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Electronics and Communication Engineering - Communication Systems

Exercise : Communication Systems - Section 4
16.
A 1000 kHz carrier is simultaneously modulated with 300 Hz, 800 Hz and 2 kHz audio sine waves. Which of the following frequency is least likely to be present in the output?
1002 kHz
1000 kHz
999.2 kHz
998.0 kHz
Answer: Option
Explanation:

Frequency present in the sidebands is equal to = fc ± fm, fc ± 2fm, fc ± 3fm .

17.
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.
The instantaneous frequency variations will be between 1500.2 and 1499.8 kHz, 1500 times a second
The instantaneous frequency variations will be between 1501 and 1499 kHz, 1500 times a second
The instantaneous frequency variations will be between 1501 and 1499 kHz, 300 times a second
The instantaneous frequency variations will be between 1501 and 1499 kHz, 60 times a second
Answer: Option
Explanation:

Since, the intensity of audio wave has been increased five times, the instantaneous frequency variation is also increased five times.

18.
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?
0.002
0.02
0.001
0.01
Answer: Option
Explanation:

.

19.
An analog signal is band limited to B Hz sampled at Nyquist rate and samples are quantized into 4 levels each with probability . The information rate is
4 bits/s
B bits/s
4 B bits/s
0.25 B bits/s
Answer: Option
Explanation:

.

Signal should be sampled at frequency 2B Hz.

Hence, information rate = 2 (2B) = 4B bits/s.

20.
The HV supply to a TV receiver is 25 kV and the total beam current for the three electron guns is 2000 μA. The power handled by IHVT is
5W
0.5 W
50 W
500 W
Answer: Option
Explanation:

25 x 103 x 2000 x 10-6 = 50 W.

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