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Electronics and Communication Engineering - Communication Systems

Exercise : Communication Systems - Section 7
41.
An AM signal is detected using an envelope detector, the carrier frequency and modulating signal frequency are 1 MHz and 2 kHz respectively. An appropriate value for the time constant of the envelope detector is
500 μ sec
20 μ sec
0.2 μ sec
1 μ sec
Answer: Option
Explanation:

In envelope detector, for proper detection

10-6 ≤ RC ≤ 250 μ sec.

Only 20 m sec satisfies the condition.

42.
For signal amplitude modulated to a depth of 100% by a sinusoidal signal, power is
same as that of unmodulated carrier
twice the power of unmodulated carrier
times the power of unmodulated carrier
times the power of unmodulated carrier
Answer: Option
Explanation:

When m = pt = 1.5 Pc

43.
If transmitted power is 10 kW, the field strength at a distance d is 50 mV/m. If transmitted power is reduced to 2.5 kW, the field strength at the same point will be
25 mV/m
12.5 mV/m
6.25 mV/m
3.125 mV/m
Answer: Option
Explanation:

E ∝ Pt .

44.
Some features of advanced mobile phone service are
  1. To provide telephone communication to a large number of telephone users within the area of a metropolitan only
  2. To use minimum amount of frequency spectrum
  3. To permit hand held as well as vehicle operation of telephone
Which of the above are true?
1 and 2 only
1, 2, 3
1 and 3 only
2 and 3 only
Answer: Option
Explanation:

All the three features are necessary.

45.
The carrier frequency will be if L = 50 µH and C = 1 nF.
512
612 kHz
712 kHz
812 kHz
Answer: Option
Explanation:

L = 50 µH, C = 1 nF

.

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