Ac Lab Manual

AMPLITUDE MODULATION &DEMODULATION

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

modulation & over modulation) and also to calculated the modulation index.

APPARATUS:

THEORY:

Amplitude modulation can be produced by a circuit where the output is product oftwo input signals. Multiplication produces sum and difference frequencies and thus the sidefrequencies of the AM wave. Two general methods exist for achieving this multiplication,one involves a linear relation between voltage and current in a device and the second uses alinear device.

A linear form of modulation of modulation causes a current I, of one frequency topass through an impedance Z, whose magnitude varies at a second frequency. The voltageacross this varying impedance is then given by E = I sinù1t * z sinù2t

The above equation is the output is a result of multiplication of two frequencies. If One of them is carrier frequency and the other is the modulating frequency the result is an AM waveform. The circuit diagram of the amplitude modulation, the carrier is fed to the transistor Q1 of its base. This produces a collector current in Q1 of carrier frequency. The impedance in the collector circuit of Q1 is decided in the transistor Q2. Modulating signal is fed to the base Q2 which changes the impedance offered by Q2 at the modulation frequency output is taken through a transformer coupling. When the modulating is zero Q2 offers a fixed impedance in the collector circuit of Q2, so that the output is a constant unmodulated carrier. As the modulating signal is applied to Q2 the impedance changes and the amplitude of carrier at output also changes). Thus an amplitude modulated carrier is obtained at the output.

In a communication system a high frequency carrier is modulated by the low frequency signal. The modulated carrier is transmitted by the transmitter antenna. At the receiver we have to recover the information back from the modulated carrier. The process of separation of signal from the carrier is called demodulation or detection. The demodulation circuit diagram is a linear diode detector. In this circuit the linear portion of dynamic characteristics of diode is used and hence the circuit is a linear detector. It consists of a half wave rectifier followed by a capacitor input filter. Input to the circuit is an AM wave with a high frequency carrier and a low frequency envelope corresponding to the signal. The diode cuts-off the negative going portion of the AM wave. Capacitor ‘C’ charges up to the peak of the carrier cycle through the low resistance rd and then during negative half cycle tries to discharge through relatively high resistance RL. Capacitor value is so chosen that this discharge is very small in time between carrier half cycles. Hence the capacitor voltage tends to follow the envelope of the carrier and the voltage available across RL is simply the modulation envelope superimposed on a constant level. A dc level in the output comes because the current through diode flows in the form of pulses occurring at the peak of each carrier cycle.

S.No Name of the Equipment Qty.

1. Amplitude Modulation & Demodulation trainer kit.

1

2. C.R.O (20MHz) 1

3. Function generator (1MHz). 1

4. Connecting cords & probes.

When the input to detector circuit is a AM waveform then the one of the component in VR cannot be assumed to be constant all the time. Actually it is constant over a few cycles of carrier in which time it is assumed that modulating signal being low frequency would not have changed appreciably. Due to this reason the measurement of detection efficiency can be done on an un modulated carrier because VR would be expected to be constant.

PROCEDURE:1. Refer to block diagram and Carry out the following connections.2. Keep all the switch faults in OFF position.3. Connect SINE OUT post of FUNCTION GENERATOR SECTION (ACL-01) to The I/p of Balance

Modulator1 (ACL-01) SIGNAL IN Post.4. To connect o/p of VCO (ACL-01) RF OUT post to the input of Balance Modulator 1 CARRIER

IN post (ACL-01).5. Connect the power supply with proper polarity to the kit ACL-01 & ACL-02, while connecting

this; ensure that the power supply is OFF.6. Keep switch SW1 towards 1-10 KHz position.7. Keep Out post LEVEL about 0.5Vpp; FREQ. About 1 KHz.8. Keep switch SW2 towards 500 KHz position.9. Keep RF out LEVEL about 1 Vpp; FREQ. about 450 KHz, Switch on 500 KHz.10. BALANCED MODULATOR1: CARRIER NULL completely rotated clockwise or counter

clockwise, so as “unbalance” the modulator and to obtain an AM signal with not suppressed carrier across the output; OUT LEVEL in fully Clockwise.

11. Observe the AM Modulator wave.12.Move the probe from post SIG to post OUT (output of the modulator), where

Signal modulated in amplitude is detected. Note that the modulated signal Envelope corresponds to the wave form of the DSB AM modulating signal.

13.Vary the amplitude of the modulating signal and check the 3 following conditions: Modulation percentage lower than the 100%, equal to the 100% ,Superior to 100% (over modulation).

14.Vary the frequency and amplitude of the modulating signal, and check the Corresponding variations of the modulated signal.

15.Vary the amplitude of the modulating signal and note that the modulated Signal can result saturation or over modulation.

FREQUENCY MODULATION & DEMODULATION

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

APPARATUS:

THEROY:

PROCEDURE:• Refer to the FIG and Carry out the following connections.• Keep all the switch faults in OFF position.• Connect SINE OUT post of FUNCTION GENERATOR SECTION (ACL-01) to the I/p of Balance

Modulator1 (ACL-01) SIGNAL IN Post.• Connect o/p of VCO (ACL-01) OUT post to the input of Balance modulator (ACL-01) CARRIER

IN post.• Connect power supply with proper polarity to the kit ACL-01 & ACL -02. While connecting

this, ensure that the power supply is OFF.• Switch on the power supply.• Keep switches SW1 towards 1-10KHZ position.• Keep Sine out LEVEL about 0.5 Vpp; FREQ. About 1 KHZ.• Keep switch SW2 towards 1500 KHz position• Keep VCO Level about 0.5Vpp; FREQ. About 550 KHz.• BALANCED MODULATOR 1: CARRIER NULL completely rotates clockwise or counter

clockwise, so that the modulator is “unbalanced” and an AM signal with not suppressed carrier is obtained across the output: adjust OUTLEVEL to obtain an AM signal across the output whose amplitude is about 500mVpp.

• Keep Local Oscillator (ACL-02) 1000 KHz, 1V.• Connect local oscillator OUT post to LO IN of the mixer section.• Connect balance modulator1 out to RF IN of mixer section in ACL-02.• Connect mixer OUT to IF IN of 1st IF Amplifier in ACL-02.• Connect IF OUT1 of 1st IF to IF IN 1 and IF OUT2 of 1st IF to IFIN 2 of 2ND IF Amplifier.• Connect post AGC1 to post AGC2 and jumper position as per diagram.

• Observe the modulated signal envelope, which corresponds to the waveform of the modulating signal at OUT post of the balanced modulator1 of ACL-01. Connect the oscilloscope to the IN and OUT post of envelope detector and detect the AM signal and the detected one. If the central frequency of the amplifier and the carrier frequency of the AM signal and local oscillator frequency coincides, you obtain two signals similar to the ones of diagram.


1. Frequency Modulation & Demodulation trainer kit.

1

2. C.R.O (20MHz) 1



BALANCED MODULATOR

AIM: To study the following of the Balanced Modulator as a1. Frequency Doubler2. DSB-SC Generator.

APPARATUS:

PROCEDURE:-I-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 to 100KHZ and500KHz. And note down the output signals.NOTE:- Amplitude decreases with increase in the applied input


1. Amplitude Modulation & Demodulation trainer kit.

1

2. C.R.O (20MHz) 1



frequency.

II-Generation of DSB-SC1. For the same circuit apply the modulating signal(AF) frequency in between 1Khz to 5Khzhaving 0.4 VP-P and a carrier signal(RF) of 100KHz havinga 0.1 VP-P .2. Adjust the RF carrier null potentiometer to observe a DSB-SC waveform atthe output terminal on CRO and plot the same.3. Repeat the above process by varying the amplitude and frequency of AF but RF maintainedconstant.NOTE :- Note down all the waveforms for the applied inputs and their respective outputs.

CIRCUIT DIAGRAM:

Note: In frequency doubling If the input time period is “T” after frequency doubling the time periodshould be halfed.i.e,”T/2”.

RESULT:

PRE-EMPHASIS & DE-EMPHASIS

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

APPARATUS:

CIRCUIT DIAGRAM:

PRE-EMPHASIS CIRCUIT:

DE-EMPHASIS CIRCUIT:

S.No Name of the equipment Specifications/Range Qty.

1. Capacitor 0.047µF 1

2. Resistor 1KΩ 1

3. Function Generator 1

4. C.R.O 1

5. Bread board 1

6. Connecting wires 1

PROCEDURE:I-PRE-EMPHASIS:

1. Connect the circuit as per the circuit diagram2. Apply a sine wave to the input terminals of 2 VP-P3. 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 formula

Gain = 20 log (VO/ VI) dbWhere VO = output voltage in volts.VI = Input voltage in volts.

5. And plot the frequency response.II-DE-EMPHASIS1. Connect the circuit as per circuit diagram.2. Repeat steps 2, 3 & 4 of Pre-Emphasis to de-emphasis also.

MODEL GRAPH:

TABULAR COLUMN:-

VI =2v

Pre-emphasis De-emphasis

S.No. Input Frequency (50Hz to 20KHz)

Output voltage (volts)

Gain = 20log vo/vin

Gain (-db) Gain

(-db)

Frequency (Hz)

Frequency (Hz)

RESULT:

CHARACTERISTICS OF MIXER

AIM: To study the functioning of a frequency mixer.

APPARATUS:

CIRCUIT DIAGRAM:

PROCEDURE:1. Connect the circuit as per the given circuit diagram.2. Switch on the power supply of trainer kit.


1. Frequency Modulation & Demodulation trainer kit.

1

2. C.R.O (20MHz) 1



3. Apply a sine wave at input RF IN of 2 VP-P amplitude and 100 KHz frequency.4. Apply a sine wave at input LO IN of 2 VP-P amplitude and 100 KHz frequency.5. Observe the output waveform on the CRO.6. Repeat the steps 3, 4 and 5 by changing the values of RF IN once greater than and

less than LO IN in steps of 5 KHz (in the range 80 KHz to 120 KHz)7. Verify the output signal obtained with the theoretical value.8. Plot the graph for RF IN versus (RF IN – LO IN).NOTE: - Note down the waveform of inputs as well as the respective outputs.

TABULAR COLOUMN

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Ac Lab Manual