Communication Systems - Study Mode

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[#71] A signal m(t), band-limited to a maximum frequency of 20 kHz is sampled at a frequency f s kHz to generate s(t). An ideal low pass filter having cut-off frequency 37 kHz is used to reconstruct m(t) from s(t). The minimum of f s required to reconstruct m(t) without distortion is

(A) 20 kHz

(B) 40 kHz

(C) 57 kHz

(D) 77 kHz

Correct Answer

(C) 57 kHz

[#72] A message signal m(t) = A m sin(2πf m t) is used to modulate the phase of carrier A c cos(2πf c t) to get the modulated signal y(t) = A c cos(2πf c t + m(t)). The bandwidth of y(t)

(A) depends of A m but not on f m

(B) depends of f m but not on A m

(C) depends of A m and f m

(D) does not depends on A m or f m

Correct Answer

(C) depends of A m and f m

[#73] A bandlimited signal is sampled at the Nyquist rate. The signal can be recovered by passing the samples through

(A) an RC filter

(B) an envelope detector

(C) a PLL

(D) an ideal low-pass filter with the appropriate bandwidth

Correct Answer

(D) an ideal low-pass filter with the appropriate bandwidth

[#74] An FM signal is being broadcast in the 88-108 MHz band having a carrier swing of 125 kHz. The percentage of modulation is

(A) 100%

(B) 83%

(C) 67%

(D) 50%

Correct Answer

(B) 83%

[#75] Hilbert transform of Acos(2πf 0 t + θ) is

(A) -Asin(2πf 0 t + θ)

(B) Acos(2πf 0 t + θ)

(C) -Acos(2πf 0 t + θ)

(D) Asin(2πf 0 t + θ)

Correct Answer

(D) Asin(2πf 0 t + θ)