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Wednesday, 8 July 2020

Communication Systems Lab Viva Questions


Viva Questions on Communication Systems Lab

1. Draw the block diagram of an analog communication technique. Discuss about the drawbacks of analog communication techniques?

The block diagram of an analog communication system is:
The main drawbacks of analog communication techniques are: we cannot trust analog communication for all time. The effect of noise is more on the transmitted signals. The power required for the transmission of the signal is very high, this will yields for high length transmitting antennas which makes the transmission section more complex. The circuit complexity and cost are high for analog communication.

2.  List the main advantages of digital communication techniques?

a. The main advantages of digital communication are:
b. It is highly reliable.
c. The effect of noise is very less, hence yields to high SNR.
d. The power consumption is very less.
e. The circuit complexity is very less because various Digital ICs are used.
f. It requires less cost.
g. The error detection and correction is possible.

3. Which types of digital modulation techniques you know?

The different types of digital modulation techniques are:
a. Amplitude shift Keying (ASK)
b. Frequency Shift Keying (FSK)
c. Phase Shift Keying (PSK)
d. Pulse code Modulation (PCM)
e. Differential Pulse Code Modulation (DPCM)
f. Delta Modulation (DM)
g. Adaptive Delta Modulation etc...

4.  With the help of a block diagram, explain the process of converting analog signal to digital signal?

For the conversion of analog signal to digital, first the analog signal is passed through an anti aliasing filter. Then this signal is passed to a Sampler circuit, then to a quantizer and  encoder.

5.  Discuss the functions of a sampler and quantizer?

We know that sampler and quantizer are the basic blocks of an analog to digital converter. A sampler circuit converts a continuous time signal into discrete time signal and the function of a  quantizer is to convert a continuous in amplitude signal into discrete in amplitude signal.

6.  Which types of Coding techniques are familiar to you?

The different coding techniques in communication system are:
a. Pulse code modulation,
b. Differential pulse code modulation,
c. Delta modulation,
d. Adaptive delta modulation

7.  What you know about the Sampling process?

Simply we can say that sampling process is similar to the process of cutting a bread into slices. Technically in terms of signals, we can say that the process of converting a continuous time signal into discrete in time signal is called as Sampling. In order to perform the sampling process, the analog signal is fed to a sampler circuit (switch).

8.  State sampling theorem?

Sampling theorem states that in order to reconstruct the continuous time signal from a discrete time signal, the sampling frequency taken should be more than or equal to twice of the maximum frequency of the continuous time signal frequency.
fs  ≥ 2fm
Violation of sampling theorem yields to aliasing effect. We cannot reconstruct the original signal from its samples, if sampling theorem is not maintained.

9.  What you know about Nyquist Rate in sampling process?

Nyquist rate is the sampling rate at which the sampling frequency is twice that of the maximum frequency component of the continuous time signal.
fs  = 2fm 

If nyquist rate is maintained for the sampling process, then that is said to be a perfect sampling process.

10.  How many types of samplings are you familiar. Explain?

The different types of samplings are:

a. Impulse Sampling:

The process of impulse sampling is done by multiplying the input signal x(t) with impulse train of period 'T'.

b. Natural sampling:

The process of sampling in which pulse have finite width equal to ¤ä is known as natural sampling. Sampling process is done with respect to a carrier signal, which is in digital form.

c. Flat top sampling:

At the time of transmission of signal through channel, noise is introduced at top of the transmission pulse. This noise can be easily remove if the pulse is in the form of flat top. Thus, the top of the samples are flat, in other words, we can say that they have constant amplitude.

11.  Discuss in detail about the aliasing effect and explain how it is rectified?

At the time of sampling, if the sampling theorem is not satisfied; i.e if imperfect sampling occurs, the signals will be interfered in frequency domain. This is called aliasing effect in sampling. The aliasing effect can be rectified, if sampling theorem is satisfied;
fs  ≥ 2fm
The aliasing effect can also be rectified as first by passing signal from anti aliasing filter before sampling.

12.  How many types of analog pulse modulation methods you know. List all?

The different types of analog pulse modulation methods are:
a. Pulse Amplitude Modulation (PAM)
b. Pulse Width Modulation (PWM)
c. Pulse Position Modulation (PPM)

13.  What you know about Pulse amplitude modulation?

In pulse amplitude modulation, the height of the carrier pulse (amplitude) is proportional to amplitude of message signal. In other words, in PAM, the amplitude of the carrier and message signals are directly proportional to each other.

14.  What you know about Pulse width modulation?

In pulse width modulation, the width of the carrier pulse is proportional to the amplitude of message signal.

15.  Discuss about Pulse position modulation?

In pulse position modulation technique, the position of carrier pulse is proportional to the amplitude of message signal.

16.  Give the comparison of PAM, PWM, PPM?

The different parameters of PAM, PWM and PPM are shown below:

17.  Discuss about Amplitude shift Keying (ASK) ?

The ASK modulation technique represents digital data in carrier wave as variations in amplitudes. In other words, we can say that in ASK, '1' represented by transmitting a fixed amplitude carrier wave with constant frequency for the bit duration.

18.  Illustrate about Phase Shift keying (PSK) technique?

In PSK, the digital data is represented as variations in phase shift in the carrier wave. In other words, in PSK, '1' is represented by a 0 phase shift carrier wave and '0' is represented by a 180 phase shift carrier wave. Both the representation is for the bit duration with constant frequency.

19.  Discuss about Frequency Shift keying (FSK) ?

In FSK, the digital data is represented as variation in frequency in carrier wave. In other words, we can say that for '1' more than carrier frequency is used and for '0' less than carrier frequency is used.

20.  Discuss about Binary Phase Shift Keying (BPSK). Show the technique?

For BPSK modulation technique, for each bit of binary data (0 & 1), the carrier phase will be changed. The two different shifts are 0 and 180 degrees.

21.  Discuss in detail about Quadrature Phase Shift Keying (QPSK). Show the technique?

In QPSK, we know that for each two bits of binary data (00, 01, 10 & 11), carrier phase will be changed to four different shifts : (45, 135, -45, -135).

22.  Give the differences between Bit Rate and Baud Rate?

We know that the bit rate represents the number of bits per second. Baud rate can be defined as the number of symbols per second.  i.e., we can say that in communications the no. of bits transmitted per sec is known as Bit Rate (units bps). The number of times a signal (carrier) changes its state per sec is known as the Baud rate.

23.  What is bandwidth of BPSK signal?

The bandwidth of a BPSK signal is 2Fc, where Fc is the carrier frequency.

24.  Give the comparison of ASK, PSK and FSK ?

Bandwidth
ASK< PSK < FSK
Power
ASK <PSK = FSK
Probability of error
ASK > PSK > FSK
Signal to Noise Ratio
ASK < PSK < FSK

25.  Justify the reason behind calling Amplitude Shift keying (ASK) as ON-OFF keying?

Simply we can say that ASK acts like a switch. The reason is that when input data is than in ASK if the symbol is 1, then output is carrier and if input is 0, then the output is also zero. Hence its looks like a switch. This switches to ON state when input is 1 and to OFF state when input is zero.

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