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Electronics and Communication Engineering - Communication Systems

Exercise : Communication Systems - Section 8
21.
At night the ionosphere can be considered to be consisting of
D, E, F1 and F2 layers
D, E, F1 layers
D, E, F2 layers
E and F2 layers
Answer: Option
Explanation:

During night D layer is almost absent. Moreover F1 and F2 layers merge to form single F2 layer. Hence only D and F2 layers at night.

22.
A balun transformer gives an impedance transformation of
4 : 1
2 : 1
1 : 4
1 : 2
Answer: Option
Explanation:

Balun gives 4 : 1 impedance transformation and can be used to interconnect twin wire feeder cable to coaxial cable.

23.
Which one of the following is true?
Characteristic impedance of twin wire feeder and coaxial cable are nearly the. same
The characteristic impedance of coaxial cable is more than that of twin wire cable
The characteristic impedance of twin wire cable is more than that of coaxial cable
The characteristic impedance of a twin wire feeder may be more or less than that of coaxial cable
Answer: Option
Explanation:

Z0 for twin wire cable is about 300 ohms while that for coaxial cable is about 75 ohms.

24.
A 1000 kHz carrier wave modulated 40% at 40000 Hz is applied to a resonant circuit tuned to a carrier frequency and having Q = 140. What is the degree of modulator after transmission through the circuit?
40%
20%
0.27
0.54
Answer: Option
Explanation:

m0 = m(1 + 4Q2δ2)1/2

.

25.
The EHT voltage in a picture tube of TV is
about 1000 kV
about 100 kV
about 15 kV
about 1 kV
Answer: Option
Explanation:

EHT voltage is 1 kV for each diagonal inch of CRT screen.

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