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Electronics and Communication Engineering - Communication Systems

Exercise : Communication Systems - Section 7
21.
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
0.1 kHz sinusoidal
10.1 kHz sinusoidal
a linear function
a constant
Answer: Option
Explanation:

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.

22.
In Vidicon camera tube, the resistance of photoconductive material on the target plate
is constant
is more in darkness and less in brightness
is more in brightness but less in darkness and brightness
may be more or less in darkness and brightness
Answer: Option
Explanation:

Light dependent resistor (LDR) has high resistance in darkness and low resistance in brightness.

23.
In a satellite transponder the power amplifier feeds power P to the transmitting antenna of gain G. If distance of earth is d, the power flux density is
PG/d
PG/4pd2
P/G(4pd2)
Answer: Option
Explanation:

Available power is PG and is spread over 4 nd2.

24.
The percentage saving in power of 100% modulated suppressed carrier AM signal is
100
75
66.6
50
Answer: Option
Explanation:

Pc = 400 W, m = 0.75

25.
Which of the following are useful in comparing the noise performance of receivers?
  1. Input noise voltage
  2. Noise figure
  3. Noise resistance
  4. Noise temperature
Select answer as per the following codes:
1, 2 and 3
1, 3 and 4
2, 3, 4
1, 2, 4
Answer: Option
Explanation:

Noise figure, noise resistance and noise temperature are useful quantities. Input noise voltage alone is not sufficient unless gain of amplifier stages is also known.

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