COMMUNICATION INTERVIEW QUESTIONS AND ANSWERS

Q.1. What is the difference between correlation and auto-correlation?

Answer: Correlation is a tool in communication to check the similarity between two signals whereas the auto-correlation function is used to check the similarity between the original signal and its time delayed version.

Q.2. What is Flicker noise and how it depends on frequency?

Answer: Flicker noise is due to imperfect surface behavior of semiconductor devices and due to imperfection in cathode surface of electron tubes. The power spectral density of flicker noise is inversely proportional to frequency. This noise is significant at very low frequencies, generally below a few KHz.

Q.3. Which method is more advantageous: producing FM from PM or direct FM?

Answer: FM signals produced by the direct method have a tendency to undergo a shift in centre frequency during the modulation process which may cause distortion whereas centre frequency is extremely stable in the FM signals produced from PM.

Q.4. What do you mean by Gibbs phenomenon?

Answer: Gibbs phenomenon says that whenever there is abrupt discontinuity in the signal which is being sampled, the reconstructed signal will always have high frequency oscillations and as the number of samples increases the oscillations compress towards discontinuity but there maximum value remains the same.

Q.5. What is Ionospheric scintillation?

Answer: Ionospheric scintillations are variations in the amplitude, phase, polarization or angle of arrival of radio waves. They are caused by irregularities in the upper part of the ionosphere. The main effect of scintillation is fading of the signal.

Q.6. What is the advantage of using orthogonal transmission in digital communication?

Answer: With orthogonal transmission we can increase the duration of each symbol for a given data transmission rate. The longer the symbol duration, the greater the time for averaging each symbol in the receiver and better the signal to noise ratio at detector.

Q.7. What is the frequency reuse concept?

Answer: Frequency reuse is the core concept of the cellular mobile radio system. In this frequency reuse system, users in different geographic locations (different cells) may simultaneously use the same frequency channel.

In short, the designing process of selecting and assigning channel groups for all of the cellular base stations within a system is termed as frequency planning or frequency reuse concept.

Q.8. Why is companding used in PCM transmission?

Answer: The companding in PCM transmission is used to improve the signal-to-noise ratio of the low signals. Basically it is used to overcome non linearity’s of the system.

Q.9. What is bit padding?

Answer: In the bit padding technique the multiplexer adds extra bits to the bit stream of a source so as to force the integer relationship between all the sources to be multiplexed. For example if the bit rate of one source is 3.5 times the bit rate of the other source then by using the bit padding, we can make it 4 times the bit rate of the other.

These extra bits however do not contain any information. So they are discarded by the multiplexer.

Q.10. Why we cannot send the digital signal as it is on the telephone lines?

Answer: The digital data consists of binary 0’s and 1’s, therefore the waveform changes its value abruptly from high to low or low to high. In order to carry such a signal without any distortion being introduced, the communication medium needs to have a larger bandwidth.

Unfortunately the telephone lines do not have high bandwidth. Therefore we have to convert the digital signal first into an analog signal which needs lower bandwidth by means of the modulation process.

Q.11. Why matched filter is so called?

Answer: A matched filter is so called because the impulse response of matched filter is matched to the input signal i.e. h(t) = x(T-t).

Q.12. What is the difference between Hilbert transform and Fourier transform?

Answer: The two transforms are used to separate signals in different domain. Fourier transform provides a mean of analyzing and designing frequency selective filters while Hilbert transform is based upon separating signals on phase selectivity.

Q.13. What corrective measures can be used to overcome the effect of aliasing?

Answer: To overcome the effect of Aliasing we use two corrective measures as-

• Prior to sampling, a low pass pre-alias filter is used to attenuate those high frequency components of the signal that are not essential to the information being conveyed by the signal.
• The filtered signal is sampled at a rate slightly higher than Nyquist rate i.e. f_s ≥ 2f_m.

Q.14. What do you mean by diagonal clipping?

Answer: Diagonal clipping is the name given to a trouble that may arise when using diode detector. At high values of modulation index the current will change so quickly that the time constant of the load may be too slow to follow this change. This is known as diagonal clipping. It does not normally occurs when percentage modulation is below 60∘.

Q.15. What is squelch?

Answer: When no carrier is present at the input i.e. in the absence of transmission on a given channel or between stations a sensitive receiver will produce a disagreeable amount of loud noise. Squelch or muting enable’s the receiver output to remain cut off unless the carrier is present.

Q.16. Why is SSB not used for broadcasting?

Answer: There are two reasons for it-

• As the SSB transmitter and receiver require excellent frequency stability, a small frequency shift in the system can result in degradation in the quality of the transmitted signal. Thus it is not possible to transmit a good quality music using the SSB system.
• It is not possible to design a tunable receiver oscillator with very high frequency stability. Now with the advent of the frequency synthesizers, this has become possible. But such receivers are very expensive.

Q.17. Why the carrier frequencies allotted for AM broadcast are comparatively smaller than those allotted for FM broadcast?

Answer: F M signal is having a comparatively larger bandwidth. Thus to accommodate more FM signals on a particular frequency spectrum without any adjacent channel interference, higher carrier frequencies are must.

Q.18. Why should the shape of the AM wave envelope be same as that of modulating signal and how it can be achieved?

Answer: The shape of the AM wave envelope should be same as that of the modulating signal because this reduces the complexity of the demodulator circuit to a great extent.

This can be achieved if the following conditions are satisfied-

• The percentage modulation should be less than 100% to avoid the envelope distortion.
• The bandwidth of the modulating signal should be small as compared to carrier frequency. This will help to visualize the envelope satisfactorily.

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