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DIGITAL COMMUNICATIONS

LAB MANUAL (STUDENT COPY)

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

GUDLAVALLERU ENGINEERING COLLEGE SESHADRI RAO KNOWLEDGE VILLAGE::GUDLAVALLERU

INDEX

S.NO. NAME OF THE EXPERIMENT PAGE NO.

1 Time Division Multiplexing 1-4

2 Pulse Code Modulation & Demodulation 5-8

3 Differential Pulse Code Modulation & Demodulation 9-12

4 Delta Modulation 13-16

5 Frequency Shift Keying 17-19

6 Phase Shift Keying 20-22

7 Differential phase shift keying 23-26

8 Companding 27-35

9 Linear Block Code- Encoder and Decoder 36-39

10 Binary Cyclic Code- Encoder and Decoder 40-42

ADDITIONAL EXPERIMENTS

1 Amplitude Shift Keying 43-45

2 Time Division Multiplexing & De multiplexing (Digital) 46-48

3 MATLAB Program for ASK Modulation & Demodulation 49-51

4 MATLAB Program for PSK Modulation & Demodulation 52-55

5 MATLAB Program for FSK Modulation & Demodulation 56-59

6 MATLAB Program for QPSK Modulation & Demodulation 60-66

APPENDIX 67-72

REFERENCES 73

Digital Communication Lab

Additional Experiments

APPENDIX

ELECTRONICS & COMMUNICATION ENGINEERING

DIGITAL COMMUNICATIONS LAB 1

1. TIME DIVISION MULTIPLEXING AND DEMULTIPLEXING

Aim:

1. To study the 4 channel analog multiplexing and demultiplexing

2. To study the effect of sampling frequency on output signal

characteristics.

3. To study the effect of input signal amplitude on the output signal

characteristics.

Apparatus required:

1. Time Division Multiplexing and de multiplexing trainer Kit.

2. Dual Trace oscilloscope

Theory:

In PAM, PPM the pulse is present for a short duration and for most of the time

between the two pulses no signal is present. This free space between the pulses

can be occupied by pulses from other channels. This is known as Time Division

Multiplexing. Thus, time division multiplexing makes maximum utilization of the

transmission channel. Each channel to be transmitted is passed through the low

pass filter. The outputs of the low pass filters are connected to the rotating

sampling switch (or) commutator. It takes the sample from each channel per

revolution and rotates at the rate of f s. Thus the sampling frequency becomes fs

the single signal composed due to multiplexing of input channels. These

channels signals are then passed through low pass reconstruction filters. If the

highest signal frequency present in all the channels is fm, then by sampling

theorem, the sampling frequency fs must be such that fs≥2fm. Therefore, the time

space between successive samples from any one input will be Ts=1/fs, and

Ts ≤ 1/2fm.



Circuit Diagram:

Fig: 1 Time Division Multiplexing And Demultiplexing Circuit

Procedure:

There are 4 signal sources;

a) AF Signal generator

b) Triangular wave generator

c) Square wave generator and

d) Sine wave generator

1. Connect these four signals to four inputs of the Multiplexer. Adjust each signal

amplitude to be with in +/-2V (p-p) and frequency non-over lapping within a

frequency band of 300Hz.

2. Connect A, B output of 7476 to A1, B l inputs of Multiplexer.

3. Adjust the frequency of IC 8038 (Square wave, triangular wave generator) to

be around 32 KHz, so that each of the Four channels are sampled at 8 KHz.

4. Adjust the pulse width of 555 timers to be around 10µsecs.

5. Observe the 4 output pin 11 of 7476 on one channel 1and TDM output pin 13

of CD4052 on second channel of oscilloscope. Synchronize scope Internal-CH

1 mode. All the multiplexed channels are observed during the full period of the

clock (1/32 KHz).

6. Connect TDM output to comparator –ve input and saw tooth wave to +ve

Input. Observe the Comparator output. The PAM pulses are now converted in

to PWM pulses.



7. Connect the PWM pulses to TDM input of De multiplexer at pin 3 of second

CD4052. Observe the individual outputs Y0, Y1, Y2, and Y3 at pin 1, 5, 2 & 4

of CD4052 respectively. The PWM pulses corresponding to each channels

are now separated as 4 streams.

8. Take one output and connect it to Low Pass Filter and the Low Pass Filter

output to Amplifier. Observe the output of the amplifier in conjunction with the

corresponding input. Repeat this for all 4 inputs. This is the Demodulated

TDM output. Any slight variation in frequency, amplitude is reflected in the

corresponding output.

Observations:

S.No Type of Signal

Input Signal Multiplexed output

Amplitude

(Vp-p)

Time period

(ms)

Time

Slot(ms)

No. of

cycles

1 AF signal

2 Sine wave

3 Square wave

4 Triangular wave

Model Waveform:

Multiplexed Output Waveform

Inference:



Questions:

1. What is TDM?

2. Applications of TDM?

3. What is the effect of amplitude and frequency of input signals on

output?



2. PULSE CODE MODULATION AND DE MODULATION

Aim:

To obtain the pulse code modulation and de modulation signals.

Apparatus required:

1. PCM trainer kit

2. Dual Trace Oscilloscope.

Theory:

Pulse Code Modulation is known as digital pulse modulation technique. In fact,

the pulse code modulation technique that the message signal is subjected to a

great number of operations. It consists of 3 main parts i.e., transmitter,

transmission path and receiver. The essential operations in the transmitter of a

PCM system are sampling, quantizing and encoding. Sampling is the operation in

which an analog signal is sampled according to the sampling theorem resulting in

a discrete time signal. The quantizing and encoding operations are usually

performed in the same circuit which is known as an ADC. Also, the essential

operations in the receiver are regeneration of impaired signals, decoding and

demodulation of the train of quantized samples. These operations are usually

performed in the same circuit which is known as digital to analog converter.

Further at intermediate points along the transmission route from the transmitter to

the receiver, regenerative repeaters are used to reconstruct the transmitted

sequence of coded pulses in order to combat the accumulated effects of signal

distortion and noise. The quantization refers to the use of a finite set of amplitude

levels and the selection of a level nearest to a particular sample value of the

message signal as the representation the system at transmission in which

sampled and quantized values of an analog signal are transmitted via a

sequence of code words is called Pulse Code Modulation. Two most commonly

used versions are the differential pulse code modulation and delta modulation.

The PCM communication system is shown in Fig1. In the circuit is often called an

analog to digital converter. The functional block that performs the task of

accepting binary digits and generating appropriate sequences of levels is called a

digital to analog converter. The bandwidth of PCM will be much greater than that

of the message. PCM is used to convert analog signals to binary form. Low pass



filter may be used to reduce the quantization noise and it yields the original

message signal.

Circuit Diagram:

Fig: 1 Pulse Code Modulation and Demodulation Circuit

Procedure:

1. Make the connections as per the diagram as shown in the Fig.1.and

switch on the power supply of the trainer kit.

2. Clock generator generates a 20 KHz clock .This can be given as

input to the timing and control circuit and observe the sampling

frequency fs= 2 KHz approximately at the output of timing and control

circuit.

3. Apply the signal generator output of 6V(p-p) approximately to the A to

D converter input and note down the binary word from LED’s i.e. LED

“ON” represents ‘1’ & “OFF” represents ‘0’

4. Feed the PCM waveform to the demodulator circuit and observe the

waveform at the output of D/A which is quantized level.



Model Waveforms:

(a)

(b)

(c)

Fig: 2 Waveforms of (a) Modulating Signal (b) Sampling Signal (c) PCM output



Apply the DC control voltage

DC voltage(v)

Bit sequence

MSB LSB

-4

-3

-2

4

5

Questions:

1. What is the need of parallel to serial converter?

2. What is the use of Companding?

3. What are the applications of PCM?



3. DIFFERENTIAL PULSE CODE MODULATION AND DEMODULATION

Aim:

To study the differential PCM & demodulation by sending variable

frequency sine wave & variable DC signal input.

Apparatus required:

1. AF oscillator

2. DPCM modulator

3. DPCM demodulator

4. Connecting wires

5. CRO - 30MHz

6. Variable DC Source – 1

Theory:

In this DPCM instead of transmitting a base band signal m(t) we send the

difference signal of Kth sample and (k-1) th sample value. The advantage here is

fewer levels are required to quantize the difference than the required to quantize

m(t) and correspondingly, fewer bits will be needed to encode the levels. If we

know the post behaviour of a signal up to a certain time, it is possible to make

some interference about its future values this is called prediction. The filter

designed to perform the prediction is called a predictor. The difference between

the interest and the predictor o/p is called the prediction error.

Circuit Diagram:

Fig:1 Differential Pulse Code Modulation Circuit



Fig: 2 Differential Pulse Code Demodulation Circuit

Procedure:

1. Switch on the experimental kit.

2. Apply the variable DC signal of amplitude 6v(p-p) with frequency of 80Hz

to the input terminals of DPCM modulator.

3. Observe the sampling signal of amplitude 5v (p-p) with frequency 20KHz

on channel 1 of a CRO.

4. Observe the output of DPCM on the second channel.

5. By adjusting the DC voltage potentiometer, observe the DPCM output.

6. During the demodulation connect DPCM output to the input of

demodulator and observe the output of DPCM demodulator.

Model waveforms:

(a)



(b)

(c)

Fig: 3 Waveforms of (a) Sampling Signal (b) Modulating Signal (c) DPCM Output

(a)



-

(b)

Fig. 4 Output of (a) D/A Converter (b) Demodulated

Inference:

Questions:

1. What is the effect sampling signal?

2. Write the advantage of DPCM compared with PCM?

3. What is the one bit version of DPCM?



4. DELTA MODULATION

Aim:

To obtain the delta modulation and demodulation signals.

Apparatus required:

1. Delta Modulation & Demodulation Kit

2. Cathode Ray Oscilloscope 0-30MHz

Theory:

Delta modulation uses a single bit PCM code to achieve digital transmission of

analog signals with conventional PCM each code is binary representation of both

the sign and magnitude of a particular sample. With delta modulation, rather than

transmit a coded representation of the sample, only a single bit is transmitted,

which indicates whether that sample is larger or smaller than the previous

sample. The algorithm for a delta modulation system is quite simple. If the

current sample is smaller than the previous sample, a logic 0 is transmitted. If the

current sample is larger than the previous sample, a logic 1 is transmitted. The

input analog is sampled and converted to a PAM signal, which is compared to

the output of the DAC. The output of the DAC is a voltage equal to the

regenerated magnitude of the previous sample, which was stored in the up/down

counter as a binary number, The up/down counter is incremented or

decremented depending on whether the previous sample is larger or smaller than

the current sample. The up/down counter is clocked at a rate equal to the sample

rate. Therefore, the up/down counter is updated after each comparison.

Block Diagram:

Fig: 1 Delta Modulation Circuit



Fig: 2 Delta Demodulation Circuit

Procedure:

1. Switch on the experimental board.

2. Connect the clock signal of frequency of 10KHz,with amplitude of 5v(p-p)

to the delta modulator circuit.

3. Connect the modulating signal of amplitude 5v(p-p) and frequency of of

0.2KHz modulating input of the delta modulator

And observe the same on channel 1 of a Dual Trace oscilloscope.

4. Observe the Delta Modulator output on channel 2.

5. Connect this Delta modulator output to the Demodulator

6. Also connect the clock signal to the demodulator.

7. Observe the Demodulator output with and without RC filter on CRO.

Model Waveforms:

(a)



(b)

(c)

Fig: 3 Waveforms (a) Clock input (b) Delta modulation output & message signal

(c) D/A converter output

Inference:



Questions:

1. What is the slope overload effect?

2. What is granular noise?

3. Write the advantage of DM over PCM?

4. What is the effect of the Low Pass Filter cut off frequency on output of

demodulator?



5. FREQUENCY SHIFT KEYING

Aim:

To generate the waveforms of frequency shift keying

Apparatus required:

Name of the apparatus Specifications/Range Quantity

Resistors 33kΩ 2

Capacitors 0.01µF, 100pF Each one

Function Generator 0-1MHz 1

RPS 0-30V, 1A 1

CRO 0-30MHz 1

IC 8038 Supply voltage - ±18V or 36V Power dissipation – 750mW

1

CRO Probes ---- 1

Theory:

FSK signaling schemes find a wide range of applications in low-speed digital

data transmission system. FSK schemes are not as efficient as PSK interms of

power and bandwidth utilization. In binary FSK signaling the waveforms are used

to convey binary digits 0 and 1 respectively. The binary FSK waveform is a

continuous, phase constant envelope FM waveform. The FSK signal bandwidth

in this case is of order of 2MHz, which is same as the order of the bandwidth of

PSK signal.

Circuit Diagram:

Fig: 1 Frequency Shift Keying



Procedure:

1. Connect the circuit as shown in fig.1

2. Apply the (binary) Data input of amplitude 20V (p-p) with frequency of 6 KHz

from function generator to pin no.7.

3. Give the power supply of 10v to the appropriate pins.

4. Observe the FSK output at pin no.2.

5. Now note down the mark and space frequencies for different carrier

frequencies.

6. Calculate the maximum frequency deviation and modulation index.

7. Repeat the steps (5) and (6) for different pulse durations of binary input.

Model Waveforms:

(a)

(b)



(c)

Fig: 2 Waveforms of (a) Carrier wave (b) Data input (c) FSK Wave

Inference:

Questions:

1. Write the advantage of FSK compared to ASK?

2. What is the disadvantage of FSK compared with ASK & PSK?

3. What is the effect of R1, C2 values on the output?



6. PHASE SHIFT KEYING

Aim:

To generate the waveforms of phase shift keying.

Apparatus required:

Name of the apparatus Specifications/Range Quantity

Diodes(IN4007) Max Voltage:45V 4

Transformers 7V-0-7V 2

Function Generator 0-1MHz 2

CRO 0-30MHz 1

CRO Probes ---- 1

Theory:

Circuit diagram of PSK as shown in Fig.1. The phase of carrier is shifted between

two values is called Phase Shift Keying. The amplitude of carrier remains

constant. Phase Shift Keying is also called Phase Reversal Keying. The

performance of PSK is more than ASK. PSK is a non linear modulation. PSK

needs a complicated. Synchronous circuit at the receiver. The bandwidth of PSK

is 2fm.

Circuit Diagram:

Fig: 1 Phase Shift Keying Circuit



Procedure:


2. Apply the carrier signal of amplitude7v (p-p) with frequency of 4 KHz to the

modulator input and observe the signal on the channel of the CRO.

3. Apply the modulating signal of amplitude 6V (p-p) and frequency of 0.5 KHz

to pin.11.

4. Observe the output of PSK modulator on the channel 2 of the CRO.

Model Waveforms:

(a)

(b)



(c)

Fig: 2 Waveforms of (a) Carrier signal (b) Modulating signal (c) PSK output

Inference:

Questions:

1. Drawback of DPSK compared to BPSK?

2. Write the advantage of BPSK over the BPSK?

3. What is the effect of carrier amplitude on the output?

4. What is the effect of modulating signal frequency on the output?



7. DIFFERENTIAL PHASE SHIFT KEYING MODULATION

AND DEMODULATION

Aim:

To study the various steps involved in generating the differential phase

shift keyed signal and the binary signal from the received DPSK signal

Apparatus required:

1. DPSK trainer board

2. Cathode Ray Oscilloscope (0-30MHz)

Theory:

The differentially coherent PSK signaling scheme make use of a technique

designed to get around the need for a coherent reference signal at the receiver.

In the DPSK scheme, the phase reference for demodulation is derived from the

phase of the carrier during the preceding signaling interval, and the receiver

decodes the digital information based on the differential phase.

Circuit Diagram:

Fig: 1 Differential Phase Shift Keying Circuit



Procedure:


2. Check the carrier signal and the data generator signals initially.

3. Apply the carrier signal of amplitude 6v (p-p) with frequency of1KHz to the

carrier input, the data input of amplitude 5v (p-p) with frequency of 600Hz

to the data input and bit clock of amplitude 5v (p-p) with and frequency of

1 KHz to the DPSK modulator.

4. Observe the DPSK wave of amplitude 5.6v (p-p) and frequency of 1 KHz

with respect to the input data generated signal of dual trace oscilloscope.

5. Give the output of the DPSK modulator signal to the input of demodulator,

give the bit clock output to the bit clock input to the demodulator and also

give the carrier output to the carrier input of demodulator.

6. Observe the demodulator output with respect to data generator signal.

Model Waveforms:

(a)

(b)



(c)

(d)

(e)



Fig: 2 Waveforms of (a) Carrier signal (b) Bit clock (c) Data input (d) Differential

data (e) DPSK wave

Inference:

Questions:

1. Write the advantage of DPSK?

2. What is the drawback of DPSK compared to PSK system?

3. What is the effect of carrier amplitude on the output of DPSK?



8. COMPANDING

Aim:

1. Study and analysis of µ-Law Compressor and Decompressor.

2. Study and analysis of A-Law Compressor and Decompressor.

Apparatus:

1. Companding Trainer Kit (Techbook ST2805)

2. Regulated Power Supply

3. Oscilloscope/DSO

4. Test probe

Theory:

Companding:

In digital Companding, the analog signal is first sampled and converted to a

linear PCM code and then the linear code is digitally compressed. In receiver, the

compressed PCM code is expanded and then decoded (i.e., converted back to

analog). The encoded representation of µ255 PCM code words use a sign-

magnitude format wherein 1 bit identifies the sample polarity and the remaining

bits specify the magnitude of the sample. The 7 magnitude bits are conveniently

partitioned into a 3-bit segment identifier (S) and 4-bit quantizating step identifier

(Q). Thus, the basic structure of an 8-bit µ255 PCM codeword is shown in figure.

Polarity bit (P) = 0 for positive sample values

1 for negative sample values

Compressor:

The compression process is as follows. The analog signal is sampled and

converted to a linear 14-bit sign-magnitude code(1-bit(MSB) as sign bit and other

13-bits as magnitude bits). The sign bit is transferred directly to an eight-bit

compressed code. The segment number in the eight-bit code is determined by

counting the number of leading 0s in the 13-bit magnitude portion of the linear



code beginning with the most significant bit. Subtract the number of leading 0s

(not to exceed 7) from 7. The results the segment number, which is converted to

a three-bit binary number and inserted into the eight-bit compressed code as the

segment identifier. The four magnitude bits (a, b, c and d) represent the

quantization interval (i.e., subsegments) and are substituted into the least

significant four bits of the 8-bit compressed code.

In the given table using only magnitude bits.

µ-Law Binary Encoding Table:

Essentially, the expander guesses what the truncated bits were prior to

compression.

µ-Law Binary Decoding Table:



A-Law Companding:

A-law is the CCITT recommended companding standard used across Europe.

Limiting sample values to 12 magnitude bits. In digital companding, the analog

signal is first sampled and converted to a linear PCM code and then the linear

code is digitally compressed. In receiver, the compressed PCM code is

expanded and then decoded (i.e., converted back to analog). The eight-bit

compressed code consists of a sign bit, a three-bit segment identifier, and a four-

bit quantization interval.

The compression process is as follows. The analog signal is sampled and

converted to a linear 13-bit sign-magnitude code (1-bit (MSB) as sign bit and

other 12-bits as magnitude bits). The sign bit is transferred directly to an eight-bit

compressed code. The segment number in the eight-bit code is determined by

counting the number of leading 0s in the 12-bit magnitude portion of the linear

code beginning with the most significant bit. Subtract the number of leading 0s

(not to exceed 7) from 7. The results the segment number, which is converted to

a three-bit binary number and inserted into the eight-bit compressed code as the

segment identifier. The four magnitude bits (a, b, c and d) represent the

quantization interval (i.e., subsegments) and are substituted into the least

significant four bits of the 8-bit compressed code.

A-Law Binary Encoding Table:

Essentially, the expander guesses what the truncated bits were prior to

compression.



A-Law Binary Decoding Table:

Block Diagrams:

µ-Law Companding



A-Law Companding

Procedure (µ-Law):

Step 1: Connect and switch on the power supply of ST2805.

Step 2: Select µ-Law Companding using switch. Move switch towards right.

Step3: Connect CN3 to CN1 or CN2 for input Selection. If CN3 connected to

CN1 then internally generated Signal (Sine Wave) will be selected as

input. If CN3 connected to CN2 then Dip input will be selected. So user

can select input using DIP Switches.

Step 4: User can match compressed output and decompressed output for

respective input using given table.

Observations:

Observe the internally generated input signal at TP1.

Observe the binary bits of selected input signal on led at 12 bit register linear

code.

Observe the sign bit of compressed output at TP2.

Observe the binary bits of segment identifier of compressed output at TP3,

TP4, and TP5.

Observe the binary bits of Quantization interval of compressed output at TP6,

TP7, TP8, and TP9.

Observe the analog compressed output at TP10 and binary bits on led at

compressed data.

Observe the sign bit of decompressed output at TP11.



Observe the binary bits of segment identifier of decompressed output at TP12,

TP13, and TP14.

Observe the binary bits of Quantization interval of decompressed output at

TP15, TP16, TP17, and TP18.

Observe the analog decompressed output at TP19 and binary bits on led at

Decompressed data.

Table for verification of compressor and decompressor output wrt. Input selection

using DIP switch.

Model Waveforms:

Real time output of µ-Law Compressor and decompressor on DSO when sine

wave is selected as input signal



CH1: Sine Wave as Input (TP1) CH2: Compressed Signal (TP10)

CH1: Sine Wave as Input (TP1) CH2: Decompressed Signal (TP19)

CH1: Compressed Signal (TP10) CH2: Decompressed Signal (TP19)

Procedure (A-Law):

Step 1: Connect and switch on the power supply of ST2805.

Step 2: Select A-Law Companding using switch. Move switch towards left.

Step 3: Connect CN3 to CN1 or CN2 for input Selection. If CN3 connected to

CN1 then internally generated Signal (Sine Wave) will be selected as

input. If CN3 connected to CN2 then Dip input will be selected. So user

can select input using DIP Switches.

Step 4: User can match compressed output and decompressed output for

respective input using given table.



Observations:

Observe the internally generated input signal at TP1.

Observe the binary bits of selected input signal on led at 12 bit register linear

code.

Observe the sign bit of compressed output at TP20.

Observe the binary bits of segment identifier of compressed output at TP21,

TP22, and TP23.

Observe the binary bits of Quantization interval of compressed output at TP24,

TP25, TP26, and TP27.

Observe the analog compressed output at TP28 and binary bits on led at

compressed data.

Observe the sign bit of decompressed output at TP29.

Observe the binary bits of segment identifier of decompressed output at TP30,

TP31, and TP32.

Observe the binary bits of Quantization interval of decompressed output at

TP33, TP34, TP35, and TP36.

Observe the analog decompressed output at TP37 and binary bits on led at

Decompressed data.

Table for verification of compressor and decompressor output wrt input

selection using DIP switch.



Model Waveforms:

Real time output of A-Law Compressor and decompressor on DSO when sine

wave is selected as input signal

CH1: Sine Wave as Input (TP1) CH2: Compressed Signal (TP28)

CH1: Sine Wave as Input (TP28) CH2: Decompressed Signal (TP37)

CH1: Compressed Signal (TP1) CH2: Decompressed Signal (TP37)



9. LINEAR BLOCK CODE - ENCODER & DECODER

Aim:

To Study the Hamming Code 7-bit Generation.

Apparatus:

1. Linear Block Code- Encoder & Decoder Trainer Kit (Scientech 2121A &

2121B)

2. 2 mm Banana Cable

3. Regulated Power Supply

Theory:

Error Detection and Correction:

Error detection is the ability to detect the presence of errors caused by noise or

other impairments during transmission from the transmitter to the receiver. Error

correction is the additional ability to reconstruct the original, error-free data.

There are two basic ways to design the channel code and protocol for an error

correcting system.

Linear block codes:

Linear block codes are conceptually simple codes that are basically an extension

of single-bit parity check codes for error detection. A single-bit parity check code

is one of the most common forms of detecting transmission errors. This code

uses one extra bit in a block of n data bits to indicate whether the number of 1s in

a block is odd or even. Thus, if a single error occurs, either the parity bit is

corrupted or the number of detected 1s in the information bit sequence will be

different from the number used to compute the parity bit: in either case the parity

bit will not correspond to the number of detected 1s in the information bit

sequence, so the single error is detected. Linear block codes extend this notion

by using a larger number of parity bits to either detect more than one error or

correct for one or more errors. Unfortunately linear block codes, along with

convolutional codes, trade their error detection or correction capability for either

andwidth expansion or a lower data rate, as will be discussed in more detail

below. We will estrict our attention to binary codes, where both the original

information and the corresponding code consist of bits taking a value of either 0

or 1.



Block Diagram:

Procedure:

1. Connect the power supply mains cord to the Scientech 2121A and

Scientech 2121B but do not turn ON the power supply until connections

are made for this experiment.

2. Keep default/manual switch in Manual mode.

3. There are some conditions regarding H-Matrix selection manually which

are:

a. Any row should not be identically selected like there should not all 1’s

or all 0’s.

b. Each row selection should be different from other row.

c. The matrix should be so chosen that all the rows are distinct and

consist of at least three 1’s in them.

4. Switch ‘On’ the power supply and press reset button.

5. Check the clock pulse of 2 KHz on Oscilloscope at given test point.

6. At Scientech 2121A Block Code Encoder unit now select the data at

seven Segment display with the help of BCD (binary coded decimal)

switch.

7. Check the data at seven segment display and its binary equivalent (d3,

d2, d1, d0), in the Code Word Generator block T where bit pattern is

selected in the form of 8, 4, 2, 1 format.

8. Now set the H matrix are per the condition given in step 3. In Observation

Table 3.1, some example sets are given (Set 1, Set2, Set3 and Set4).

You can set your own matrix or you can choose any set from example

sets and select the H Matrix as per the table.



9. After that check the H matrix in the form of H= [Ik] [P]; Identity matrix and

Parity matrix corresponding to the selected set as given in the

Observation Table 3.1.

10. Check the massage signal in the form of (d3, d2, d1, d0, p3, p2, p1) and

verify the status of ‘Parity Bits’ (p3, p2, p1) as per the equations given for

parity generation (see bservation Table 3.1).

11. Connect 2mm patch cords between horizontal bit stream and p/s block as

per the connections diagram.

12. Observe the bit pattern output of codeword Generator at vertical 7-bit

stream.

13. Now connect the Data output to the Data In of 2121B which is block code

decoder.

14. Now connect the clock and ground of 2121A to 2121B via a 2mm patch

cord.

15. Now set the H-Matrix section of 2121B Block code Decoder unit as per

the same set what you have chose for 2121A Encoder unit. Refer the

Observation Table 3.1.

16. Now first set Data ‘0’ at Encoder unit and press reset switch until you get

same decoded data on LED display and as well as at the seven segment

display in numeric form. Once you get the same data 0 at decoder unit

you can vary BCD switch to get the sequential data from 0-9.

17. For any selected data from 0-9, check the H matrix in the form of H= [P]

[Ik]; Parity matrix and Identity matrix as given in the Observation Table

3.1

18. Also check the massage signal in vertical matrix ‘R’ in the form of (d3, d2,

d1, d0, p3, p2, p1) and check the status of Syndrome Em. As there is no

error in the bits it will show (0 0 0).

19. Check the corrected code word and match it with the code word of

Encoder unit.

20. Also check the Decode Bits (d3, d2, d1, d0) and match with the data at

Encoder unit.



Equations for Parity Generation:

Code Word Generator:

Observation Table:



10. BINARY CYCLIC CODE- ENCODER AND

DECODER

Aim:

To study the Binary Cyclic Encoding and Decoding and to verify the input and

output.

Apparatus Required:

1. ST2120 Error Detection & Correction Cyclic Codes Kit.

2. 2 mm Banana cable

3. Cathode Ray Oscilloscope

Theory:

The linear code C of length n is a cyclic code if it is invariant under a cyclic Cyclic

code shift:

if and only if

As C is invariant under this single right cyclic shift, by iteration it is invariant under

any number of right cyclic shifts. As a single left cyclic shift is the same as n-1

right cyclic shifts, C is also invariant under a single left cyclic shift and hence all

left cyclic shifts. Therefore the linear code C is cyclic precisely when it is invariant

under all cyclic shifts.

Encoder Diagram:

Fig.: Encoder for (7, 4) Cyclic Code Generator by g(x) =1+x2+x3



Table: Register Contents of Encoder

Block Diagram:

Procedure:

1. Connect the power supply mains cord to the ST2120, but do not turn ON

the power supply until connections are made for this experiment.

2. From Clock Section, connect 16 KHz Clock output to Clock Generator

(Clock input).

3. Connect the Data Clock of clock generation section to Data Clock of Data

Source.

4. Connect the Data Out of Data Source to Data In of Cyclic Encoder.

5. Connect the Code Clock of Clock Generation section to Code Clock of

Cyclic Encoder.



6. Switch ‘On’ the power supply and oscilloscope.

7. Observe the bit pattern of Code Word output of Cyclic Encoder. The bit

stream is a repeating 8 bit serial sequence of the input data selected

through BCD switches.

8. Change input data through BCD switches and observe the output of

Cyclic Encoder.

9. Now connect the Code Word output of Cyclic Encoder to Decoder and

output of Decoder to Input of display.

10. Change the clock input to 8 KHz to 1 KHz and then observe the data

output.

Observations:

1. The data output of Data Source is a repeating sequence of input data

selected through BCD switches.

2. The 8 bits of output data are binary coded decimal values on BCD

switches.

3. The output data rate of Data Sources is selected through the input data

clock.

4. Encoded and decoded data stream is same as observed on segmental

display.

Result:



1. AMPLITUDE SHIFT KEYING

Aim:

To generate the waveforms of Amplitude Shift Keying.

Apparatus required:

Name of the Apparatus Specifications/Range Quantity

Resistors 1.2KΩ, 3

Transistor BC 107 2

CRO 30MHz 1

Function generator 0-1MHz 1

Regulated Power Supply 0-30V, 1A 1

CRO Probes --- 1

Theory:

The binary ASK system was one of the earliest form of digital modulation used in

wireless telegraphy. In an binary ASK system binary symbol 1 is represented by

transmitting a sinusoidal carrier wave of fixed amplitude Ac and fixed frequency fc

for the bit duration Tb where as binary symbol 0 is represented by switching of the

carrier for Tb seconds. This signal can be generated simply by turning the carrier

of a sinusoidal oscillator ON and OFF for the prescribed periods indicated by the

modulating pulse train. For this reason the scheme is also known as on-off shift

testing. Let the sinusoidal carrier can be represented by Ec (t) =Ac cos (2Πfct)

then the binary ASK signal can be represented by a wave s(t) given by S(t) =

Accos(2Πfct), symbol 1 ASK signal can be generated by applying the incoming

binary data and the sinusoidal carrier to the two inputs of a product modulator.

The resulting output is the ASK wave. The ASK signal which is basically product

of the binary sequence and carrier signal has a same as that of base band signal

but shifted in the frequency domain by ±fc. The band width of ASK signal is

infinite but practically it is 3/Tb.



Circuit Diagram:

Fig: 1 Amplitude Shift Keying Circuit

Procedure:

1. Connect the circuit as per the circuit diagram.

2. Switch on the supply.

3. Apply the sinusoidal carrier signal from the function generator to the collector

terminal of the transistor with 10v (p-p) amplitude and10KHz frequency.

4. Apply the Binary signal from the pulse generator to the Base terminal of the

transistor with 5v (p-p) amplitude and 2 KHz frequency.

5. Observe the output of ON/OFF keying from ASK modulator circuit using CRO.

6. Now vary the Amplitude and frequency of the binary signal and observe the

output changes of ASK modulated Wave & compare it with the modulating

data signal applied to the modulator input.

Model Waveforms:

(a)



(b)

Fig: 2 Waveforms of (a) Carrier signal (b) Data signal & ASK wave

Questions:

1. Why we are not preferred ASK over PSK and FSK?

2. What is another name of ASK modulation scheme?

3. What is the Effect of carrier amplitude, frequency, V cc on the output?



2. TIME DIVISION MULTIPLEXING AND

DEMULTIPLEXING

Aim:

1. To study the 4 channel analog multiplexing and de multiplexing

2. To study the effect of sampling frequency on output signal

characteristics.

Apparatus Required:

1. Time Division Multiplexing & De multiplexing (Digital) Trainer Kit

(Or)

a. Data Generator IC 7490

b. 8-1 Multiplexer IC 74151

c. 3-bit Address Generator IC 74163

d. 1-8 De multiplexer IC 74138

e. Clock Generator IC 555

f. DC Regulated Power Supply +5V.

2. Set of Patch chords

3. Dual Trace Cathode Ray Oscilloscope.

Theory:

8 TO 1 MULTIPLIXER

For this Multiplexing process, 8 bit Digital Multiplexer 74151 IC is used. L1 to L8

are the 8 input channels, data Multiplexed is given to them. A0, A1 and A2 are

the address data finder. Depending upon the address data at any instant, input

channel corresponding to the address location is multiplexed. For example, if the

address bit is 101, channel L6 is multiplexed at that instant.

1 TO 8 DE MULTIPLEXER

For this De multiplexing process, 74138 digital De multiplexer IC is used. L1 to

L8 are the corresponding. 8-channel outputs, which are in synchronization to the

multiplexed channels ar multiplexing process. The address data generator for

both Multiplexer and De multiplexer is same. So both Multiplexer and

Demultiplexer are in synchronization.



Block Diagram:

Procedure:

1. Switch ‘ON’ the experimental kit.

2. Observe the MSB bit of the address generator to one channel of a dual

trace CRO and trigger the CRO w.r.t . the same channel.

3. Observe the output of the 8-1 line multiplexer on the second channel of

the CRO.

4. Apply a low (GND) signal to the 8 multiplexing input one by one and

observe how the total time is divided by each channel w.r.t. the address

generator. Fig.2 shows the working principle.

5. Now connect the 8-1 line multiplexer to the 1-8 line demultiplexer.

6. Give any data available from the data generator to any multiplexing input

and observe the output at corresponding demultiplexer output.

Ex. Suppose if we are giving input at L1 input, observe the output of the

Demultiplexing output at L1 only.

7. Now connect different data to the different inputs and observe the outputs

at corresponding demultiplexed outputs and compare the multiplexing

inputs and corresponding demultiplexed outputs.

8. Data provided at D1 is not multiplexed because its frequency is greater

than the Address generators frequency (sampling theorem).



Model Output Waveforms:



3. BINARY AMPLITUDE SHIFT KEYING

MODULATION AND DEMODULATION

Aim:

To Generate the Amplitude Shift Keying Modulation and Demodulation signals

using MATLAB 7.0.4

Apparatus required: Software MATLAB7.0.4

Algorithm:

Step1: The binary sequence is taken as input into a variable.

Step2: The carrier signal with required frequency is selected.

Step3: The required carrier wave is generated.

Step4: The input binary data is given to serial to parallel converter.

Step5: The parallel sequence is converted to analog signal using D/A converter.

Step6: The analog signal is combined with the generated carrier signal.

Step7: The required ASK signal is generated.

Step8: (Detection) The ASK wave is combined with the carrier wave.

Step9: The output is given to the comparator circuit.

Step10: The output is converted into digital by using A/D converter.

Step11: This gives us the binary sequence as the output of the demodulator.

Block diagrams:

BASK MODULATOR BASK DEMODULATOR

Program:

%Matlab program for ASK wave

clc;

clf;

clear all;



close all;

b=input('enter binary data:');

fc=4000;

t=linspace(0,1/1000,50);

ec=cos(2*pi*fc*t);

ook=[ ];

bin=[ ];car=[ ];

for i=1:length(b);

ook=[ook,b(i)*ec];

bin=[bin,b(i)*ones(1,50)];

car=[car,ec];

end

%ASK detection

balout=[ ];%sync det output

demod=[ ];%demodulation output

for i=1:length(ook);

balout=[balout,car(i)*ook(i)];

end;

for i=1:50:length(ook);

if sum(balout(i):balout(i+49))>0.5

demod=[demod,ones(1,50)];

else demod=[demod,zeros(1,50)];

end;

end;

%ploting the graph

subplot(5,1,1);

plot(ec,'linewidth',2);

title('carrier');

xlabel('time');

ylabel('amplitude');

subplot(5,1,2);

plot(0:length(bin)-1,bin,'k','linewidth',2);

title('input data');

xlabel('time');




subplot(5,1,3);

plot(ook,'r-','linewidth',2);

title('modulated data');

xlabel('time');


subplot(5,1,4);

plot(balout,'r-','linewidth',2);

title('balanced modulator data');

xlabel('time');


subplot(5,1,5);

plot(demod,'r-','linewidth',2);

title('demodulated output data');

xlabel('time');





4. BINARY PHASE SHIFT KEYING


Aim:

To Generate the Binary Phase Shift Keying Modulation and Demodulation

signals using MATLAB 7.0.4

Software required: MATLAB7.0.4

Algorithm:

Step1: The binary sequence is taken as input into a variable.

Step2: This binary data is converted into the polar form either in RZ or NRZ form.

Step3: The carrier signal with required frequency is selected.

Step4: The required carrier wave is generated.

Step5: Both carrier and the binary data is given to the balanced modulator circuit.

Step6: The balanced modulator output gives us the BPSK waveform.

Step7: (Demodulator) The BPSK wave is given to the multiplier along with the

carrier signal.

Step8: The multiplier output is passed through the low pass filter.

Step9: This gives us the binary sequence as the output of the demodulator.

Block diagram:



Program:

%Matlab program for BPSK wave

clc;

clf;

clear all;

close all;


fc=4000;

t=linspace(0,1/1000,50);

ec=cos(2*pi*fc*t);

pskout=[ ];

bin=[ ];car=[ ];

for i=1:length(b);

if b(i)==1

pskout=[pskout,b(i)*ec];

else pskout=[pskout,(b(i)-1)*ec];

end;


car=[car,ec];

end

%BPSK detection

balout=[ ];%sync det output

demod=[ ];%demodulation output

for i=1:length(pskout);

balout=[balout,car(i)*pskout(i)];

end;

for i=1:50:length(pskout);



if sum(balout(i):balout(i+49))>0


else demod=[demod,zeros(1,50)];

end;

end;

%ploting the graph

subplot(5,1,1);

plot(ec,'linewidth',3);

title('carrier');

xlabel('time');


subplot(5,1,2);



xlabel('time');


subplot(5,1,3);

plot(pskout,'r-','linewidth',3);

title('BPSK output');

xlabel('time');


subplot(5,1,4);

plot(balout,'r-','linewidth',3);

title('balanced modulater data');

xlabel('time');


subplot(5,1,5);


title('Demodulated output');

xlabel('time');




5. FREQUENCY SHIFT KEYING MODULATION

AND DEMODULATION

Aim:

To Generate the Frequency Shift Keying Modulation and Demodulation signals

using MATLAB 7.0.4


Algorithm:

Step1: The binary input bit sequence is taken and stored into a variable.

Step2: The carrier frequencies for both low frequency and high frequency bits are

selected and the respective carriers are generated.

Step3: The input bits are given separately to two level shifters as one as the

direct bits and for another through the inverter.

Step4: Both the level shifter outputs are multiplied with the respective carrier

(High frequency carrier for bit ‘1’ and low frequency carrier for bit’0’).

Step5: Both the multiplier outputs are added up to get the output FSK wave form.

Step6: In the FSK detector process at first the input FSK wave is given to the

band pass filter and then passed through the limiting circuit.

Step7: The limiting circuit output is given to the FM detector circuit.

Step8: The FM detector output is then passed through low pass filter which acts

as an integrator that adds up all the values of the signal.

Step9: The low pass filter output is then given to the decision device which gives

us the output as the input bit stream.

Block diagrams:

Binary FSK Transmitter:



Binary FSK Receiver

Program:

%Matlab program for FSK wave

clc;

clf;

clear all;

close all;


f1=4000;f2=12000;

t=linspace(0,1/4000,50);

ec1=cos(2*pi*f1*t);

ec2=cos(2*pi*f2*t);

fskout=[ ];

bin=[ ];car1=[ ];car2=[ ];

for i=1:length(b);

if b(i)==1

fskout=[fskout,b(i)*ec2];

else fskout=[fskout,(1-b(i))*ec1];

end;


car1=[car1,ec1];

car2=[car2,ec2];

end

%FSK detection

bal1=[ ];bal2=[ ];demod=[ ];



for i=1:length(fskout);

bal1=[bal1,car1(i)*fskout(i)];

bal2=[bal2,car2(i)*fskout(i)];

end;

for i=1:50:length(fskout);

sum1=0,sum2=0;sum=0;

for i=i:i+49

sum1=sum1+bal1(i);

sum2=sum2+bal2(i);

end;

sum=-sum1+sum2;

if sum>0


else

demod=[demod,zeros(1,50)];

end;

end;

%ploting the graph

subplot(5,1,1);

plot(car1,'linewidth',3);

title('carrier 1');

xlabel('time');


subplot(5,1,2);


title('carrier 2');

xlabel('time');


subplot(5,1,3);



xlabel('time');


subplot(5,1,4);

plot(fskout,'r-','linewidth',3);

title('FSK output');

xlabel('time');




subplot(5,1,5);


title('Demodulated output');

xlabel ('time');

ylabel ('amplitude');




6. QUADRATURE PHASE SHIFT KEYING


Aim:

To Generate the Quadrature phase Shift Keying Modulation and Demodulation

signals using MATLAB 7.0.4


Algorithm:

Step1: The binary bit sequence is first taken into a variable.

Step2: The suitable carrier frequency is selected and two carriers one with zero

phase and other with 90o phase are generated.

Step3: The input bit stream is given to the de-multiplexer and is decoded.

Step4: The de-multiplexer output is divided into two and one is multiplied with in

phase component and the other with the out of phase component.

Step5: Both the multiplexer outputs are taken and are added together which

gives us the QPSK output wave.

Step6: In the de-modulation process the input QPSK wave is multiplied with both

the in phase and out phase components by an multiplier.

Step7: Both the outputs are passed through an low pass filter which acts as an

integrator that adds up all the values of the signal.

Step8: The low pass filter outputs are given to the decision device which assigns

the values to the wave.

Step9: The decision device output is given to the multiplexer circuit which gives

the output as the input bit stream.

Block Diagrams:



Program:

%Matlab program for QPSK wave

clc;

clf;

clear all;

close all;


fc=4000;

t=linspace(0,1/4000,50);

ec2=cos(2*pi*fc*t);

ec1=sin(2*pi*fc*t);

qpskout=[ ];bin=[ ];car1=[ ];car2=[ ];be=[ ];bo=[ ];bal1=[ ];bal2=[ ];

for i=1:length(b);


car1=[car1,ec1];

car2=[car2,ec2];

if mod(i,2)==0

if b(i)==0

be=[be,-ones(1,100)];

else be=[be,ones(1,100)];

end;

bal1=[bal1,be(i*50-1)*ec1,be(i*50-1)*ec1];

else

if b(i)==1

bo=[bo,ones(1,100)];

else bo=[bo,-ones(1,100)];



end;

bal2=[bal2,bo(i*50-1)*ec2,bo(i*50-1)*ec2];

end;

end;

for i=1:2:length(b)

if b(i)==0 && b(i+1)==0

qpskout=[qpskout,-ec1-ec2,-ec1-ec2];

elseif b(i)==0 && b(i+1)==1

qpskout=[qpskout,-ec1+ec2,-ec1+ec2];

elseif b(i)==1 && b(i+1)==0

qpskout=[qpskout,ec1-ec2,ec1-ec2];

else

qpskout=[qpskout,ec1+ec2,ec1+ec2];

end;

end;

%ploting the graph

subplot(4,1,1);

plot(bin,'linewidth',3);

title('binary data');

xlabel('time');


subplot(4,1,2);

plot(be,'r','linewidth',3);

title('even data');

xlabel('time');


subplot(4,1,3);

plot(bo,'g','linewidth',3);

title('odd data');



xlabel('time');


subplot(4,1,4);


title('carrier 1');

xlabel('time');


figure;

subplot(4,1,1);


title('carrier2');

xlabel('time');


subplot(4,1,2);

plot(bal1,'r','linewidth',3);

title('bal mod 1 data');

xlabel('time');


subplot(4,1,3);

plot(bal2,'g','linewidth',3);

title('bal mod 2 data');

xlabel('time');


subplot(4,1,4);

plot(qpskout,'linewidth',3);

title('modulated');

xlabel('time');


%demodulation of qpsk

demod1=[ ];demod2=[ ];demod=[ ];synd1=[ ];synd2=[ ];

for i=1:length(qpskout);



synd1=[synd1,car1(i)*qpskout(i)];

synd2=[synd2,car2(i)*qpskout(i)];

end;

for i=1:100:length(qpskout);

sum1=0;sum2=0;

for i=i:i+99

sum1=sum1+synd1(i);

sum2=sum2+synd2(i);

end;

if sum1 > 0

demod1=[demod1,ones(1,50)];

else

demod1=[demod1,zeros(1,50)];

end;

if sum2 > 0

demod2=[demod2,ones(1,50)];

else

demod2=[demod2,zeros(1,50)];

end;

end;

demod1

demod2

for i=1:50:length(demod1);

demod=[demod,demod1(i:i+49),demod2(i:i+49)];

end;

%ploting the graph

figure;

subplot(3,1,1)

plot(synd1,'linewidth',3);

title('sync detector 2');

xlabel('time');


subplot(3,1,2)

plot(synd2,'r','linewidth',3);

title('sync detector 2');

xlabel('time');




subplot(3,1,3)

plot(demod,'g','linewidth',3);

title('demodulated');

xlabel('time');





APPENDIX

Name of the component

Important Specifications Pin Diagram

74LS00

Supply Voltage Min – 4.75V

Supply Voltage Max – 5.25V

Operating temperature Range – 0oC to

+70oC

Output Current High max - -0.4mA

Output Current Low Max- 80mA

74LS08




+70oC

Output Current High Max - -0.4mA


74LS74




+70oC

Power supply current - 8.0mA

74138




+70oC

Output Current High Max - -0.4mA


74194




+70oC


Output Current Low max - 80mA



74LS374



High level input voltage min – 2V

Low level input voltage max– 0.8V

High level output current max – -2.4mA

Low level output current max- 24mA


+70oC

74151




+70oC



74LS163




+70oC





74LS164




+70oC



74165



Free air operating temperature- 0oC to

+70oC

Supply Current max – 36mA

Clock frequency – 25MHz

Pulse width (Clock) – 25ns

74168




+70oC

Power supply current – 34mA



74193




+70oC

Clock frequency – 25MHz

Supply Current max – 34mA

74LS86




+70oC



74LS90




+70oC



LM311

Input voltage Range - -14.5V to 13V

Voltage gain – 40V/mV

Saturation voltage – 1.5V

Positive Supply Current – 7.5mA

Negative Supply Current – 5mA

LM324

Wide power supply rating – 3V to 32V


+70oC

Storage temperature - (-65oC to +150oC)



DAC0800

Supply Voltage – 5V

Operating temperature Range – (-55oC to

+125oC)

Power Dissipation -500mW

Input current – 5mA

Storage temperature - (-65oC to +150oC)

ADC0800

Supply voltage – ±5V

Clock range – 50 to 800KHz

Operating temperature Range – (-55oC to

+125oC)

Power supply current – 20mA

CD4051

Supply voltage - +5V to 18V

Operating temperature Range – (-40 oC to

+80oC)

Storage temperature - (-65 oC to +150oC)

Power dissipation – 700mW

CD4052

Supply voltage - +5V to 18V

Operating temperature Range – (-40 oC to

+80oC)

Storage temperature - (-65 oC to +150oC)


8038

Simultaneous outputs – sine wave

Square wave and Triangle

Low distortion – 1%

High linearity – 01%

Easy to use – 50% reduction in external

components

Wide frequency range of operation 0.001

Hz to 1.0Mhz

Variable duty cycle – 2% to 98%



Supply voltage - ±18V or 36V total


Input voltage (any pin) Not to exceed

supply voltages

Input current (pins 4 and 5 ) – 25mA

Operating temperature range: 55oC to

+125oC

µA741

Supply Voltage ±22V

Power Dissipation 500mW

Differential input voltage ±30V

Input voltage ±15V

Operating Temperature -55o to +125oC

Storage Temperature range -55o to

+150oC

TL084

Supply Voltage ±18V

Power Dissipation 680mW

Input voltage ±15V

Operating Temperature -0o to +70oC

Storage Temperature range -65o to

+150oC

IC 555

Operating tem :SE 555 -55oC to 125oC

NE 555 0o to 70oC

Supply voltage :+5V to +18V

Timing :µSec to Hours

Sink current :200mA

Temperature stability :50 PPM/oC change

in temp or 0-005% /oC.



REFERENCES:

• Digital Communications - by John Proakis, TMH

• Communication Systems - by Simon Hay kin, John Wiley

• Communication Systems –by Sanjay Sharma

• Digital Communication Fundamentals & Applications –by

Bernard Sklar

• Principles of Digital Communications – by P.Chakrabarthi

• Theory and Design of Digital communication Systems

–by Tri T HA

DIGITAL COMMUNICATIONSece.gecgudlavalleru.ac.in/pdf/manuals/DC-Labwithout... · 2015-07-27 ·...

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