Exercise: Communication Systems

Questions for: Communication Systems

In a tape recorder the output from RF oscillator is given to
A:
only erase head coil
B:
only record head coil
C:
coils of both erase head and record head
D:
none of the above
Answer: A

RF waves (about 75 kHz frequency) are fed to erase head for erasing.

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A superheterodyne receiver is to operate in the frequency range 550 kHz - 1650 kHz, with the intermediate frequency of 450 kHz. Let denote the required capacitance ratio of the local oscillator and I denote the image frequency (in kHz) of the incoming signal. If the receiver is tuned to 700 kHz, then
A:
R = 4.41 J =1600
B:
R = 2.10 J =1150
C:
R = 3.0 J =1600
D:
R = 9.0 J =1150
Answer: A

Image frequency = 700 + 2 x 450 1600 kHz

fmax = 1650 + 450 2100 kHz

fmin = 550 kHz + 450 1000 kHz

R = 4.41, I = 1600 kHz.

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Assertion (A): An electronic tuner uses varactor diode

Reason (R): The capacitance of a varactor diode depends on the magnitude of reverse bias.

A:
Both A and R are correct and R is correct explanation of A
B:
Both A and R are correct but R is not correct explanation of A
C:
A is correct but R is wrong
D:
A is wrong but R is correct
Answer: A

A varactor diode has a variable capacitance which depends on reverse bias. It is a very convenient device for tuning.

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In an AM wave the carrier and one of the side bands is suppressed. If m = 1, the percentage saving in power is
A:
25%
B:
50%
C:
66.6%
D:
83.3%
Answer: D

.

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A noiseless 3 kHz channel can't transmit binary bit at the rate
A:
12000 bps
B:
1000 bps
C:
6000 bps
D:
3000 bps
Answer: A

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).

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