9. Set the amplitude of R.F signal to 0. 2Vp-p and A.F signal amplitude to

8Vp-p and connect AF-00and RF-900to inputs of balanced modulator A and

Observe DSB-SC(A) output on CRO. Connect AF-900and RF-00to inputs of

Balanced modulator B and observe the DSB-SC (B)out put on CRO and plot the

same on graph.

10. To get SSB lower side band signal connect balanced modulator outputs (DSB-SC)

to subtract or and observe the output wave form on CRO and plot the same on

graph.

11. To get SSB upper side band signal, connect the output of balanced modulator

Outputs to summer circuit and observe the output waveform on CRO and plot the

same on graph.

12. Calculate theoretical frequency of SSB (LSB & USB) and compare it with

Practical value.

USB = RF frequency + AF frequency.

LSB = RF frequencyAF frequency.


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EXPECTED WAVE FORMS:


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Demodulated Signal:

RESULT:


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FREQUENCY SYNTHESIZER

AIM: To study the operation of frequency synthesizer using PLL

APPARATUS:

1. Frequency synthesizer trainer AET -26A

2. Dual trace C.R.O (20MHz)

3. Digital frequency counter or multimeter

4. Patch chords

CIRCUIT DIAGRAM:

Phase

Comparator

Amplifier Low pass

filterV C O

Div. N Network

frequency divider

Fin= fout

finFout=N.f in


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1. Switch on the trainer ad verify the output of the regulated power supply i.e. 5V.These supplies are internally connected to the circuit so no extra connections are

required.

2. Observe output of the square wave generator using oscilloscope and measure therange with the help of frequency counter, frequency range should be around 1 KHz

to 10 KHz.

3. Calculate the free running frequency range of the circuit (VCO output between 4 thpinand ground). For different values of timing resistor R1 ( to measure Rtswitch off the

trainer and measure Rtvalue using digital multimeter between given test points). And

record the frequency values in tabular 1. Fout = 0.3 / (RtCt) where Rt is the timing

resistor and Ct is the timing capacitor = 0.01 f.

4. Connect 4thpin of LM 565 (Fout) to the driver stage and 5thpin (Phase comparator)connected to 11th pin of 7490. Output can be taken at the 11th pin of the 7490. It

should be divided by the 10, 2 times of the fout.


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EXPECTED WAVEFORMS:

TABULAR COLUMN:

Fin KHz Fout = N fin KHz Divided by 10, 2

RESULT:


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PULSE AMPLITUDE MODULATION & DEMODULATION

AIM: To study the characteristics of Pulse amplitude modulation & Demodulation

APPARATUS:

1. Pulse amplitude modulation & Demodulation trainer kit.

2. C.R.O (20MHz)


CIRCUIT DIAGRAM:

PAM Modulator Circuit Diagram:

PAM Demodulator Circuit Diagram:


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1. As the circuitry is already wired you just have to trace the circuit according to thecircuit diagram given above.

2. Connect trainer to mains and switch on the power.3. Measure the output voltages of regulated power supply circuit i.e. +12V.4. Observe the output of AF generator and pulse generator using CRO and note that AF

signal is approximately 3V P-P of 400Hz frequency and pulse generator output is

pulse train of 10V P-P with frequency between 1 KHz and 6 KHz.

Modulator:

5. Connect pulse output and AF output to the respective inputs of modulator circuit.6. Connect one of the input of oscilloscope to the modulator output and another to AF

signal.

7. Initially set the amplitude of the AF generator to minimum level and samplingfrequency to 1 KHz (by adjusting the preset provided in pulse generator block). Note

down the output of modulator, by varying amplitude of modulating signal observe the

modulator output so that you can notice the amplitude of the sampling pulses is

varying in accordance with the modulating signal.

Demodulator:

8. Connect PAM wave input to demodulator input and set sampling pulse frequency tomaximum (6 KHz).

9. Observe demodulated signals at output of demodulator, compare it with the originalAF signal.

i. (Note: Only shape, amplitude will be attenuated ).10.You can observe the amplified signal by applying demodulated signal to amp.11.Find the detected signal is same as the AF signal applied. Thus no information

lost in the process of modulation.


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INPUT & OUTPUT WAVE FORMS

AF Signal

Clock

PAM Output

Demodulated output

RESULT:


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PULSE WIDTH MODULATION & DEMODULATION

AIM: To study the characteristics of Pulse width modulation & Demodulation

APPARATUS:

1. Pulse width modulation & Demodulation trainer kit.

2. C.R.O (20MHz)


CIRCUIT DIAGRAM:

PWM Modulator:

PWM Demodulator:


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Observation of PWM with DC input voltage:

1. Study circuit operation thoroughly.2. Switch on the trainer and measure the output voltages of the regulated power supply

i.e. +5V and5V.

3. Observe the output of the AF generator using CRO, note that the output is 5V pp @400 Hz frequency.

4. Observe the output of the control signal generator i.e. ramp and reference pulse usingCRO.

5. Connect ramp signal to the ramp input of the PWM modulator and dc source output toAF input.

6. Connect one DMM to the dc source output and CH 1 input of the scope to the PWMmodulator output.

7. Measure the output pulse width at different input voltages starting from zero and notedown the readings. (By this we can observe the output pulse width is varying in

accordance with the input voltage as per theory of PWM, the amplitude and position

are fixed only width is varying)

Observation of PWM with AC input signal:

8. Now connect AF signal instead of dc voltage to the modulator and observe outputwaveform (condition: scope is in dual mode, CH 1 is connected to AF signal and CH

2 is connected to PWM output, trigger source in CH 1,if you are using storage

oscilloscope after setting AF input voltage observe output in stop mode). .

PWM Demodulation:

9. Remove connection from monostable input and connect it to PWM demodulatorinput.

10.Connect CH 1 to input AF signal and CH 2 to demodulator output and observe theoutput, compare it with original AF signal


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INPUT & OUTPUT WAVE FORMS:

AF Signal

PWM Output

RESULT:


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PULSE POSITION MODULATION & DE MODULATION

AIM: To study the characteristics of Pulse Position modulation & Demodulation

APPARATUS:

1. Pulse Position modulation & Demodulation trainer kit.

2. C.R.O (20MHz)


CIRCUIT DIAGRAM:

PWM Modulator Circuit Diagram:

PWMPPM (Monostable)


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PPMPWM (JK Flip-Flop)

PWM Demodulator


Observation of PWM and PPM with DC input voltage:

1. Study circuit operation thoroughly.2. Switch on the trainer and measure the output voltages of the regulated power

supply i.e. +5V and5V.

3. Observe the output of the AF generator using CRO, note that the output is 5V pp@ 400 Hz frequency.

4. Observe the output of the control signal generator i.e. ramp and reference pulseusing CRO.

5. Connect ramp signal to the ramp input of the PWM modulator and dc sourceoutput to AF input.

6. Connect one DMM to the dc source output and CH 1 input of the scope to thePWM modulator output.

7. Measure the output pulse width at different input voltages starting from zero andnote down the readings. (By this we can observe the output pulse width is varying


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in accordance with the input voltage as per theory of PWM, the amplitude and

position are fixed only width is varying)

8. Now connect output of the PWM modulator to monostable multivibrator input andCH 2 input of the oscilloscope to the monostable output i.e. PPM output (Set

scope in dual mode and trigger source in CH 1).

9. Observe PWM and PPM waveforms for different values of the input voltagestarting from zero (By this we can notice the output of monostable is PPM i.e. the

pulse width is fixed and amplitude is constant only position is varying).

Observation of PWM and PPM with AC input signal:

10.Now connect AF signal instead of dc voltage to the modulator and observe outputwaveform (condition: scope is in dual mode, CH 1 is connected to AF signal and

CH 2 is connected to PWM output, trigger source in CH 1,if you are using storage

oscilloscope after setting AF input voltage observe output in stop mode). Similarly

PPM waveform.

PWM Demodulation:

11.Remove connection from monostable input and connect it to PWM demodulatorinput.

12.Connect CH 1 to input AF signal and CH 2 to demodulator output and observe theoutput, compare it with original AF signal

PPM demodulation:

13.Connect PPM and reference pulse signals to respective inputs of PPM PWMconverter circuit and output of the same circuit to PWM demodulator. (Scope

should be set in dual mode, CH 1 is connected to input AF Signal, CH 2 to

demodulator output and trigger source to CH 1). Observe the output signal and

compare it with input AF signal.


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Note:The main problem in this experiment will be in triggering the oscilloscope to

observe the waveforms, especially PPM

INPUT & OUTPUT WAVE FORMS:

AF Signal

PWM Output

PPM Output

RESULT:


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SAMPLING THEOREM VERIFICATION

AIM: To verify sampling theorem.

APPARATUS:

1. Sampling theorem verification trainer2. C.R.O (30MHz)3. Patch cords.

CRICUIT DIAGRAM:


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1.

Connect trainer to mains and switch on the power.2. Observe the output of AF generator and pulse generator using CRO and note that3. AF signal is approximately 3Vp-pof 100HZ frequency and pulse generator4. output is pulse train of 10VP-Pwith frequency between 200 HZ and 4KHz.5. Connect pulse output and AF output to the respective inputs of sampling circuit.6. Connect one of the input of oscilloscope to the sampling circuit output and another to

AF signal.

7. Initially set the amplitude of the AF generator to minimum level and samplingfrequency to 200Hz(by adjusting the potentiometer). Observe the output of sampling

circuit by varying the amplitude of modulating signal. You can notice the amplitude

of sampling pulse is varying in accordance with the amplitude of the modulating

signal.

8. Connect sampling circuit output to reconstructing circuit.9. Observe the output of reconstructing circuit (AF signal) at 200Hz sampling frequency

until you get the original signal.

10.Statement: The Nyquist Theorem states that in order to recover the original signalfrom the samples of the signal, the signal must be sampled at a minimum of twice the

maximum frequency of the signal.

11.Note:The sampling output is uni-polar sampled i.e, only positive side


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EXPECTED WAVEFORMS:

Waveforms for fs > 2fm


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Demodulated Output:

RESULT:


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TIME DIVISION MULTIPLEXING & DEMULTIPLEXING

AIM:To study the characteristics of multiplexing & Demultiplexing for the given signal

EQUIPMENT REQUIRED:

1. TDM Multiplexer trainer2. TDM De-Multiplexer trainer3. Storage Oscilloscope

(Note: Storage oscilloscope is desired for satisfactory observation of TDM wave forms)

4. Digital Multimeter.5. 2 Nos co-axial cables (standard accessories with trainer)

BLOCK DIAGRAM:


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Multiplexer:

1. Study the theory of operation.2. Connect the trainer TDM multiplexer to the mains and switch on the power supply.3. Measure the output of the regulated power supply i.e. +5v and -5v with the help of

digital multimeter.

4. Observe the output of the AF generator-1 using CRO; it should be a sine wave of400 Hz frequency with 3vpp amplitude.

5. Observe the output of the AF generator-2 using CRO; it should be a sine wave of200 Hz frequency with 3vpp amplitude.

6. Verify the operation of logic source with multimeter/scope, output should be +5v inlogic1 position and 0v in logic0 position.

7. Observe the output of clock generator using CRO; it should be a square wave of 500Hz to 15 KHz frequency with 5Vpp amplitude.

8. Now connect the CH1 and CH2 inputs of the TDM multiplexer to the outputs of theAF generator1 and 2 respectively.

9. Connect control input of the TDM multiplexer to the output of the logic source.10.Put control signal (logic source) at logic 1 condition and observe the output of the

TDM multiplexer with the help of oscilloscope, by this we can notice that the output

of the TDM multiplexer is a signal which has been connected to CH1 input. In this

condition the signal at CH2 input has no effect on multiplexer output.

11.Similarly Put control signal (logic source) at logic 0 condition and observe the outputof the TDM multiplexer, now notice that the output of the TDM multiplexer is a

signal which has been connected to CH2 input and the signal at CH1 input has no

effect on multiplexer output.

12.Now disconnect logic source and connect clock output to the control input.13.Observe TDM wave form using CRO at different values of clock frequency, input

signal voltage levels and sketch them.

NOTE1: After setting the clock frequency and input signals to desire values put storage

scope in STOP mode so that you can view stable display of waveforms.

NOTE2: sample waveforms given in figure 1:3, 1:4 are drawn at 1 KHz sampling clock,

you can take at any clock frequency.


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14.Similarly you can observe and plot the TDM waveforms for different inputs i.e. DCsignals alone, AC&DC instead of AC signals.

NOTE1: DC signals (voltages) can be connected from an external sources and care

should be taken in case voltage levels i.e. maximum voltage input voltage must be in

range of 4.8v.

NOTE2: you can use even normal scope, when you observe the TDM waveform for DC

inputs.

De- Multiplexer:

1. Study the theory of operation.2. Connect the trainer TDM de-multiplexer to the mains and switch on the power supply.3. Measure the output of the regulated power supply i.e. +5v and -5v with the help of

digital multimeter.

4. Verify the operation of logic source with multimeter/scope, output should be +5v inlogic1 position and 0V in logic 0 position.

5. Observe the output of clock generator using CRO; it should be a square wave of 500Hz to 15 KHz frequency with 5Vpp amplitude.

6. Connect TDM-PAM signal to input of the TDM de-multiplexer from TDMmultiplexer with the help of co-axial cable (supplied with trainer)

7. Connect control input to the output of the logic source.8. Keep CRO in dual mode; connect one input to CH1 output and another input to CH2

output.

9. Put control signal (logic source) at logic 1 condition and observe CH1 and CH2outputs. You can notice that the entire TDM signal is transferred to CH1 output and

no signal at CH2 output.

10.Similarly Put control signal (logic source) at logic 0 condition and observe CH1 andCH2 outputs. The entire TDM signal is transferred to CH2 output and no signal at

CH1 output. By the above two steps you can notice that the entire TDM signal is

transferred to CH1 output when control input is 1 and to CH2 when control input is 0.

11.Now disconnect logic source and connect clock from transmitter through a co-axialcable.

12.Observe CH1 and CH2 outputs, you will notice that the outputs are natural topsampled PAM signals.


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13.Connect CH1, CH2 outputs to low pass filters and observe the output of the filters andcompare them with the original AF signals (at multiplexer inputs) using CRO. You

will notice that both the signals are same in frequency and shape. Signal amplitude

may be attenuated during smoothening process and this can be achieved by taking

amplifier output. Select AC/DC coupling depending on the input signal.

EXPECTED WAVEFORMS:

RESULT:


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AGC CHARACTERISTICS

AIM: To study the AGC Characteristics

APPARATUS REQUIRED:

1. AGC Characteristics circuit kit consists of wired circuitry:2. RF Generator3. AF Generator4. Regulated power supply5. AM Modulator6. Demodulator (simple diode detector)7. AGC circuit8. Dual trace C.R.O9. Connecting wires

CIRCUIT DIAGRAM:


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EXPERIMENTRAL PROCEDURE:

1. As the circuit is already wired you just have to trace the circuit according to thecircuit diagram

2. Connect the trainer to the mains and switch on the power supply.3. Measures the output voltages of the regulated power supply circuit i.e. +12v and -12v,4. Observe outputs of RF and AF signal generator using CRO, note that RF voltage is

approximately 50mVpp of 455 KHz frequency and AF voltage is 5Vpp of1 KHz

frequency.

5. Now vary the amplitude of AF signal and observe the AM wave at output, note thepercentage of modulation for different values of AF signal.

i. % Modulation= (Emax -Emin) /(Emax+Emin) 1006. Now adjust the modulation index to 30% by varying the amplitudes of RF & AF

signals simultaneously.

7. Connect AM output to the input of AGC and also to the CRO channel -18. Connect AGC link to the feedback network through OA79 diode9. Now connect CRO channel - 2 at output. The detected audio signal of 1 KHz will be

observed.

10.Calculate the voltage gain by measuring the amplitude of output signal (Vo)waveform, using Formula A = Vo/V i

11.Now vary input level of 455 KHz IF signal and observe detected 1 KHz audio signalwith and Without AGC link. The output will be distorted when AGC link removed i.e.

there is no AGC action.

12.This explains AGC effect in Radio circuit.


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EXPECTED WAVEFORMS:

AM OUTPUT

AGC OUTPUT

RESULT:


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