DIGITAL COMMUNICATION LAB

STATE INSTITUTE OF TECHNICAL TEACHERS TRAINING AND RESEARCH

 GENERAL INSTRUCTIONS

Rough record and Fair record are needed to record the experiments conducted in the laboratory.
Rough records are needed to be certified immediately on completion of the experiment. Fair records are
due at the beginning of the next lab period. Fair records must be submitted as neat, legible, and complete.

INSTRUCTIONS TO STUDENTS FOR WRITING THE FAIR RECORD

In the fair record, the index page should be filled properly by writing the corresponding experiment
number, experiment name , date on which it was done and the page number.

On the right side page of the record following has to be written:

1. Title: The title of the experiment should be written in the page in capital letters.
2. In the left top margin, experiment number and date should be written.
3. Aim: The purpose of the experiment should be written clearly.
4. Apparatus/Tools/Equipments/Components used: A list of the Apparatus/Tools/ Equipments
/Components used for doing the experiment should be entered.
5. Principle: Simple working of the circuit/experimental set up/algorithm should be written.
6. Procedure: steps for doing the experiment and recording the readings should be briefly
described(flow chart/programs in the case of computer/processor related experiments)
7. Results: The results of the experiment must be summarized in writing and should be fulfilling
the aim.
8. Inference: Inference from the results is to be mentioned.
On the Left side page of the record following has to be recorded:

1. Circuit/Program: Neatly drawn circuit diagrams/experimental set up.

2. Design: The design of the circuit/experimental set up for selecting the components

should be clearly shown if necessary.

3. Observations:

i) Data should be clearly recorded using Tabular Columns.

ii) Unit of the observed data should be clearly mentioned

iii) Relevant calculations should be shown. If repetitive calculations are needed, only show a sample
calculation and summarize the others in a table.

4. Graphs: Graphs can used to present data in a form that show the results obtained, as one or more of
the parameters are varied. A graph has the advantage of presenting large amounts of data in a
concise visual form. Graph should be in a square format.

GENERAL RULES FOR PERSONAL SAFETY

1. Always wear tight shirt/lab coat, pants and shoes inside workshops.
2. REMOVE ALL METAL JEWELLERY since rings, wrist watches or bands, necklaces, etc. make excellent
electrodes in the event of accidental contact with electric power sources.
3. DO NOT MAKE CIRCUIT CHANGES without turning off the power.
4. Make sure that equipment working on electrical power are grounded properly.
5. Avoid standing on metal surfaces or wet concrete. Keep your shoes dry.
6. Never handle electrical equipment with wet skin.
7. Hot soldering irons should be rested in its holder. Never leave a hot iron unattended.
8. Avoid use of loose clothing and hair near machines and avoid running around inside lab.
TO PROTECT EQUIPMENT AND MINIMIZE MAINTENANCE:

DO: 1. SET MULTIRANGE METERS to highest range before connecting to an unknown source.

2. INFORM YOUR INSTRUCTOR about faulty equipment so that it can be sent for

repair.

DO NOT: 1. Do not MOVE EQUIPMENT around the room except under the supervision of

an instructor.
Experiment No.1

SAMPLING AND RECONSTRUCTION OF SINE WAVE

AIM:

To set up sampling and reconstruction circuits to study the sampling theorem and to plot
waveforms for different sampling rates.

OBJECTIVES:

After the completion of this experiment, students will have good knowledge about
sampling, effect of sampling rate, method of sampling and reconstruction of signals.

COMPONENTS AND EQUIPMENTS REQUIRED:

Sl. No. Item & Specification Quantity

1 FET BFW 10 1No.
2 Resistor- 1K, 56 1No.
3 Capacitor- 1F  1No.
4 Signal Generator 2Nos.
5 CRO 1No.
6 Bread Board 1No.
7 Wires and probes

THEORY:

As a first step to convert analog signals into digital form, the samples of the analog
signals are taken at regular intervals. The levels of these samples are then encoded and send to
the receiver. At the receiver these samples are recovered and from that the original signal is
reconstructed. Sampling theorem states that the original signal can be faithfully reconstructed
only if the sampling frequency is at least double that of the highest frequency component in the
sampled signal.

A sampling and reconstruction circuit is shown in figure. An FET is used as a switch to

take samples of the sine wave input. Sampling pulses are applied to the gate of the FET that
switches it ON and OFF. The input signal is sent to the output only when the transistor is ON.
Thus the output of the FET is a sampled form of the input signal.

The reconstruction circuit is a low pass filter having a cut off frequency equal to the
frequency of the analog input signal.
PROCEDURE:

1. Test all the components and probes.

2. Set up the circuit as shown in figure on the bread board.
3. Feed 5Vpp, 100Hz sine wave as input.
4. Apply 5 Vpp square wave signal as sampling pulse.
5. Observe sampled output and reconstructed signals for the following sampling
frequencies; a) 100Hz b) 200Hz c) 500Hz d) 1 KHz and e) 2KHz. Vary the amplitude of
the sampling pulses to obtain the optimum output.
6. Plot the above waveforms.

CIRCUIT DIAGRAM:

WAVEFORMS:
RESULT:

Analog Input is sampled at different sampling rates and then reconstructed. Observed the
waveforms and plotted.

INFERENCE:

1) To reconstruct the original signal, the sampling frequency should at least be double that
of the input frequency.
2) Reconstructed signal quality improves with increase in sampling frequency.

Experiment No.2
PULSE AMPLITUDE MODULATOR AND DEMODULATOR
AIM: To set up pulse amplitude modulator and demodulator circuits and to observe the
waveforms.
OBJECTIVES: After completing this experiment, students will be able to set up PAM
modulator and demodulator circuits and identify the waveforms.

COMPONENTS AND EQUIPMENTS REQUIRED:

Sl. No. Item & Specification Quantity

1 Analog Switch CD 4016 1No.
2 Resistor- 1.5K 1No.
3 Capacitor- 1F  1No.
4 Signal Generator 2Nos.
5 CRO 1No.
6 Bread Board 1No.
7 Power supply +/- 10V 1No.

THEORY:
Pulse Amplitude Modulation (PAM) is the simplest pulse modulation scheme. In pulse
amplitude modulation system the amplitude of a carrier pulse train is varied in accordance with
the instantaneous level of the modulating signal.
The simplest form of the PAM modulator is an analog switch that is turned on and off at
the RF carrier pulse rate. As the switch changes state, the modulating signal is connected and
disconnected from the output. Thus the output PAM signal is a sampled version of the
modulating signal. If the sampling frequency is sufficiently high (at least twice that of the highest
modulating frequency), the original signal can be recovered at the receiver by simply passing it
through a low pass filter having a cut-off frequency equal to the highest frequency in the
modulating signal.
CIRCUIT DIAGRAM:

Modulator Demodulator

CD 4016 Pinout

PROCEDURE:

1. Test all the components and probes.

2. Set up the modulator circuit using CD 4016 as shown in figure. Switch on the power
supplies.
3. Feed 5Vpp, 100Hz modulating signal at IN 1 input and 5V, 1KHz square wave signal at
Control 1 input of CD 4016.
4. Observe the PAM output signal at OUT 1 pin on one channel of the CRO.
5. Set up the demodulator circuit as shown in figure. Feed the PAM signal as the input and
observe the demodulated output on the other channel of the CRO. Plot the waveforms.

WAVEFORMS:

Modulating
Signal

Pulse
Input

PAM
Output
RESULT:

The PAM modulator and demodulator circuits were set up and the waveforms were plotted.

INFERENCE: Analog switch can be used as a PAM modulator and a simple RC low pass filter
can be used as a PAM demodulator.

Experiment No.3

PULSE WIDTH MODULATOR AND DEMODULATOR

AIM: To set up pulse width modulator and demodulator circuits and to observe and plot the
waveforms.

OBJECTIVES: After completing this experiment the students will be able to set up pulse width
modulator and demodulator circuits and to identify PWM waveform.

COMPONENTS AND EQUIPMENTS REQUIRED:

Sl. No. Item & Specification Quantity
1 IC 555, IC 741 1No. each
2 Resistor- 1.2K, 4.7K, 10K, 1.5K 1No. each
1K 2Nos.
3 Capacitor- 0.1F, 0.001F 1No. each
10F, 1F 2Nos.each
4 Diode 1N 4001 1No.
5 CRO 1No.
6 Signal Generator 2Nos.
7 Bread Board 1No.
8 Power Supply 10V 1No.
9 Wires and connectors

THEORY:

Pulse Width Modulation (PWM) is a form of pulse modulation where the width of the
pulses in a carrier pulse train is made proportional to the instantaneous amplitude of the
modulating signal.

A pulse width modulator circuit made up of 555 Timer is shown in figure. Here the 555
timer is working in monostable mode. A negative trigger pulse at pin 2 sets the output. The
modulating signal is applied to the control pin of the 555 which varies the threshold voltage. This
in turn varies the charging time of capacitor C2 and makes the trailing edge of the output pulse
proportional to the modulating signal. Thus the leading edge of the output pulse is decided by the
trigger pulse which occurs periodically and the trailing edge is proportional to the amplitude of
the modulating signal. The resulting output will be pulse width modulated.

The pulse width demodulator circuit consists of an integrator and a low pass filter with a
cut off frequency of 100Hz. The integrator reconstructs the modulating signal which is further
smoothened by the low pass filter. The series capacitor eliminates the dc component from the
demodulated signal.

PROCEDURE:

1. Test all the components and probes.

2. Set up the circuit as shown in the figure on the bread board. Switch on the power supply.
3. Feed 10Vpp, 1 KHz square wave signal at the trigger input and 5Vpp, 100Hz sine wave
at the control input.
4. Observe the input modulating signal, carrier pulse train and the PWM output signal on
CRO. Vary the modulating signal amplitude to get the optimal output. Plot the
waveforms.

CIRCUIT DIAGRAM:

Modulator
 Demodulator

WAVEFORM:

RESULT:

Pulse width modulator circuit was set up and the waveforms were observed and plotted.

INFERENCE: Learned the usage of 555 timer as a pulse width modulator.

Experiment No.4

PULSE POSITION MODULATOR AND DEMODULATOR

AIM: To set up pulse position (PPM) modulator and demodulator circuits and to observe and
plot the waveforms.

OBJECTIVES: After completing this experiment the students will be able to set up pulse
position modulator circuit using IC 555, demodulator using transistor and to identify PPM
waveform.
COMPONENTS AND EQUIPMENTS REQUIRED:
Modulator

Sl. No. Item & Specification Quantity

1 IC 555 2Nos.
2 Resistor- 4.7K,10K, 1.2K 2No. each
3 Capacitor- 10F, 0.01F 1No. each
0.1F, 0.001F 2Nos.
4 Diode 1N 4001 2Nos.
5 CRO 1No.
6 Signal Generator 1Nos.
7 Bread Board 1No.
8 Power Supply 10V 1No.
9 Wires and connectors

Demodulator

Sl. No. Item & Specification Quantity

1 IC 741, IC LM324 1No.each
2 Resistor-1.5K, 47K, 15K, 100K 1No. each
1K,10 2Nos.
3 Capacitor- 10F 1No.
1F 2Nos.
4 CRO 1No.
5 Bread Board 1No.
6 Power Supply 10V, +/- 15V 1No.each
7 Wires and connectors

THEORY:

Pulse Position Modulation (PPM) is one of the pulse modulation schemes where the
relative position of the pulses in a carrier pulse train is made proportional to the instantaneous
value of the modulating signal.

A pulse position modulator made up of IC 555 is shown in figure. Both the 555s are
working in monostable mode. The first monostabe generates a PWM signal and this PWM
output is used as the trigger input of the second monostable. Since the monastable triggers at the
trailing edge of the PWM signal, the position of the resulting pulses will have position shift
compared to the input pulse train.

The PPM demodulator is set up using an Op Amp SR flip flop, an integrator and a low
pass filter. The flip flop is set by the carrier pulses and reset by the PPM pulses. The resulting
output is a PWM signal. This PWM signal is then demodulated using the integrator-low pass
filter combination.

PROCEDURE:

1. Test all the components and probes.

2. Set up the circuit as shown in the figure on the bread board. Switch on the power
supplies.
3. Feed the 10Vpp, 1KHz, 0.2 duty cycle carrier pulse train and the 5Vpp, 100Hz
modulating signal (Sine wave) at the trigger and control inputs of the first 555
respectively.
4. Make sure that the PWM signal is available at pin 3 of the first 555. Vary the amplitude
of the modulating signal to get a proper PWM output if needed.
5. Observe the waveforms of the input pulse train, modulating input, PWM output and PPM
output on the CRO.
6. Observe the following waveforms in pairs on both the channels of the CRO; a)
Modulating input and PWM output b) PWM output and PPM output c) Modulating input
and PPM output
7. Plot the waveforms.
8. Set up the demodulator circuit as shown in figure. Switch on the power supply.
9. Feed the PPM signal input and the carrier pulse input as shown in figure. Observe the
waveforms at various points on CRO and plot.

CIRCUIT DIAGRAM:

Modulator
 Demodulator

WAVEFRM:

RESULT:

The PPM modulator and demodulator circuits were set up and the waveforms were observed and
plotted.

INFERENCE: Studied the usage of 555 timer as PPM modulator.

Experiment No.5

PULSE CODE MODULATION

AIM: To set up a PCM modulator and observe the waveforms.

OBJECTIVES: After completing this experiment, the students will be able to a) Set up a PCM
modulator and to generate a PCM encoded output for a given analog input.

COMPONENTS AND EQUIPMENTS REQUIRED:

Sl. No. Item & Specification Quantity
1 Resistor- 2.2K 7Nos.
1K 3Nos.
2 IC CD4016, LM324, 7493, 7400 1No.each
3 Signal Generator 3Nos.
4 CRO 1No.
5 Power Supply- +5V 1No.
6 Bread Board 1No.
7 Wires and probes

THEORY:
Pulse Code Modulation is a digital modulation technique by which an analog signal is
converted to an equivalent sequence of binary codes. The analog signal is first sampled at regular
intervals and these samples are then quantized to predefined levels. An analog to digital
convertor converts these quantized symbols to their corresponding binary codes.
In the circuit an analog switch is used to sample the input signal. These samples are
compared to the output of a DAC circuit which is initially zero. So the comparator output goes
‘high’ and strobes the clock input to the counter. This signal also disables the reset inputs of the
counter. The counter starts to count up. An R-2R ladder DAC simultaneously converts the
counter output to its equivalent analog value. When the DAC output goes above the input
sample, the comparator output switches to ‘low’ and cuts off the clock input from the counter.
The reset inputs are also enabled causing the counter output to reset. When the next sample
reaches the comparator input the whole process starts over again.

PROCEDURE:
1. Test all the components and probes.
2. Set up the circuit as shown in figure on a bread board.
3. Feed 2Vpp, 100Hz unipolar sine wave as the analog input (Set dc level at 2V to obtain a
signal that varies between +1V and +3V). Make sure that the input peak voltage never
exceeds the peak DAC output.
 4. Use the dc offset knob on the function generator to add dc offset to make unipolar sine
wave.
5. Use 4Vpp, 500Hz square wave with 20% duty cycle as sampling clock (clock 1) and 5V,
5KHz square wave as the clock input of the counter (clock 2).
6. Observe the input sine wave, sampled output and the PCM output (DAC output; staircase
waveform) on CRO. Vary the analog input and clock 1 input amplitudes to obtain the
optimum result, if needed.
7. Plot the waveforms.
8. The binary output can be checked by giving discrete dc input voltages (less than 5V).

CIRCUIT DIAGRAM:

LM 324 Pin Connection

PCM Modulator
WAVEFORM:

RESULT:
The PCM modulator circuit was set up and the waveforms were plotted.
INFERENCE:
PCM is the actual digital modulation scheme where the analog input is sampled and converted
into sequence of binary codes corresponding to the sample amplitudes.

Experiment No.6

ASK MODULATOR AND DEMODULATOR

AIM:

To set up ASK modulator and demodulator circuits and to observe the waveforms.

OBJECTIVES:

After completing this experiment the students will be able to a) Set up ASK modulator and
demodulator circuits and b) Identify ASK waveform.

COMPONENTS AND EQUIPMENTS REQUIRED:

Sl. No. Item & Specification Quantity

1 Transistor- BC 107 1No.
2 Resistor- 4.7K, 10K, 2.2K, 1K 1No. each
10K pot 1No.
 3 Capacitor- 0.01F  1No.
4 Diode- 1N 4001 1No.
5 Zener Diode- 5Z1 2Nos.
6 IC 741 1No.
7 Signal Generator 2Nos.
8 CRO 1No.
9 Power Supply- +/- 15V, 5V 1No. each
10 Bread Board 1No.
11 Wires and probes

THEORY:
Amplitude Shift Keying (ASK) is a digital modulation scheme where the binary data is
transmitted using a carrier signal with two different amplitude levels. For binary 0 and 1, the
carrier switches between these two levels. In its simplest form, a carrier is sent during one input
and no carrier is sent during the other. This kind of modulation scheme is called on-off keying.
A simple ASK modulator circuit is shown in figure. Here a sinusoidal high frequency
carrier signal is sent for logic ‘0’ (-5V) and no carrier is sent for logic ‘1’ (+5V). The transistor
works as a switch closes when the input (base) voltage is +5V (logic ‘1’) and shorts the output.
When the input voltage is -5V (logic ‘0’), the switch opens and the carrier signal is directly
connected to the output.
The demodulator circuit consists of an envelope detector and a comparator. The diode D
selects the positive half cycle of the ASK input. The envelop detector formed by 2.2K resistor
and 0.01uF capacitor detects the data out of the ASK input. The Op Amp comparator and the
zener diode amplitude limiter convert this detected signal to its original logic levels. The 10K
potentiometer may be varied to set suitable reference voltage for the comparator.

PROCEDURE:
1. Test all the components, bread board and probes.
2. Set up the circuits as shown in figure on the bread board.
3. Feed 10Vpp, 500Hz square wave as the message/data input and 2Vpp, 5KHz sine wave
as the carrier input.
4. Observe both the message input and ASK output simultaneously on CRO and plot.
5. Apply the ASK output of the modulator to the demodulator input.
6. Observe both the ASK input and the demodulated output simultaneously on CRO. Adjust
the reference voltage of the comparator if needed.
7. Plot the waveforms.
CIRCUIT DIAGRAM:

ASK
Modulator

ASK Demodulator
WAVEFORMS:
RESULT:
ASK modulator and demodulator circuits were set up and the waveforms were plotted.
INFERENCE: Studied the simplest digital modulation scheme.

Experiment No.7

FREQUENCY SHIFT KEYING

AIM:

To set up FSK modulator and demodulator circuits and to observe the waveforms.

OBJECTIVES:

After completing this experiment, the students will be able to a) Set up FSK modulator and
demodulator circuits and b) Identify FSK waveform.

COMPONENTS AND EQUIPMENTS REQUIRED:

Sl. No. Item & Specification Quantity

1 Transistor- BC 177 1No.
2 Resistor- 47K, 47K pot, 10K pot 1No. each
10K 7Nos.
3 Capacitor- 0.01F 3Nos.
0.02F 4Nos.
0.047F, 0.001F  1No.each
4 IC LM311 1No.
5 IC 555, IC 565 1No. each
6 Signal Generator 1Nos.
7 CRO 1No.
8 Power Supply- +/- 5V 1No.
 9 Bread Board 1No.
10 Wires and probes

THEORY:
Frequency Shift Keying (FSK) is a digital modulation scheme where the digital data is
transmitted using a high frequency carrier signal. For logic ‘0’ and ‘1’ the carrier signal switches
between two preset frequencies, hence the name FSK.
A frequency shift keying modulator circuit made up of 555 timer is shown in figure. The
555 works in monostable mode. For logic ‘1’ transistor BC 177 is OFF and the monostable
works in normal mode and capacitor C1 charges through R A and RB and the output frequency is
at the first preset level. For logic ‘0’ the transistor is ON and the resistor RC comes in parallel
with RA and reduces its effective resistance. The charging rate of the capacitor increases and a
higher frequency signal is obtained at the output. This is the second preset frequency level. Thus
the output signal switches between the two preset frequencies for logic ‘0’ and ‘1’. The resulting
signal is FSK modulated.
PLL IC 565 is used to demodulate the FSK signal. As the signal appears at the input of
565 PLL, the PLL locks to the input frequency and tracks it between the two possible
frequencies with a corresponding dc shift at the output. A three stage RC ladder filter is
employed for removing the sum frequency component from the output. The demodulated output
is applied to the comparator to restore the required logic levels.

PROCEDURE:
1. Test all the components and probes.
2. Set up the FSK modulator and demodulator circuits on the bread board. Switch on the
power supplies.
3. Feed 5V, 100Hz (10Vpp, 100Hz) square wave as the data input. Vary the pot RC to adjust
the output frequencies if needed.
4. Observe both the input and output waveforms on CRO and plot. The waveform of the
FSK output will be rectangular in nature for 555 modulator.
5. Apply the FSK output of the modulator to the input of the demodulator, and observe the
output. Vary the 10K pot to get the PLL locked with the input signal. Plot the waveforms.

CIRCUIT DIAGRAM:
 FSK Modulator

FSK Demodulator

Alternate Circuits
 FSK Modulator

FSK Demodulator

Pinout LM 565 Pinout LM 311

WAVEFORM:
RESULT: The FSK modulator and demodulator circuits were set up and the waveforms were
plotted.
INFERENCE: Studied the usage of IC 555 as FSK modulator and LM 565 as FSK
demodulator.

Experiment No.8
BINARY PHASE SHIFT KEYING
AIM: To set up Binary Phase Shift Keying (BPSK) modulator and demodulator circuits and to
observe the waveforms.

OBJECTIVES: After completing this experiment, the students will be able to a) Set up BPSK
modulator and demodulator circuits and b) Identify BPSK waveform.

COMPONENTS AND EQUIPMENTS REQUIRED:

Sl. No. Item & Specification Quantity

1 Transistor- SL100, SK100 1No. each
2 Resistor- 10K pot, 100K 1No. each
1K 3No.
10K 7Nos.
3 Capacitor- 0.01F 1No.
4 IC 741 3No.
5 Diode- 1N4001 1No.
Zener Diode- 5Z1 2Nos.
6 Signal Generator 2Nos.
7 CRO 1No.
8 Power Supply- +/- 15V, 2V 1No.each
9 Bread Board 1No.
 10 Wires and probes

THEORY:
Binary Phase Shift Keying (BPSK) is digital transmission scheme where the binary data
is transmitted using out of phase signals. During logic ‘0’ a preset number of cycles of a
sinusoidal carrier signal is transmitted and during logic ‘1’ the same number of cycles of the
carrier signal is transmitted but with 180o phase shift.

Modulator
A simple BPSK modulator circuit using an NPN-PNP transistor pair and an Op amp is
shown in figure. The transistors work as switches and the Op amp works as inverting/non-
inverting amplifier. The carrier signal is fed to the collectors and the message signal is fed to the
bases of the two transistors simultaneously. The emitters of the transistors are grounded. When
the message signal is at logic ‘1’ (+5V), the NPN transistor is ON and works as a closed switch.
The PNP transistor is OFF and works as an open switch. The Op amp now works as a non-
inverting amplifier with the carrier signal fed to its non-inverting input. The carrier signal
reaches the output without any phase shift. When the message signal is at logic ‘0’ (-5V), the
NPN transistor is OFF and the PNP transistor ON. The Op amp works as an inverting amplifier
with the carrier signal fed to its inverting pin. The carrier signal now reaches the output with
180o phase shift. Thus the carrier signal switches its phase as the message signal switches
between ‘0’ and ‘1’. The resulting output is BPSK modulated.

Demodulator
The BPSK demodulator circuit shown in figure consists of an Op Amp difference
amplifier, a rectifier, an envelope detector and a comparator. The difference amplifier which is
fed with the unmodulated carrier signal at the non-inverting input and the BPSK modulated
signal at the inverting input passes only the phase shifted signal to the output. The in phase
signals get subtracted completely. The envelope detector removes the carrier content and
recovers the data information. The comparator inverts and level limits the signal to regain the
correct logic level.

PROCEDURE:
1. Test all the components and probes.
2. Set up the circuits on the bread board as shown in figure.
3. Feed 2Vpp, 1KHz sine wave as carrier input and 10Vpp, 200Hz square wave signal as
the message input.
4. Observe the BPSK output on CRO and plot the waveforms.
5. Feed this BPSK modulated signal to the inverting input of the demodulator. Also feed the
unmodulated carrier signal (2Vpp, 1KHz) to the non-inverting input.
6. Observe waveforms on CRO. Adjust the potentiometer to obtain the correct output (if
needed).
7. Plot the waveforms.

CIRCUIT DIAGRAM:
 BPSK Modulator

BPSK Demodulator
WAVEFORMS:
RESULT:
BPSK modulator and demodulator circuits were set up and the waveforms were plotted.
INFERENCE: Studied how digital data is transmitted using BPSK.

Experiment No.9

TIME DIVISION MULTIPLEXER AND DEMULTIPLEXER

AIM:

To set up Time Division Multiplexer and Demultiplexer circuits and to observe the waveforms.

OBJECTIVES: After completing this experiment, the students will be able to a) Set up TDM
multiplexer and demultiplexer circuits.

COMPONENTS AND EQUIPMENTS REQUIRED:

Sl. No. Item & Specification Quantity
1 Transistor- SL100, SK100 1No. each
2 Resistor- 1K 7Nos.
10K 4Nos.
3 Capacitor- 1F 2No.
4 IC 741 1No.
5 Signal Generator 3Nos.
6 CRO 1No.
7 Power Supply- +/- 15V 1No.
8 Bread Board 1No.
9 Wires and probes
THEORY:
Time Division Multiplexing (TDM) is widely used in digital communication networks to
transmit multiple signals simultaneously through the same channel. Different signals are
transmitted in a time shared manner. Each signal is allotted a fixed time slot and a sample of the
corresponding signal is transmitted during that period. After one sample each of all the signals is
sent, the time slot is given back to the first signal and this process repeats.

TDM Multiplexer
A simple TDM multiplexer circuit using an NPN-PNP transistor pair and an Op amp is
shown in figure. The transistors work as switches and the Op amp works as an adder. The signals
to be sent are fed to the collectors of the two transistors. The switching signal is applied to the
bases the transistors. During the ON time of the switching signal, the NPN transistor is ON and
the PNP transistor is OFF. Signal 1 alone is connected to the adder input and reaches the output.
During OFF time of the switching signal, the NPN transistor is OFF and the PNP transistor is
ON. Signal 2 alone is connected to the adder input and reaches the output. Thus the two signals
reach the output one after the other as the switching signal changes state. The resulting signal is a
time division multiplexed one. The on-off period of the switching signal decides the time slot.

TDM Demultiplexer
In the demodulator circuit the two transistors act as switches. They connect the input TDM signal
to the respective outputs alternately as the switching signal changes state. A square wave signal
with the same phase and frequency as the one used at the TDM modulator is used as the
switching signal. During the ON time of the switching signal, the NPN transistor is ON and the
PNP transistor is OFF. TDM input is now connected to signal 1 output. During the OFF time of
the switching signal, the NPN transistor is OFF and the PNP transistor is ON. TDM input is now
connected to signal 2 output. The RC networks act as low pass filters.

PROCEDURE:
1. Test all the components and probes.
2. Set up the circuits on the bread board as shown in figure.
3. Connect 5Vpp, 2KHz square wave signal as the switching input.
4. Connect 2Vpp, 100Hz sine wave as signal 1 and 2Vpp, 100Hz square wave as signal 2.
5. Observe the TDM output on CRO and plot the waveforms.
6. Feed this TDM output to the input of the demultiplexer. Use the same square wave signal
used at the modulator as the switching signal.
7. Observe signal 1 and signal 2 outputs of the demultiplexer on CRO.
8. Plot the waveforms.

CIRCUIT DIAGRAM:
 TDM Multiplexer

TDM Demultiplexer
WAVEFORM:
RESULT:
TDM Multiplexer and demultiplexer circuits were set up and the waveforms were plotted.
INFERENCE: Learned the basic idea of TDM multiplexing and demultiplexing.
Note: The demultiplexed outputs should be perfect sine and square waves. But with the given
demultiplexer, the signal 2 output won’t be a perfect square wave. An Op Amp comparator can
be employed to make the output a perfect square wave.

Experiment No.10

PRE-EMPHASIS AND DE-EMPHASIS

AIM:

To design and set up pre-emphasis and de-emphasis circuits using Op amp and to plot the gain
Vs frequency characteristics.

OBJECTIVES: After completing this experiment, the students will be able to a) design and set
up pre-emphasis and de-emphasis circuits and to b) plot the response curve.

COMPONENTS AND EQUIPMENTS REQUIRED:

Sl. No. Item & Specification Quantity

1 IC 741 1No.
2 Resistor- 4.7K, 1K, 560 1No.
3 Capacitor- 0.01F 1No.
4 Signal Generator 1No.
5 CRO 1No.
6 Power Supply- +/- 15V 1No.
7 Bread Board 1No.
 8 Wires and probes

THEORY:
In FM Broadcasting, the effect of noise is more intense on higher frequencies than on low
frequencies. Therefore, in order to have high signal-to-noise ratio(low noise), the high
frequencies are amplified at the transmitter side (pre-emphasis) and for compensation, de-
emphasis (decreasing the amplitude of those boosted frequencies ) is done at receiver.
The pre-emphasis circuit is actually a high pass filter and de-emphasis circuit a low pass
filter. The amount of pre-emphasis and de-emphasis used is defined by the time constant of a
simple RC filter circuit. As per European standards 50S is the time constant. In North America,
75S is the standard value. Simple pre-emphasis and de-emphasis circuits using Op Amp are
given in the diagram.

DESIGN:
Time constant chosen, T = 50S
Therefore the time constant RC = 50 S
Take C = 0.01F, then R = 4.7K
For Butterworth filters, Gain A = 1.586
Gain of non-inverting amplifier = 1+Rf/R1
1.586 = 1+Rf/R1
Rf/R1 = 0.586
Take R1 = 1K, then Rf = 560

PROCEDURE:
1. Test all the components and probes.
2. Set up the pre- emphasis circuit on a bread board as shown in figure.
3. Feed 5Vpp sine wave as input. Vary the frequency from 50Hz to 3MHz and note down
the values of the corresponding output voltage on a tabular column.
4. Plot frequency response on a graph sheet with log f on x-axis and gain in dB on y-axis.
Mark the cut-off frequencies corresponding to 3dB points.
5. Repeat the above steps for de-emphasis circuit.

CIRCUIT DIAGRAM:
 Pre-emphasis Circuit

De-emphasis circuit

TABULAR COLUMN:
Gain in dB
f (Hz) Vo log f
(20logVo/Vi)

FREQUENCY RESPONSE:
RESULT:
Pre-emphasis and de-emphasis circuits were set up and frequency response curves were drawn.
Cut-off frequency (3dB) for pre-emphasis circuit =…………………..
Cut-off frequency (3dB) for de-emphasis circuit =…………………..
INFERENCE: Signal to noise ratio at high frequencies can be considerably improved by
employing pre-emphasis and de-emphasis.

Experiment No.11

TRANSISTOR MIXER
AIM:

To set up a transistor mixer circuit to produce IF from RF and oscillator frequency inputs
and to measure the conversion transconductance.

OBJECTIVES:

After completing this experiment, the students will be able to a) set up a transistor mixer
circuit and to b) study the relation between the RF, oscillator frequency and IF and to c) measure
the conversion transconductance.

COMPONENTS AND EQUIPMENTS REQUIRED:

Sl. No. Item & Specification Quantity

1 Resistor- 22K, 33K, 3.3K 1No. each
2 Transistor BF194 1No.
3 Capacitor- 0.1F 1No.
4 IFT 1No.
5 Signal Generator 2Nos.
 6 CRO 1No.
7 Power Supply- =12V 1No.
8 Multimeter 1No.
9 Bread Board 1No.
10 Wires and probes

THEORY:
Mixer or frequency convertor is actually a non linear resistor having two sets of input
terminals and one set of output terminal. The two inputs to the mixer are the input signal and the
local oscillator signal. The output of the mixer contains many frequencies including the sum and
difference frequencies between the two input signals. The mixer output is commonly tuned to the
difference frequency. This frequency is called the intermediate frequency (IF).
The input to the mixer is the input signal voltage with magnitude VS and frequency fS.
The output is usually a current component at IF frequency having a magnitude IIF proportional to
VS. The proportionality constant is called transconductance and is given by
gC = IIF/ VS
The conversion transconductance of a transistor mixer is of the order of 6ms. Mixing takes place
when the transconductance of the mixer is caused to vary with the local oscillator voltage.

PROCEDURE:
1. Test all the components and probes.
2. Set up the circuit as shown in figure on a breadboard.
3. Switch on the power supply.
4. Check the dc conditions of the transistor and make sure that it is working in the active
region.
5. Feed a 2Vpp, 100KHz sine wave signal at the base of the transistor as shown in figure.
6. Feed a 2Vpp, 555KHz sine wave signal at the emitter of the transistor as shown in figure.
7. Observe the output waveform on a CRO and measure the frequency. Adjust the IFT to
obtain 455KHz as the peak output frequency.
8. Plot the input/output waveforms.
9. Measure the output ac current (IIF) and the input ac voltage (VS) using a multimeter.
Calculate the transconductance using the equation gC = IIF/ VS
10. Check the output for 100KHz and 355KHz inputs.

Note: The IFT centre tap point should be connected to Vcc. Connect one of the other two
terminals of the IFT primary to the collector of the transistor. Try both terminals and select the
one that gives the better output.
CIRCUIT DIAGRAM:

Transistor Mixer

WAVEFORMS:

RESULT:
The Mixer circuit was set up and the waveforms were plotted.
Frequency of the IF signal = …………………..
Conversion transconductance, gC = …………...

INFERENCE:
1) The transistor in a mixer circuit is acting as a non linear resistor.
2) The IF is the frequency difference between the local oscillator frequency f LO and the
input signal frequency fS.
PIN DIAGRAMS

BC 107, SL 100, SK 100 BF 194 BFW 10

IC 741 IC 555 IC 7493

CD 4016 LM 324 LM 311