334 + Interview Questions in Communication Systems in Electronics and Communication Engineering Page 1 InterviewSolution

1.	The maximum permissible distance between two samples of a 2 kHz signal is
Answer» .

Discussion

2.	Which camera tube has highest dark current?
Answer» Dark current is the small signal current even when there is no illumination on the face plate of camera tube.

Discussion

3.	If a sound wave has a frequency of 50 Hz, the wavelength is
Answer»

Discussion

4.	Skin depth at 1000 MHz in comparison with that at 500 MHz is
Answer» Skin depth decreases as square root increase in frequency.

Discussion

5.	The dc voltage on plate modulated class C amplifier is 8 kV. If the peak modulating voltage is 4 kV, the peak RF voltage delivered to load is
Answer» Peak RF voltage delivered to load = 2 (8 + 4) = 24 kV.

Discussion

6.	The output resistance of a microphone is 20 ohms. It is to feed a line of impedance 320 ohms. To ensure maximum power transfer, the turn ratio of transformer is
Answer» (20)(4)2 = 320 ohms.

Discussion

7.	In a reactance FET, = 9 x 10 Siemens, gate to source resistance is one-ninth of reactance of gate to drain capacitor and frequency is 4 MHz. The capacitive-reactance is
Answer» .

Discussion

8.	24 telephone channels, each band limited to 3.4 kHz are to be time division multiplexed using PCM. If sampling frequency is 10 kHz and number of quantization levels is 128, the required bandwidth of PCM is
Answer» Bandwidth ≈ 24 (ln2 128) 10 kHz = 1.68 MHz.

Discussion

9.	A total of 16 equally probable events exist. If it is required to select one out of them, the number of required bits is
Answer» 24 = 16.

Discussion

10.	When modulation index of an AM wave is increased from 0.5 to 1, the transmitted power
Answer» .

Discussion

11.	A Discone antenna is
Answer» It is a combination of disc and cone in close approximity.

Discussion

12.	Audio cassettes used in household tape recorders have a width of
Answer» Home tape recorders use in (i.e., 6.3 mm) tape having two tracks of 2.5 mm width each with an in between separation of 1.3 mm.

Discussion

13.	The receiver of a pulsed RADAR can detect signal power in the order of
Answer» It can detect very low power signals.

Discussion

14.	The total number of pulses in a TV remote control is
Answer» The 14 pulses are preliminary pulse, start pulse, stop pulse and 11 other pulses.

Discussion

15.	Three point tracking is achieved with
Answer» To obtain three point tracking a capacitor is connected in series with local oscillator coil. This capacitor is called padder capacitor.

Discussion

16.	In TV transmitter, visual exciter may consist of Which of the above are actually a part of visual exciter?
Answer» All these are part of visual exciter.

Discussion

17.	The positive peak of an AM wave is 16 V and the minimum value is 4 V Assuming single tone modulation, the modulation index is
Answer» Average value = 10 V and .

Discussion

18.	CATV stands for
Answer» In USA the first cable service was called community antenna television.

Discussion

19.	Household tape recorders use
Answer» For reasons of economy only one motor is used.

Discussion

20.	The antenna diameter of a radar system is doubled. The maximum range will
Answer» Range is proportional to antenna diameter.

Discussion

21.	In PAL system weighting of R - Y and B - Y signals is done to
Answer» Weighting reduces the chances of over-modulation of the transmitter carrier in the presence of highly saturated colours when picture has very light or very dark parts.

Discussion

22.	In a CE amplifier connection, the emitter is at ground potential for
Answer» XC for ac frequencies is very low and therefore emitter is nearly at ground potential.

Discussion

23.	In telephone systems the relation between load , number of calls initiated during a specified interval λ and average duration of a call () is
Answer» Load = (number of calls) (duration of call).

Discussion

24.	In a Vidicon camera tube
Answer» Grid G2 accelerates the electrons. Grid G3, is focus grid and slows the electrons so that they fall perpendicularly on the target at a low speed without causing secondary emission.

Discussion

25.	In a CD the sampling frequency is nearly
Answer» For proper sampling and faithful reproduction sampling frequency must be at least twice the highest frequency in the signal.

Discussion

26.	A line has L = 0.8 µH/ and C = 32 pF/. Z =
Answer» .

Discussion

27.	In an AM wave with = 1, the carrier is suppressed. The resulting power saving will be
Answer» .

Discussion

28.	FSK is a system involving
Answer» FSK, i.e., frequency shift keying is a system of frequency modulation.

Discussion

29.	A 200 Hz sound wave is to be transmitted by a 1000 kHz radio wave using frequency modulation. The instantaneous frequency variation will be
Answer» The instantaneous frequency variation is equal to frequency of audio signal.

Discussion

30.	The number of channels in a stereophonic system is
Answer» Left and right channels.

Discussion

31.	A sinusoidal voltage of 2 kV peak value is amplitude modulated to give 20% modulation. The peak value of each side band term is
Answer» .

Discussion

32.	A 300 Hz sound wave of intensity 20 dB is transmitted by a 1500 kHz radio wave by frequency modulation. It is seen that instantaneous frequency varies between 1500.2 kHz and 1499.8 kHz, 300 times a second. If the intensity of sound wave is 100 dB.
Answer» Since, the intensity of audio wave has been increased five times, the instantaneous frequency variation is also increased five times.

Discussion

33.	If the average power of radar transmitters is 2 kW and the peak power of the transmitter is 1000 kW, what will be the duty cycle?
Answer» .

Discussion

34.	In a tape recorder the output from RF oscillator is given to
Answer» RF waves (about 75 kHz frequency) are fed to erase head for erasing.

Discussion

35.	An amplifier has an input resistance of 10 k&ohm; and the noise voltage is 10 μV. If two such resistances of 10 k&ohm; each are connected in series at the input, the total noise voltage is about
Answer» Noise voltage = 10 = 14.14 μV.

Discussion

36.	In a high speed modem the data rate handled is about
Answer» When data rate is in the range of 2400 to about 10800 bits per second, modem is designated as high speed.

Discussion

37.	Trinitron is a monochrome picture tube.
Answer» Frequency band for local programs is fixed.

Discussion

38.	The first pulse in a TV remote control is
Answer» The first pulse is preliminary pulse followed by start pulse.

Discussion

39.	If the ratio of ac to dc load impedance of a diode detector is low, the result will be
Answer» Negative peak will be clipped.

Discussion

40.	SWR for a transmission line is infinity when the load is Of the above statements:
Answer» SWR is infinite if \|rv\| = 1.

Discussion

41.	Two sinusoidal signals of same amplitude and frequencies 10 kHz and 10.1 kHz are added together. The combined signal is given to an ideal frequency detector. The output of the detector is
Answer» Y(t) = A cos (2p x 104 t) + A cos (2p x 10.1 x 103 t) when passed through high pass filter, Output will be A cos (2p x 10.1 x 103 t) only.

Discussion

42.	If D is mouth diameter of a paraboloid reflector, its directivity is proportional to
Answer» Directivity of parabolic reflector = .

Discussion

43.	A noiseless 3 kHz channel can't transmit binary bit at the rate
Answer» Noiseless channel: Nyquist's Theorem - If the signal has V discrete levels over a transmission medium of bandwidth H, the maximum data rate = 2H log2 V bits/sec. A noiseless 3-kHz channel cannot transmit binary signals at a rate exceeding 6000 bps (= 2 x 3000 log2 2).

Discussion

44.	The minimum step size required for a Delta modulator operating at 36 samples/sec to track the signal (here () is the unit step function) () = 125 (() - ( - 1)) + (250 - 125) (( - 1) - ( - 2)) So that slope overload is avoided, would be
Answer» To avoid slope overload \|m(t)\| ≥ Δ . fs or \|x(t)\| ≥ Δ . fs 125 ≤ Δ . 25 . 210 27 ≤ Δ . 215 Δ ≥ 2-8.

Discussion

45.	An FM signal with a deviation δ is passed through a mixer, and has its frequency reduced 5 fold. The deviation in the output of the mixer is
Answer» Deviation depend on amplified of signal not on frequency.

Discussion

46.	For an ideal 3000 Hz channel, Nyquist rate is
Answer» For 3000 Hz. Nyquist rate is twice, i.e., 6000 bps.

Discussion

47.	In the FM wave described by equation = 15 sin (4 x 10 + 3 sin 1100 ), the maximum frequency deviation is
Answer» . Maximum frequency deviation = 3 x 175 = 525 Hz.

Discussion

48.	The radar cross-section of a target depends an
Answer» Effective area of target is called radar cross-section. It depends on frequency polarization of incident wave and aspect of target.

Discussion

49.	The Nyquist sampling interval, for the signal sin c (700) + sin c (5000 ) is
Answer» g(t) = sin c (700 t) + sin c (500 t) [sin (p 700 t) + sin (p 500 t)] fm = 350 Hz fs = 700 Hz, Nyquist rate = .

Discussion

50.	In PCM, if the number of quantization levels is increased from 4 to 64, then the bandwidth requirement will approximately be increased __________ times.
Answer» Bandwidth for PCM system = 2nN fm Hz for N >> 1, n >> 1 where n is no. of channel, N is no. of bit. B1 = 2n N1 fm B2 = 2n N2 fm 2N1 = 4 N1 = log24 N1 = 2 2N2 = 64 N2 = log264 6 .

Discussion

Explore topic-wise InterviewSolutions in .

The maximum permissible distance between two samples of a 2 kHz signal is

Which camera tube has highest dark current?

If a sound wave has a frequency of 50 Hz, the wavelength is

Skin depth at 1000 MHz in comparison with that at 500 MHz is

The dc voltage on plate modulated class C amplifier is 8 kV. If the peak modulating voltage is 4 kV, the peak RF voltage delivered to load is

The output resistance of a microphone is 20 ohms. It is to feed a line of impedance 320 ohms. To ensure maximum power transfer, the turn ratio of transformer is

In a reactance FET, = 9 x 10 Siemens, gate to source resistance is one-ninth of reactance of gate to drain capacitor and frequency is 4 MHz. The capacitive-reactance is

24 telephone channels, each band limited to 3.4 kHz are to be time division multiplexed using PCM. If sampling frequency is 10 kHz and number of quantization levels is 128, the required bandwidth of PCM is

A total of 16 equally probable events exist. If it is required to select one out of them, the number of required bits is

When modulation index of an AM wave is increased from 0.5 to 1, the transmitted power

A Discone antenna is

Audio cassettes used in household tape recorders have a width of

The receiver of a pulsed RADAR can detect signal power in the order of

The total number of pulses in a TV remote control is

Three point tracking is achieved with

In TV transmitter, visual exciter may consist of Which of the above are actually a part of visual exciter?

The positive peak of an AM wave is 16 V and the minimum value is 4 V Assuming single tone modulation, the modulation index is

CATV stands for

Household tape recorders use

The antenna diameter of a radar system is doubled. The maximum range will

In PAL system weighting of R - Y and B - Y signals is done to

In a CE amplifier connection, the emitter is at ground potential for

In telephone systems the relation between load , number of calls initiated during a specified interval λ and average duration of a call () is

In a Vidicon camera tube

In a CD the sampling frequency is nearly

A line has L = 0.8 µH/ and C = 32 pF/. Z =

In an AM wave with = 1, the carrier is suppressed. The resulting power saving will be

FSK is a system involving

A 200 Hz sound wave is to be transmitted by a 1000 kHz radio wave using frequency modulation. The instantaneous frequency variation will be

The number of channels in a stereophonic system is

A sinusoidal voltage of 2 kV peak value is amplitude modulated to give 20% modulation. The peak value of each side band term is

A 300 Hz sound wave of intensity 20 dB is transmitted by a 1500 kHz radio wave by frequency modulation. It is seen that instantaneous frequency varies between 1500.2 kHz and 1499.8 kHz, 300 times a second. If the intensity of sound wave is 100 dB.

If the average power of radar transmitters is 2 kW and the peak power of the transmitter is 1000 kW, what will be the duty cycle?

In a tape recorder the output from RF oscillator is given to

An amplifier has an input resistance of 10 k&ohm; and the noise voltage is 10 μV. If two such resistances of 10 k&ohm; each are connected in series at the input, the total noise voltage is about

In a high speed modem the data rate handled is about

Trinitron is a monochrome picture tube.

The first pulse in a TV remote control is

If the ratio of ac to dc load impedance of a diode detector is low, the result will be

SWR for a transmission line is infinity when the load is Of the above statements:

Two sinusoidal signals of same amplitude and frequencies 10 kHz and 10.1 kHz are added together. The combined signal is given to an ideal frequency detector. The output of the detector is

If D is mouth diameter of a paraboloid reflector, its directivity is proportional to

A noiseless 3 kHz channel can't transmit binary bit at the rate

The minimum step size required for a Delta modulator operating at 36 samples/sec to track the signal (here () is the unit step function) () = 125 (() - ( - 1)) + (250 - 125) (( - 1) - ( - 2)) So that slope overload is avoided, would be

An FM signal with a deviation δ is passed through a mixer, and has its frequency reduced 5 fold. The deviation in the output of the mixer is

For an ideal 3000 Hz channel, Nyquist rate is

In the FM wave described by equation = 15 sin (4 x 10 + 3 sin 1100 ), the maximum frequency deviation is

The radar cross-section of a target depends an

The Nyquist sampling interval, for the signal sin c (700) + sin c (5000 ) is

In PCM, if the number of quantization levels is increased from 4 to 64, then the bandwidth requirement will approximately be increased __________ times.