Department of Computer Engineering · What are the Draw backs in Pulse amplitude modulated signal? 4. What do you mean by synchronization in PAM? 5. Write the standard equation of

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CE3701-DIGITAL COMMUNICATION LAB

Department of Computer Engineering

COURSE INCHARGE:

MOHAMMED ALTAF AHMED

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List of Experiments

1. STUDY OF MODULATION PROCESSES

2. PULSE AMPLITUDE MODULATION (PAM)

3. TIME DIVISION MULTIPLEXING (TDM)

4. PULSE POSITION MODULATION (PPM)

5. PULSE WIDTH MODULATION(PWM)

6. AMPLITUDE SHIFT KEYING(ASK)

7. FREQUENCY SHIFT KEYING(FSK)

8. PHASE SHIFT KEYING(PSK)

9. PULSE CODE MODULATION (PCM)

10. OPTICAL SIGNAL TRANSMISSION

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Title:Experiment 1: Study of Modulation Processes

Objective & Scope

To understand the Modulation Process.Experiment deals with basic concepts of modulation and demodulation. Students can getbenefited by studying basic concept of modulation processes.

Background & Terminologies

Knowledge of Digital Communication.Knowledge of Analog Communication.Familiar with kits using to perform experiments.Familiar with Lab equipments like CRO, Function Generator etc.

Safety Issues

• Before start of experiments students should understand the procedure to operate tooland kits.

• Through understanding of Lab Equipments and Components.

Tools & Equipments

Computer System with at least 40 GB Hard Disk , 1 GB Ram, and windows Operating SystemCOM3LAB (70071 TX433 Transmitting Technology, 70072 RX433 Receiving Technology,70073,70074 Modem Technology)Kits are required to perform the Experiments.

Procedure

1. Connections are made as per the circuit diagram.2. Modulating signal and carrier signal of High frequency is given to aboard. and

switch ON the power supply.3. Then the modulated output is observed.4. The output characteristics are plotted on a graph.

Troubleshooting

• While performing experiment instruction given by tool have to be noticed.• Avoid short circuiting.

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• Connections must be tight.

The width of the signal must be noted carefully.

THEORY:

Modulation procedures are characterized as those having a sinusoidal carrier signal and those with apulse shaped carrier signal. On the other hand, the modulation signal representing the message can beanalog or digital. This course deals with those modulation procedures in which a digital modulationsignal is modulated on to a sinusoidal carrier (shown lighter in the adjacent overview). The basics of thevarious modulation procedures are provided by the corresponding instructional systems.

• T 7.2.1.3 Amplitude Modulation• T 7.2.1.5 Frequency and Phase modulation• T 7.2.2.1 Pulse Code Modulation• 700 71 TX 433 Transmission Technique• 700 72 RX 433 Receiving Technique

In pulse Amplitude Modulation the amplitude of the carrier pulses varies in accordancewith the instantaneous values of message signal and the pulse width is fixed, in practice thisis realized by a simple mechanical commutator or by electronic circuit. Natural PAM signalsampling occurs when finite width is used in the modulators and tops of the pulses are forcedto follow the magnitude of modulating waveform. Flat tapped PAM system is quite often used

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because of the generating the modulating waveform and spectrum. Finite width pulses areused but they are flat topped after modulation. Reconstruction of original signal is possible bypassing the modulated signal through a low pass filter.

Comment

Quiz

1. What are the classifications of pulse modulation techniques?2. What is the transmission bandwidth of Pulse amplitude modulation?3. What are the Draw backs in Pulse amplitude modulated signal?4. What do you mean by synchronization in PAM?5. Write the standard equation of a PAM in frequency domain?6. What is meant by Aperture effect?7. Draw the frequency spectrum of a PAM signal?8. What is the time domain representation of a PAM signal?9. What are the major differences between PAM &PWM?10. Which type of sampling technique is used in PAM signal?

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Title:Experiment 2: Pulse Amplitude Modulation

Objective & Scope

To understand the Modulation Process.Experiment deals with basic concepts of Pulse Amplitude modulation and demodulation.Students can get benefited by studying basic concept of PAM processes.



Safety Issues



Tools & Equipments


Procedure1. Connections are made as per the circuit diagram.2. Modulating signal and carrier signal of High frequency is given to aboard. and

switch ON the power supply.3. Then the modulated output is observed.4. The output characteristics are plotted on a graph.

Troubleshooting

• While performing experiment instruction given by tool have to be noticed.• Avoid short circuiting.• Connections must be tight.

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THEORY:

In pulse Amplitude Modulation the amplitude of the carrier pulses varies in accordancewith the instantaneous values of message signal and the pulse width is fixed, in practice thisis realized by a simple mechanical commutator or by electronic circuit. Natural PAM signalsampling occurs when finite width is used in the modulators and tops of the pulses are forcedto follow the magnitude of modulating waveform. Flat tapped PAM system is quite often usedbecause of the generating the modulating waveform and spectrum. Finite width pulses areused but they are flat topped after modulation. Reconstruction of original signal is possible bypassing the modulated signal through a low pass filter.

Results\Graph\

Comments:

Quiz

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1. What are the classifications of pulse modulation techniques?2. What is the transmission bandwidth of Pulse amplitude modulation?3. What are the Draw backs in Pulse amplitude modulated signal?4. What do you mean by synchronization in PAM?5. Write the standard equation of a PAM in frequency domain?6. What is meant by Aperture effect?7. Draw the frequency spectrum of a PAM signal?8. What is the time domain representation of a PAM signal?9. What are the major differences between PAM &PWM?

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Title:Experiment 3:Time Division Multiplexing

Objective & Scope

To understand the Time Division Multiplexing.Experiment deals with basic concepts of Multiplexing. Students can get benefited by studyingbasic concept of Multiplexing.



Safety Issues



Tools & Equipments


Procedure(AT TRANSMITTING BLOCK)

1. Place the duty cycle controlled switch in position-52. Turn the potentiometer in function generator block fully in clock wise3. 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

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generator block to indicate which sample belongs to which output channel andthen 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 passfilters at receiver block.

1. .

Troubleshooting



THEORY:

Time division multiplexing is a technique used for transmitting several analog messagesignals 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 theinput Low pass filters, and all these are sequentially sampled at the transmitter by using arotary switch i.e. commutator. This commutator makes fs revolutions per second and extractsone sample from each input during each revolution. The out put of the switch is a PAM waveform containing samples of the input signals periodically interfaced with time.

CIRCUIT DIAGRAM:

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OUTPUT WAVEFORMS: (Transmitting Signals)

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DEMULTIPLEXED OUTPUT:

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Results\Graph\

Comments:

Quiz

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

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?

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Title:Experiment 4: Pulse Position Modulation(PPM)

Objective & Scope

To understand the Modulation Process.Experiment deals with basic concepts of Pulse Position modulation and demodulation.Students can get benefited by studying basic concept of PPM processes.



Safety Issues



Tools & Equipments


Procedure1. Connections are made as per circuit diagram.

2. The message signal of 500Hz, 2v pp amp- is applied .

3. The output is observed on CRO

4. The input and output waveforms are rated from CRO and are plotted on graph

sheet.

Troubleshooting

• While performing experiment instruction given by tool have to be noticed.

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• Avoid short circuiting.• Connections must be tight.


THEORY:

PPM can be considered version of PDM in PDM, long pulses expend considerablepower during the pulse while bearing no additional information. If an arrangement is made sothat the unused power could be subtracted from the PDM, we get a more efficient pulsemodulation. In PPM the position of a pulse relative to its unmodulated time of occurrence isvaried in accordance with the message signal PPM may be obtained from PWM, in which theposition of PWM pulses are position modulated. Thus these pulses will have timedisplacement proportional to the instantaneous value of the signal voltage. The simplestmethod of generation of PPM from PWM is to use a monostable multivibrator. It is to bedesigned in such away that it triggers the trailing edges of a PWM signal. If a PWM signal isapplied at the inputs, the output will be obviously a pulse position modulated signal whoseduration will be determined by the timing circuit of multivibrator. For demodulation of PPM,first it converted into PWM with help of flip-flop, and then it is demodulated.

OUTPUT WAVEFORMS:

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Results\Graph\

Comments:

Quiz1. Define PPM?

2. What are the differences between PPM & PWM?

3. Which type of sampling technique is used in PPM?

4. The Multivibrator used in PPM is?

5. Pulse duration of PPM is?

6. In PPM the position is proportionally varied in which parameter of the modulating signal?

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Title:Experiment 5: Pulse Width Modulation(PWM)

Objective & Scope

To understand the Modulation Process.Experiment deals with basic concepts of Pulse Width modulation. Students can get benefitedby studying basic concept of PPM processes.



Safety Issues



Tools & Equipments


Procedure1. Connection is made as per the circuit diagram.2. Vary the control voltage (0-5) v and observe the corresponding change in output

square Waveform using CRO.3. Change in control voltage changes the width of the square wave.4. Note down the T-ON and T-OFF.5. Plot the observed waveform.

Troubleshooting

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THEORY:

If the widths of the pulses are varying in accordance with the modulating signal it iscalled pulse width modulation. In Pulse width modulation, the amplitude of the pulses isconstant. Generation of PWM the in put modulating signal is given to non - inverting terminalof op-amp .the op-amp now compares with both the input signals. The output of thecomparator is high only when instantaneous value of input modulating signal is grater thenthat of saw tooth waveform. When saw tooth waveform voltage is grater then input modulatingsignal at that instant the out put of the comparator remains zero i.e. in negative saturation.Thus out put of comparator is PWM signal.

OUTPUT WAVEFORMS:

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Results\Graph\

Comments:

Quiz1. What are the different types of PTM systems?2. What is the other name of Pulse width modulation?3. What do you mean by pulse time modulation?4. What is the comparison between the PAM and PWM?5. What is the definition of PWM?6. What is the transmission bandwidth of PWM signal?7. Which type sampling technique is used in PWM? What are the applications of PWM

modulation technique?

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Title:Experiment 6: Amplitude Shift Keying

Objective & Scope

To understand the Process of Shift keying.Experiment deals with basic concepts of shift keying. Students can get benefited by studyingbasic concept of ASK process.



Safety Issues



Tools & Equipments


Procedure6. Connection is made as per the circuit diagram.7. Vary the control voltage (0-5) v and observe the corresponding change in output

square Waveform using CRO.8. Change in control voltage changes the width of the square wave.9. Note down the T-ON and T-OFF.10.Plot the observed waveform.

Troubleshooting


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THEORY:

ASK is one in which the amplitude of a carrier is switched between two values i.e, onand off. The resultant waveform consists of on pulses representing binary 1 and off pulsesrepresenting binary 0.The binary ASKS signaling scheme was one of the earliest forms ofdigital modulation used in wireless telegraphy at the beginning of this century. It is thesimplest form of digital modulation & serves as a useful model for introducing certainconcepts.

In Amplitude Shift Keying (Amplitude Shift Keying ASK1 ) a sinusoidal carrier signal having frequency f0isturned on and off by the data signal (which is why this procedure is also known as ON/OFF keying). Ifthe data signal has the value logical 1 (HIGH), the carrier signal is turned on, and a data signal oflogical 0 (LOW) turns it off. ASK can therefore be implemented by means of a switch.

ASKAmplitude Shift Keying is a modulation method whereby the amplitude of a sinusoidal carriers signal ischanged as a function of the level of the (digital) modulation signal. The carrier signal is usually turnedon and off alternately (ON/OFF Keying).

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In the following experiment you will first study the modulated signal in amplitude shift keying.Periodically a constant data byte will be sent as a data signal and modulated on to the sinusoidal carriersignal. Both signals will be compared using the oscilloscope.

Result:

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SQ format generates a square-wave signal with a pulse width of 800 μs (upper curve), whereby thepulse width corresponds to the bit width. There are exactly eight oscillations of the carrier signal foreach pulse (lower curve). The carrier signal thus has a frequency of f0 = 10 kHz.

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Results\Graph\

Comments:

Quiz1. 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?

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Title:Experiment 7: Frequency Shift Keying

Objective & Scope

To understand the Process of Shift keying.Experiment deals with basic concepts of shift keying. Students can get benefited by studyingbasic concept of FSK process.



Safety Issues



Tools & Equipments


Procedure11.Connection is made as per the circuit diagram.12.Vary the control voltage (0-5) v and observe the corresponding change in output

square Waveform using CRO.13.Change in control voltage changes the width of the square wave.14.Note down the T-ON and T-OFF.15.Plot the observed waveform.

Troubleshooting


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THEORY:

In FSK, the waveform is generated by switching the frequency of the carrier betweentwo values corresponding to the binary information which is to be transmitted. Here the carrierfrequency varies from lowest to highest point i.e. carrier swing is known as Frequency shiftkeying. FSK signaling schemes find a wide range of applications in low speed digital datatransmission systems.

FSK Demodulation

InFrequency Shift Keying (FSK1 ) depending on the data signal the shift is made between twofrequencies f0 -Δf (data bit is logical 0) and f0 +Δf (data bit is logical 1). The frequency f0 is termed thecenter frequency2 , the frequencyΔf the frequency deviation3 . Compared with ASK, FSK issignificantly less noisesensitive.

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1 FSKFrequency Shift Keying is a modulation technique whereby the frequency of a sinusoidal carrier signalis changed as a function of the level of the (digital) modulation signal.

2 Center frequencyThe center frequency f0 = 1/2 (f1 + f2) is the average of the two characteristic frequencies f1 and f2whichare alternated in frequency shift keying (FSK).

3 Frequency deviationThe frequency deviationΔf indicates by what value the two characteristic frequencies used infrequency shift keying (FSK) deviate from the center frequency f0.

In the following experiment we will first study the modulated signal in frequency shift keying.Periodically a constant data byte will be sent as the data signal and modulated on to the sinusoidalcarrier signal. The center frequency and frequency deviation can then be determined from the signals.

Result:

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In SQ data format (symmetrical square-wave signal) alternating LOW and HIGH pulses are sent; eachhas a duration of 800 μs (upper curve). At logical 0 there are four oscillations during this time, or eightat logical 1.This corresponds to frequencies of 5 kHz and 10 kHz. The center frequency is therefore 7.5kHz and thefrequency deviation 2.5 kHz.

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Results\Graph\

Comments:

Quiz1. 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?

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Title:Experiment 8:Phase Shift Keying

Objective & Scope

To understand the Process of Shift keying.Experiment deals with basic concepts of shift keying. Students can get benefited by studyingbasic concept of PSK process.



Safety Issues



Tools & Equipments


Procedure16.Connection is made as per the circuit diagram.17.Vary the control voltage (0-5) v and observe the corresponding change in output

square Waveform using CRO.18.Change in control voltage changes the width of the square wave.19.Note down the T-ON and T-OFF.20.Plot the observed waveform.

Troubleshooting


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THEORY:

Phase shift keying or discrete phase modulation is another technique available forcommunicating digital information over band pass channels. In PSK signaling schemes thewaveforms s1(t) = -Acoswct & S2(T) = Acoswct are used to convey binary digits 0& 1respectively. 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 differenceb/w the ASK&PSK waveform is that in the ASK scheme the carrier is switched on &offwhereas in the PSK scheme the carrier is switched b/w levels +A & -A.

The differentially coherent PSK signaling scheme makes use of a clever techniquedesigned to get around the need for a coherent reference signal at the receiver.

In two-phase shift keying (2PSK1 ) a sinusoidal carrier signal with frequency f0 is toggled between twodifferent phase positions depending on the data signal. Since disturbances generally affect only theamplitude of the signal and not its phase position, Phase Shift Keying is very noise-immune.1 PSKPhase Shift Keying is a modulation procedure whereby the phase position of a sinusoidal carrier signalis changed depending on the level of the (digital) modulation signal. In 2PSK two and in 4PSK four

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different phase positions are toggled. There are also variations with more phase positions.

In the following experiment we will first study the modulated signal in two-phase shift keying. A periodicsquare-wave signal (SQ format) will be sent as the data signal and modulated on to the sinusoidalcarrier signal. Both signals will be compared on the oscilloscope. Both the phase positions for logical 0and logical 1 will be determined from the signals.

Results:

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With the selected square-wave data signal alternating LOW and HIGH pulses are sent; each has adurationof 800 μs (upper curve). The frequency of the carrier is constant at a value of 10 kHz. For adata bit of logical 0 the carrier has a phase shift of 0°, and for a data bit of logical 1 a phase position of180°.

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Results\Graph\

Comments:

Quiz1. 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?

Documents

Department of Computer Engineering · What are the Draw backs in Pulse amplitude modulated signal? 4. What do you mean by synchronization in PAM? 5. Write the standard equation of