Seth Doc: Jai Parkash Mukand lal Institute of...

Seth Jai Parkash Mukand lal

Institute of Technology,Radaur

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JMIT/ECE-312 E

DIGITAL COMMUNICATION LAB

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Subject Name: Digital Communication Lab – ECE-312E

Sr. No. Work Instruction Sheet No. Aim of Experiment

1. ECE/ECE-312E/WI-601 To study PSK.

2. ECE/ECE-312E/WI-602 To study FSK.

3. ECE/ECE-312E/WI-603 To study IF amplifier.

4. ECE/ECE-312E/WI-604 To study balanced modulator and demodulator.

5. ECE/ECE-312E/WI-605 To study PCM.

6. ECE/ECE-312E/WI-606 Setting up a fiber optic analog link.

7. ECE/ECE-312E/WI-607 Setting up a fiber optic digital link.

8. ECE/ECE-312E/WI-608 Losses in optical fiber.

9. ECE/ECE-312E/WI-609 Measurement of numerical aperture.

10. ECE/ECE-312E/WI-610 Time division multiplexing of signals.



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EXPERIMENT NO.- 1

Aim of Experiment: - To study Phase Shift Keying (PSK).

Apparatus:- Experimental kit of PSK , connecting wires, CRO

Theory: - Phase shift keying (PSK) involves the phase shift change of the carrier sine wave

between 0º and 180º in accordance with the data stream to be transmitted. PSK is also known as

phase reversal keying (PRK) .

Functionally , the PSK modulator is very similar to the ASK modulator . both uses balanced

modulator to multiply the carrier with the modulating signal. Bit in cobtrast to ASK technique ,

the digital signal applied to the modulation input for PSK generation is bipolar i.e. have equal

positive and negative voltage levels.When the modulating input is positive the output of

modulator is a sine wave in phase with the carrier input. Where as for the negative voltage levels

, the output of modulator is a sine wave which is shifted out of phase 180º from the carrier input.

Fig (1) shows the block diagram of the PSK modulation and demodulation

The unipolar – Bipolar convertes the unipolar data stream to bipolar data. At receiver , the square

loop detector circuit is used to demodulation the transmitted PSK signal. Functionally , the

demodulator is shown in Fig. (2)

The incoming PSK signal with 0º & 180º phase changes is first fed to the signal square , which

multiplies the input signal by itself. The PLL blocks locks to the frequency of the signal square

o/p & produces a clean square wave o/p of same frequency. To derive the square wave of same

frequency as the incoming PSK signal, the PLL’s o/p is divided by 2 in frequency domain is the

divided by 2 circuit.

The following phase adjust circuit allows the phase of the digital signal to be adjusted with

respect to the input PSK signal. Also its o/p controls the closing of an analog switch.



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PSK Waveform

Fig. (1) Block diagram of PSK Modulator

PSK Modulated Output

Fig. (2) Block Diagram of PSK Demodulator

Carrier Input

Output

Modulation Input

Unipolar

Bipolar

Converter



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Procedure:-

1. The experiment makes use of two trainers namely ST 2156 and ST 2157. ST 2156 serves

as data formatting device while ST 2157 reformats the data.

2. Ensure that all trainers are swiched off, until the complete connections are made.

3. Set up the following conditions on ST 2156 & st 2157 trainers.

a) Mode switch set in FAST position .

b) Pseudo- random sync code generator swiched ‘OFF’.

c) Error check code selector switches A=B IN A=0 & B=0 positions.

d) All swiched faults ‘OFF’.

Step1. PSK modulation

a. Connect carrier output of carrier generator to carrier input to modulator 1 on ST 2156.

b. Connect NRZ(M) data output to unipolar to bipolar convertor.

c. Connect the o/p of unipolar to bipolar convertor to modulatator.

d. Switch on experimental kit..

e. Observe the PSK output at modulator 1.

f. And observe PSK output changes accordingly.

Step2. PSK demodulation

a. Connect the PSK modulated output of ST 2156 to the the PSK demodulator of ST 157.

b. Connect the output of PSK demodulator to LPF and observe the o/p on CRO.

c. Then connect the o/p of LPF to the Data Squaring Circuit and observe the o/p and trace

the waveform.

d. Fig 2 shows the block diagram of observing the PSK demodulation output on CRO.

Precautions:-

1. Don’t make interconnections on the board when power supply is ON.

2. No external connections for DC power supply to the circuit are to be made modulating

signal is input I.

3. If an external modulating signal is used that it should not have any DC.

Result: - PSK has been studied successfully.



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EXPERIMENT NO.- 2

Aim of Experiment: - To study frequency shift keying (FSK).

Apparatus: - Experimental kit of FSK, CRO, connecting wires etc.

Theory: - The technique where the binary information is transmitted by the shifting of the

sinusoidal carrier frequency b/w two frequency corresponding to binary information is called

FSK. The FSK on frequency F1 is transmitted for high input and another frequency F2 is

transmitted for two input.

The functionally blocks required in order to generate the FSK signal is shown in Fig. (1). The

two carriers have different frequencies & the digital data is inverted in one case.

The demodulation of FSK waveforms can be carried out by a phase locked loop. As known , the

phase locked loop tries to ‘lock’ to the input frequency . it achieves this by generating

corresponding output voltage to be fed to the voltage controlled oscillator , if any frequency

deviation at its input is encounted . Thus the PLL detector follows the frequency changes &

generates proportional o/p voltage. The o/p voltage from PLL contains the carrier components .

therefore the signal is passed through the LPF to remove them. The resulting wave is too

rounded to be used for digital data processing . Also , the amplitude level may be very low due to

channel attenuation.



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Fig. 1 Block Diagram of FSK

Modulated Output of FSK

Carrier Input

Output

Modulation Input

Carrier Input

Output

Modulation Input

f

1

f

2

Data

Stream

Data

Stream

Summing

Amplifier

FSK

Waveform



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Procedure:

1 The experiment makes use of two trainers namely ST 2156 and ST 2157. ST 2156

serves as data formatting device while ST 2157 reformats the data.

2 Ensure that all trainers are swiched off, until the complete connections are made.

3 Set up the following conditions on ST 2156 & st 2157 trainers.

a) Mode switch set in FAST position .

b) Pseudo- random sync code generator swiched ‘OFF’.

c) Error check code selector switches A=B IN A=0 & B=0 positions.

d) All swiched faults ‘OFF’.

For FSK Modulation

Make the additional connections on trainer ST 2156

a) NRZ(L) output to modulation input of modulator 1.

b) Carrier output of 1.6 MHz of carrier generator to carrier input of modulator 1.

c) Modulator 1 o/p to summing amplifier input .

d) Modulation input to data inverter input.

e) And inverted data to modulation input of modulator 2.

f) Carrier output of 960 KHz.of carrier generator to carrier input of modulator 2.

g) Modulator 2 o/p to summing amplifier input .

h) observe the summing amplifier o/p on CRO and trace the waveform.

For demodulation :

a) Connect the FSK modulated output of ST 2156 to the FSK demodulator of ST 157.

b) Connect the output of FSK demodulator to LPF and observe the o/p on CRO.

c) Then connect the o/p of LPF to the Data Squaring Circuit and observe the o/p and trace

the waveform.

Fig shows the block diagram of the FSK demodulation output on CRO

.

Precautions:-

4. Don’t make interconnections on the board when power supply is ON.

5. No external connections for DC power supply to the circuit are to be made modulating

signal is input I.

6. If an external modulating signal is used that it should not have any DC.

Result: -- FSK has been studied successfully.



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EXPERIMENT NO.- 3

Aim of Experiment:-To study IF amplifiers

Equipment: - Experimental kit, CRO, Function generator, wires



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Th



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Precautions:-

1. Supply should be proper.

2. Make proper connection.

Result :- IF Amplifier has been studied.



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EXPERIMENT NO.- 4

Aim of Experiment: - To study balanced modulator and demodulator.

Apparatus used: - Experimental kit of DSB-SC, CRO, connecting wires

Theory:-

Balanced modulator

It is essentially and amplitude modulator with no carrier power at the output and is the heart of

all the methods of the single side band suppressed carrier modulation and demodulation. In AM,

the amplitude of carrier is changed in accordance with the modulating wave and carrier wave are

sine wave .

In balanced modulator only side branch are transmitted while carrier is fully suppressed. The

carrier however does have a function at the receiving end where the demodulation occurs.

For commercial broadcasting , this penalty is tolerated since the radio receivers are much

simpler, cheap and easier to tune.

Balanced modulator is a multiplier. It contains two preprocess. If modulating signal counties a

frequency band, there is balanced modulator two separate frequency branch will be observed.

The carrier is considered to be a switching voltage .

Assuming that base currents are negativable balanced demodulator.

Balanced modulated signal is demodulated using a product detector circuit. Demodulator is same

as of modulator. At the output of the demodulator,

Low pass filter is used at the output to obtain demodulated signal. It is important to note that

carrier used at the receiver should either in phase or 180 degree out of phase of the original

carrier.



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Procedure: For Modulation

1. Connect the AF output of generator to the input signal of balanced modulator (DSB).

2 connect the carrier o/p of generator for modulation to the carrier input.

3 observe the output on CRO and trace the waveform.

For Demodulation

1 Connect the modulated output to the demodulator input. 2 connect the carrier o/p of generator for demodulation to the carrier input.

3 Connect the filter to the demodulator circuit.

3 Check the demodulated waveform at the end of filter and trace it.

Precautions:-

1. Do not make connections on the kit with power switched ON.

2. No external connection for the DC power supply to the circuit is to be made.

Result: - The balanced modulation and demodulation has been studied successfully.



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EXPERIMENT NO.- 5

Aim of Experiment: - To study Pulse Code Modulation (PCM).

Apparatus: - PCM coding kit, connecting wire, CRO, Probes.

Theory: -PCM is used to convert analog signals to binary form. In the absence of noise it is

possible to completely recover a continuous analog modulator signals. But in real time they

suffer from transmission and noise to an appreciate extent. In the PCM system, groups of pulses

or codes are through which represent binary numbers corresponding to modulating signal voltage

levels. Recovery of transmitted information does not depend on the height, width or energy

extent of the individual pulses, but only on the absence or presence since it is relatively easy

recover pulses. Under these conditions even in the presence of large amount of distortions and

noise. PCM system tends to be very immune to interference and noise. Regeneration of pulses in

route is also early resulting in system that produces explants in system that produces excellent

result for being distance communication.

Quantization: -The 1st step in PCM system is to quantize the modulating signal. The modulating

signal is assumed to be an infinite number of different levels b/w the two limits levels which

define the range of signal. In PCM, a code number is transmitted for each level sampled in

modulated signal. If the exact number corresponding to exact voltage were to be transmitted for

every sample an infinite number of different code symbols would be needed. Quantization has

the effect of reducing this infinite large number of levels to relatively small number which can be

coded without difficulty. In quantization process, the total range of modulating signal is divided

into a number of small sub ranges. The number will depend on the nature of modulating signal

and will be shown as few as 8 to as many as 128 level, a number that is integral power of 2 is

chosen because of the case of binary codes. A new signal is generated by producing for each

sample a voltage level corresponding to mid point level of sub range in which the samples fall.

The result is a stepped waveform which follows original modulating signal with each step

synchronized to the sampling period.



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PCM Encoding: -The modulating signal is applied to the input of A/D converter which

performs the two functions of quantization and encoding producing a 8 bit binary coded number.

The modulating signal is sampled at regular intervals (0.345 kHz). If the max amplifier +5V is

represented by 8 bit, the lsb amplitude corresponding to 5×1/128 = 39 mv and MSB represent the

sign. So that values of the sampled signal at the output of A/D, digital connectors are

00000000,00 \\ \\ \ \\ , 0\ \\ \\ \\ , 00 \\ \\ \\, 000000000000,10 \\ \\ \\ , \\ \\ \\ \\ , 10 \\ \\ \\ , 00 00

0000 , to transmit all the bits in one channel parallel to series converter is used and transmitted.

PCM Decoding: -Now at the receiver data will be passed through desired series to parallel

counter and turn to D/A converter for decoding which maintains the pulse level for duration of

sampling period, which with the

help of amplifier recreating the staircase waveform which is approximate of modulating signal.

A LPF may be used to reduce the quantization noise and yield the original signal.

Procedure: -

1. Switch ON power of kit of PCM.

2. Measure the frequency of sampling clock.

3. Provide DC voltage as modulating signal.

4. Connect the DC at the input of A/D converter and measure the voltage.

5. Connect the clock to timing and control kit.

6. Note down the binary data word from LED’s



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LED ON represents 1.

LED OFF represents 0.

7. The data which is transmitted through the channel is decoded by connecting the

sampling clock (351 kHz) to that CH-1 of CRO of serial data to the CH-2 of the digital

world.

8. Observe the same binary waveform at the o/p of D/A converter, which is shown as

quantization level.

9. Observe the same binary word at the receiver.

10. Now apply the modulating signal at the input.

11. Observe the waveform at the o/p of D/A converter and then LPF.

12. Repeat the above steps for varying the modulating signal from 0 Hz to 500 Hz and verify

the sampling theorem.

Precautions: -

1. Connections should be tight.

2. The circuit should be checked by the prescribed authority before giving the power supply.

Result: Pulse Code Modulation (PCM) has been studied.



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EXPERIMENT NO.- 6

Aim of Experiment: - Setting up fiber optic analog link.

Theory: -Fiber optics link can be used for transmission of digital as well as analog signals.

Basically a fiber optic link contains three main elements, a transmitter, an optical fiber and a

receiver. The transmitter module takes the input signal in electrical form and then transforms it

into optical energy containing the same information. The optical fiber is the medium, which

takes the energy to the receiver. At the receiver light is converted back into electrical form with

the same pattern as originally fed to the transmitter.

Transmitter: Fiber optic transmitters are typically composed of a buffer, driver and optical

source. The buffer provides both an electrical connection and isolation b/w the transmitter and

the electrical system supplying the data. The driver provides electrical power to the optical

source. Finally, the optical source converts the electrical current to the light energy with the same

pattern. Commonly used optical sources are light emitting diodes (LEDs) and laser beam. Simple

LED circuits, for digital and analog transmissions are shown below:

Fig 1

Above fig shows Trans conductance drive circuits for analog transmission-common emitter

configuration. The transmitter section comprises of:

1. Function generator

2. Frequency modulator

3. Pulse width modulator block.



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The function generator generates the input signals that are going to be used as information to

transmit through the fiber optic link. The output voltage available is

1 kHz sinusoidal signal of adjustable amplitude and fixed amplitude 1 KHz square wave signal.

The modulator section accepts the information signal and converts it into suitable form for

transmission through the fiber optic link.

The fiber optic link: Emitter and detector circuit on the board form the fiber optic link. This

section provides the light source for the optic fiber and the light detector at the far end of the

fiber optic links. The optic fiber plugs into the connectors provided in this part of the board. Two

separate links are provided.

The receiver: The comparator circuit, LPF, phase locked loop, AC amplifier circuits form

receiver on the board. It is able to undo the modulation process in order to recover the original

information signal. In this experiment the trainer board is used to illustrate one-way

communication b/w digital transmitter and receiver circuits.

Procedure: -

1. Connect the power supply to the board.

2. Ensure that all switched faults are off.

3. Make the following connections(as shown in fig)

a. Connect the F.G. 1 KHz sine wave output to emitter’s input.

b. Connect the F.O. cable b/w emitter output and detector’s input.

c. Detector output to AC amplifier input.

4. On the board, switch emitter driver to analog mode.

5. Switch ON the power.

6. Observe the input to emitter (t.p.5) with the output from AC amplifier (t.p.19) and note

that the two signals are same.



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Precautions: -

1 Connections should be tight.

2 The circuit should be checked by the prescribed authority before giving the power supply.

Result :- Fiber optic analog link has been performed.



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EXPERIMENT NO.- 7

Aim of Experiment: - Setting up fiber optic digital link.

Theory: -Fiber optics link can be used for transmission of digital as well as analog signals.

Basically a fiber optic link contains three main elements, a transmitter, an optical fiber and a

receiver. The transmitter module takes the input signal in electrical form and then transforms it

into optical energy containing the same information. The optical fiber is the medium, which

takes the energy to the receiver. At the receiver light is converted back into electrical form with

the same pattern as originally fed to the transmitter.

Transmitter: Fiber optic transmitters are typically composed of a buffer, driver and optical

source. The buffer provides both an electrical connection and isolation b/w the transmitter and

the electrical system supplying the data. The driver provides electrical power to the optical

source. Finally, the optical source converts the electrical current to the light energy with the same

pattern. Commonly used optical sources are light emitting diodes (LEDs) and laser beam. Fig

below shows a simple drive circuit for binary digital transmission consisting a common emitter-

saturating switch.



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The transmitter section comprises of:

1. Function generator

2. Frequency modulator

3. Pulse width modulator block.

The function generator generates the input signals that are going to be used as information to

transmit through the fiber optic link. The output voltage available is 1 kHz sinusoidal signal of

adjustable amplitude and fixed amplitude 1 KHz square wave signal. The modulator section

accepts the information signal and converts it into suitable form for transmission through the

fiber optic link.

The fiber optic link: Emitter and detector circuit on the board form the fiber optic link. This

section provides the light source for the optic fiber and the light detector at the far end of the

fiber optic links. The optic fiber plugs into the connectors provided in this part of the board. Two

separate links are provided.

The receiver: The comparator circuit, LPF, phase locked loop, AC amplifier circuits form

receiver on the board. It is able to undo the modulation process in order to recover the original

information signal. In this experiment the trainer board is used to illustrate one-way

communication b/w digital transmitter and receiver circuits.

Procedure: -

1. Connect the power supply to the experimental kit.

2. Ensure that all switched faults are off.

3. On the kit, switch emitter driver to digital mode.

4. Make the following connections(as shown in fig)

a. Connect the F.G. 1 KHz sine wave output to emitter’s input.

b. Connect the F.O. cable b/w emitter output and detector’s input.

c Switch ON the power.

c. Observe the o/p at detector’s o/p. Amplitude of waveform is very less .

d. Then connect Detector output to comparator’s input.

5. Observe the output at comparator’s end note that the two signals are same at input terminal

and at this terminal.

6 Trace this signal.



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Precautions: -



Result :- Fiber optic digital link has been performed.



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EXPERIMENT NO. - 8

Aim of Experiment: - study the loss in the optical fiber.

Theory: - Whenever the condition for angle of incidence of the incident light is violated the

losses are introduced due to refraction of light. This occurs when fiber is subjected to bending.

Lower the radius of curvature more is the loss.

Procedure:-

1. Connect the power supply to board.

2. Make the following connections as shown in fig.

a. Function generators 1 kHz sine wave output to input socket of emitter circuit via 4

mm lead.

b. Connect 1 m optic fiber b/w emitter o/p and detector i/p.

c. Connect detector o/p to amplifier input socket via 4 mm lead

3. Switch ON the power supply.

4. Set the oscilloscope channel 1 to 0.5 V/Div and adjust 4-6 div amplitude by using X1

probe with the help of variable pot in function generator block at input of emitter.

5. Observe the o/p signal from detector on CRO.

6. Adjust the amplitude of the received signal as that of transmitted one with the help of

gain adjusts pot in AC amplifier block. Note this amplitude and name it V1.

7. Wind the FO cable on the mandrel and observe the corresponding AC amplifier o/p on

CRO, it will be gradually reducing showing loss due to bends.



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Precautions: -



Result :-Bending Losses in Fiber optic has been verified..



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EXPERIMENT NO. - 9

Aim of Experiment: - To measure the numerical aperture (NA) of the fiber.

Apparatus: - NA measurement jig , fiber optic kit, cable.

Theory: - NA refers to the maximum angle at which the light incident on the fiber end is totally

internally reflected and is transmitted properly along the fiber. The cone formed by the rotation

of this angle along the axis of the fiber is the cone of the acceptance of the fiber. The light ray

should strike the fiber end within its cone of the acceptance else it is refracted out of the fiber.

Consideration in NA measurement: -

It is very important that the optical source should be properly aligned with the cable and the

distance from the launched point and cable be properly selected to ensure that the maximum

amount of optical power is transferred to the cable.

Procedure: -

1. Connect power supply to the board.

2. Connect the frequency generator of 1 kHz sine wave output to input of emitter circuit. Adjust

its amplitude at 5Vpp.

3. Connect one end of fiber cable to the output socket of emitter circuit and the other end to the

NA measurement jig. Hold the white screen facing the fiber such that its cut face is

perpendicular to the axis of the fiber.

4. Hold the white screen with 4 concentric circles(10,15,20 & 25 mm diameter) vertically at a

suitable distance to make the red spot from the fiber coincide with 10 mm circle.

5. Record the distance of screen from the fiber end L and note the diameter W of the spot.

6. Compute the NA from the formula given below:

NA = W/ (√4L2 +W

2)

= Sin Ө max (acceptance angle)



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7. Vary the distance b/w in screen and fiber optic cable and make it coincide with one of the

concentric circles. Note its distance.

8. Tabulate the various distances and diameter of the circles made on the white screen and

computes the NA from the formula given above.

Inferences: -The NA recorded in the manufacturer’s data sheet is 0.5 typical. The variation in

the observation is due to fiber under filled. The acceptance angle is given by 8max. The

deviation from the data sheet is again due to fiber being under filled.

Precautions: -



Result :- The numerical aperture (NA) of the fiber optic has been measured.



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EXPERIMENT NO. - 10

Aim of Experiment: - Study of Time Division Multiplexing.

Theory: -TDM is a technique of transmitting more than one information on the same channel.

As can be notices from fig-1, samples consist of short pulses followed by another pulse after a

long time intervals. This no-activity time intervals can be used to include samples from the other

channels as well. This means that several information signals can be transmitted over a single

channel by sending samples from different information sources at different moments in time.

This technique is known as TDM.

TDM is widely used in digital communication systems to increase the efficiency of the

transmitting medium. TDM can be achieved by electronically switching the samples such that

they interleave sequentially at correct instant in time without mutual interference. The basic 4

channel TDM is shown in fig-2.

Fig-1



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Fig-2

Procedure: -

1. Set up the following initial conditions on ST2103:

a. Mode switch in fast position.

b. DC1 & DC 2 controls in function generator block fully clockwise.

c. 1 KHz and 2 KHz control levels set to give 10Vpp.

d. Pseudo-random sync code generator on/off switch in OFF position.

e. Error check code generator switch A & B in A = 0 & B = 0 position (OFF Mode).

f. All switched faults off.

2. First, connect only the 1 KHz output to CH 0.

3. Turn ON the power. Check that the PAM output of 1 KHz sine wave .

4. Connect channel 1 of the oscilloscope and observe the wave.

5. Now connect the 2 KHz output to CH 1.

6. Connect channel 2 of the oscilloscope and check the waveform.

7. Now check the TDM form of both the input signal simultaneously

8. And trace the waveform

Precautions:-

1 Do not make connections on the kit with power switched ON.

2 No external connection for the DC power supply to the circuit is to be made

Result :- TDM has been studied successfully.

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