DIGITAL COMMUNICATIONS

&

MICROWAVE ENGINEERING

LAB MANUAL

IV BTECH, ECE

1ST SEMESTER

SYLLABUS

DIGITAL COMMUNICATIONS LAB

List of Experiments:

1. AMPLITUDE SHIFT KEYING

2. FREQUENCY SHIFT KEYING

3. PHASE SHIFT KEYING

4. DIFFERENTIAL PHASE SHIFT KEYING

5. PULSE CODE MODULATION AND DEMODULATION

6. DELTA MODULATION AND DEMODULATION

7. TIME DIVISION MULTIPLEXING AND DEMULTIPLEXING

MICROWAVE ENGINEERING LAB

List of Experiments:

1. REFLEX KLYSTRON CHARACTERISTICS

2. GUNN DIODE CHARACTERISTICS

3. ATTENUATION MEASUREMENT

4. MEASUREMENT OF FREQUENCY AND WAVELENGTH

5. DIRECTIONAL COUPLER CHARACTERISTICS

6. VSWR MEASUREMENT

1. AMPLITUDE SHIFT KEYING

AIM: To generate the ASK signal for given binary data and also modulate the transmitted

binary data.

APPARATUS:

1. SL100 transistor 2. Op-amp 3. 1K resistors 4. 100 K resistors 5. +5v power supply 6. Function generator 7. CRO 8. Bread board 9. Connecting wires, probes

THEORY:

ASK is one in which the amplitude of a carrier is switched between two values i.e, on

and off. The resultant waveform consists of on pulses representing binary 1 and off pulses

representing binary 0.The binary ASKS signaling scheme was one of the earliest forms of

digital modulation used in wireless telegraphy at the beginning of this century. It is the

simplest form of digital modulation & serves as a useful model for introducing certain

concepts.

CIRCUIT DIAGRAM:

PROCEDURE:

1. Connections are made as per the circuit diagram. 2. Set function generator (square wave) of 1v, 1 kHz for input modulating signal

then switch ON the power supply.

3. Now observe ASK output waveform on the CRO. 4. Plot the observed waveform on the graph.

OUTPUT WAVEFORMS:

VIVA QUESTIONS:

1. What is the other name of ASK signal? 2. Draw the o/p waveform of ASK? 3. What are the demodulation techniques of BASK? 4. Draw the power spectral density of ASK signal? 5. Write the standard equation of ASK signal? 6. What is the transmission BW of an ASK signal? 7. What are the differences between BASK&FSK? 8. What the advantages are of ASK over an AM?

RESULT: Hence the ASK signal is generated form a given binary input &also the

demodulated signal of an ASK is obtained.

2. FREQUENCY SHIFT KEYING

AIM: To generate the frequency shift keying signal for a given binary data and also

demodulate the original data input.

APPARATUS:

1. PCM transmitter trainer kit.

2. Data formatting & carrier modulation transmitter trainer kit

3. Data reformatting & carrier demodulation receiver trainer kit

4. C.R.O

5. Connecting wires and probes

THEORY:

In FSK, the waveform is generated by switching the frequency of the carrier between

two values corresponding to the binary information which is to be transmitted. Here the

carrier frequency varies from lowest to highest point i.e. carrier swing is known as Frequency

shift keying. FSK signaling schemes find a wide range of applications in low speed digital

data transmission systems.

BLOCK DIAGRAM:

PROCEDURE: (PCM TX)

1. The following connections are made at PCM TX

D.C 1 to CH.0

CH.0 to CH.1 2. The following conditions should be there

Mode switch - fast mode

Sync switch - ON

Switched faults OFF

Error check code OFF

TX .T0 - CRO external triggering.

Adjust D.C1 until the 7 bit code is displayed on A/D converter LED.

Then observe the PCM TX data output.

(DATA FORMATTING AND CARRIER MOD.&DEMOD. TRAINER)

1. From PCM Tx clock to Tx clock input terminal. 2. PCM output to Tx data input. 3. Then connect NRZ (L) output, carrier of 1.44MHz is applied at modulating

input& carrier inputs of Modulator I.

4. Now invert the NRZ (L) output, then the inverted output and 960MHz carrier signal both are given to Modulator II.

5. Both the outputs of Modulator I& Modulator II are given to a summing amplifier then we observe the FSK output across the summing amplifier

output terminal.

6. For demodulation of this FSK signal, connect this FSK output to FSK demodulator input terminal and the output of this FSK demodulator block is

given to the input of LPF.

7. The LPF output is given to the input of voltage comparator, then we observe the demodulated output across the output terminal of the voltage comparator

of DF&CDM trainer kit.

PRECAUTIONS:

1. Connections must be tight.

2. Carefully draw the output waveform

OUTPUT WAVEFORMS:

.VIVA QUESTIONS:

1. Define Binary FSK signal?

2. What is meant by carrier swing?

3. Define Frequency deviation of FSK signal?

4. What are the advantages of this FSK signal?

5. Give the differences between FSK & FM?

RESULT: Hence obtained the FSK signal for a given input data and also obtained the

demodulated data the given FSK signal.

3. PHASE SHIFT KEYING

AIM: To generate the phase shift keying signal for the given binary data & to demodulated to

receive the transmitted binary data.

APPARATUS:

1. PCM trainer kit 2. PSK trainer kit 3. CRO

THEORY:

Phase shift keying or discrete phase modulation is another technique available for

communicating digital information over band pass channels. In PSK signaling schemes the

waveforms s1(t) = -Acoswct & S2(T) = Acoswct are used to convey binary digits 0& 1

respectively. The binary PSK waveform Z (t) can be described by, Z (t) = D (t) Acoswct .

Where D (T) is a random binary waveform with period Tb& levels -1&1. The only difference

b/w the ASK&PSK waveform is that in the ASK scheme the carrier is switched on &off

whereas in the PSK scheme the carrier is switched b/w levels +A & -A.

The differentially coherent PSK signaling scheme makes use of a clever technique

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

CIRCUIT DIAGRAM:

PROCEDURE:

1. Assume that the following connections are made on PCM TX. 2. DC1 o/p is connected to channel -0 i/p. 3. Channel-0 i/p is connected to chennal-1 i/p 4. Mode switch is kept in fast mode. 5. Synch button is kept in on position. 6. Switched faults should be in OFF position. 7. Error check OFF (00). 8. TX to be connected to CRO external triggering. 9. Adjust the DC1 until the 7bit code displayed on A/D converter. 10. LED is observing the PCM o/p at PCM Tx o/p.

OUTPUT WAVEFORMS:

VIVA QUESTIONS:

1. What is the bandwidth requirement of BPSK?

2. What is the expression for error probability of BPSK reception using coherent

matched filter detection?

3. What are the draw backs of BPSK?

4. Draw the Power spectral density of BPSK?

5. What are the major differences between DPSK&BPSK?

6. What are the advantages of BPSK over a PSK signal?

RESULT: Hence the PSK output of a given binary data is generated and also the

demodulated output of a PSK signal is obtained.

4. DIFFERENTIAL PHASE SHIFT KEYING

AIM: To generate differentially phase shift keying signal and also demodulate the original

binary data.

APPARATUS:

1. DPSK modulation and demodulation trainer kit 2. CRO 3. Patch cards

THEORY:

We may view DPSK as the non-coherent vision of PSK. It eliminates the need for

adjustment coherent reference signal at the receiver by connecting two basic operations at the

transmitter.

1. Differential encoding at the transmitter. 2. Phase shift keying

Hence differential encoding means the given input data will be done EX-OR

operation with the previous encoded bit. Now the process of Phase shift keying will be done

for both differentially encoded data and the carrier signal.

BLOCK DIAGRAM:

DPSK MODULATOR

DPSK DEMODULATOR

PROCEDURE:

(MODULATOR)

1. In this DPSK trainer kit there three signal generators one is for carrier signal and the second is for clock signal and another is for electrical representation of data

bits, so give the carrier signal to CARRIER IN terminal OF MODULATOR.

2. Give the clock signal to CLOCK IN terminal and there are four different data bit combinations are available in the form of (D1, D2, D3 and D4) so connect one

of this input data signal to DATA IN terminal of the modulator.

3. Take the differential data output across the DIFF.OUT terminal of the modulator.

4. And then observe the differentially phase shifted carrier signal across the output of the modulator i.e. DPSK output.

(DEMODULATOR)

5. Connect DPSK output to DPSK input terminal of the demodulator block. 6. Give the clock signal to CLOCK IN terminal and also connect the carrier signal

to CARRIER IN terminal.

7. Ground both the modulator and demodulator circuits. 8. Observe the DPSK demodulated output across the DEMOD.OUT terminal of

the demodulator circuit.

OUTPUT WAVE FORMS:

PRECAUTIONS:

1. Connections should be tight. 2. Note the output wave forms carefully.

VIVA QUESTIONS:

1. What is the difference between PSK&DPSK?

2. What is the band width requirement of a DPSK?

3. Explain the operation of DPSK detection?

4. What are the advantages of DPSK?

5. What is meant by differential encoding?

6. In Differential encoding technique which type of logic gates are used?

RESULT: Hence the differential phase shift keying output of a given binary data and the

corresponding demodulated outputs are observed.

5. PULSE CODE MODULATION & DEMODULATION

AIM: To convert an analog signal into a pulse digital signal using PCM system and to

convert the digital signal into analog signal using PCM demodulation system.

APPARATUS:

1. PCM transmitter trainer. 2. PCM receiver trainer. 3. CRO and connecting wires.

THEORY:

In the PCM communication system, the input analog signal is sampled and these

samples are subjected to the operation of quantization. The quantized samples are applied to

an encoder. The encoder responds to each such a sample by generation unique and

identifiable binary pulse. The combination of quantize and encoder is called analog to digital

converter. It accepts analog signal and replaces it with a successive code symbol, each

symbol consists of a train of pulses in which the each pulse represents a digit in arithmetic

system.

When this digitally encoded signal arrives at the receiver, the first operation to be

performed is separation of noise which has been added during transmission along the

channel. It is possible because of quantization of the signal for each pulse interval; it has to

determine which of many possible values has been received.

BLOCK DIAGRAM:

OUTPUT WAVEFORM:

PROCEDURE:

1. The two inputs of function generator are connected to channel -0 and channel-1 simultaneously that is DC1 output to channel -0 and DC2 to channel-1.

2. With the help of oscillator DC1 output is adjusted to 0 volts. 3. Transmitter and receiver are connected by the synchronization of clock pulses and

by connecting ground transmitter to ground receiver.

4. The transmitter is connected to the input of receiver to go the original signal at the receiver output.

5. After connection is made the inputs channel 1 and channel 0 are noted. The sampled output of bit channels are taken by connecting DC1 output to channel 0

and DC2 output to channel-1.

6. The phase shift of a channel can be obtained by comparing the input and output of channels at the transmitter block.

7. Thus the output of transmitter can be noted down and input of receiver is similar to that.

8. The receiver output signals are noted down at channel 0 and channel 1 of the receiver block.

VIVA QUESTIONS:

1. What is the expression for transmission bandwidth in a PCM system?

2. What is the expression for quantization noise /error in PCM system?

3. What are the applications of PCM?

4. What are the advantages of the PCM?

5. What are the disadvantages of PCM?

RESULT: Thus the A/D and D/A are converted using PCM modulation and demodulation

systems.

6. DELTA MODULATION AND DEMODULATION

AIM: To transmit an analog message signal in its digital form and again reconstruct back the

original analog message signal at receiver by using Delta modulator.

APPARATUS:

1. Delta modulator trainer kit

2. CRO

3. Probes & patch cards

THEORY:

Delta modulator is an advanced version of PCM system, so it is also known as Single

bit PCM system. It generates the output signal by comparing the input signal with its

quantized approximated output i.e. if the step size increases to+ it gives binary value 1

and if step downs to -it gives binary value o. In this way it reduces the transmission

channel band width.

BLOCK DIAGRAM:

BLOCK DIAGRAM FOR DEMODULATOR:

PROCEDURE:

MODULATOR

1. Give the 1kHz analog input to the comparator input pin(9) and the output of the comparator is given to the bi-stable circuit input the TX clock signal is given to the

other input of the bi-stable circuit.

2. The bi-stable circuit output is internally given to the Unipolar/bipolar converter and the output of this converter will be given to the input of integrator.

3. The integrator output is given to the second input of the comparator. 4. Then plot the comparator input waveforms and the bi-stable circuit output, and the

corresponding clock signal.

DEMODULATOR

1. Connect the bi-stable circuit output to the demodulator side bi-stable circuit input, and also give the receiver clock signal to this circuit.

2. The output of this bi-stable circuit is internally given to the Unipolar/bipolar converter and the output of this converter will be given to the input of integrator at demodulator side.

3. Then the integrator output is given to the low pass filter, so finally we observe the original analog signal output across low pass filter output terminal.

OUTPUT WAVEFORMS:

PRECAUTIONS:

1. Connections must be tight. 2. Note down the comparator inputs carefully.

VIVA QUESTIONS:

1. What are the advantages of Delta modulator? 2. What are the disadvantages of delta modulator? 3. How to overcome slope overload distortion? 4. How to overcome Granular or ideal noise? 5. What are the differences between PCM & DM? 6. Define about slope over load distortion? 7. What is the other name of Granular noise? 8. What is meant by staircase approximation? 9. What are the disadvantages of Delta modulator? 10. Write the equation for error at present sample?

RESULT: Hence we obtained the digital data output of a given analog message signal by

using delta modulator

7 TIME DIVISION MULTIPLEXING AND

DEMULTIPLEXING

AIM: To transmit a multiplexed output of different frequency message signals through a single channel using TDM system and recover back the original message signals through a

demultiplexer at receiver end.

APPARATUS:

1. TDM trainer kit 2. CRO 3. Patch cards 4. Connecting wires

THEORY:

Time division multiplexing is a technique used for transmitting several analog

message signals over a single communication channel, by dividing the time frame in to

number of slots, i.e. one slot for each signal. Here there are four input signals; all are band

limited to fx by the input Low pass filters, and all these are sequentially sampled at the

transmitter by using a rotary switch i.e. commutator. This commutator makes fs revolutions

per second and extracts one sample from each input during each revolution. The out put of

the switch is a PAM wave form containing samples of the input signals periodically

interfaced with time.

CIRCUIT DIAGRAM:

PROCEDURE:

(AT TRANSMITTING BLOCK)

1. Place the duty cycle controlled switch in position-5 2. Turn the potentiometer in function generator block fully in clock wise 3. The following connections are made

250Hz to channel-o

500Hz to channel-1

1kHz to channel-2

2kHz to channel -3 4. The external triggering will be given to the channel-0 terminal. 5. Then multiplexed output is observed across Tx output terminal. 6. Vary the amplitude of input sine wave by varying the potentiometers in function

generator block to indicate which sample belongs to which output channel and

then the outputs are plotted on the graph.

(AT RECEIVER BLOCK)

1. The following connections are made

Tx output to Rx output

Tx clock to Rx clock

Tx t0 to Rx t0 2. Above connections are made sure that the Tx clock signal is used by the Rx to

Synchronize its activity

3. Then de-multiplexed original message signals are available across the low pass

filters at receiver block.

PRECAUTIONS:

1. Connections must be tight. 2. Waveforms must be noted carefully.

OUTPUT WAVEFORMS: (Transmitting Signals)

DEMULTIPLEXED OUTPUT:

VIVA QUESTIONS:

1. What is meant by multiplexing technique and what are the different types of

Multiplexers?

2. Briefly explain about TDM&FDM?

3. What is the transmission band width of a PAM/TDM signal?

4. Define crosstalk effect in PAM/TDM system?

5. What are the advantages of TDM system?

6. What are major differences between TDM&FDM?

7. Give the value of Ts in TDM system?

8. What are the applications of TDM system

and give some example?

9. What is meant by signal overlapping?

10. Which type of modulation technique will

be used in TDM?

RESULT:

Hence four message signals are transmitted at a time through a single communication

channel, using TDM system and again de-multiplex these four message signals at receiver is

observed.

www.jwjobs.net