GATE EC || COMMUNICATION SYSTEM || AMPLITUDE MODULATION ||PYQs (2000-2025)

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Question 1

The amplitude of a sinusoidal carrier is modulated by a single sinusoid to obtain the amplitude modulate signal s(t)=5cos1600 πt+20cos1800πt+5cos2000 πt. The value of the modulation index is ____. (rounded off to two decimal place)
( PYQ || 2016 NAT|| 1 MARK)

  • 0.5

Question 2

For a message signal m(t) = cos(2 π f m t) and carrier of frequency f c , which of the following represents a signal side-band (SSB) signal?

(PYQ || 2009 MCQ|| 1 MARK)

  • cos(2 πf m t)cos(2πf c t).

  • cos(2 πf c t).

  • cos[2 π(f c +f m )t].

  • [1 + cos(2πf m t)]cos(2 πf c t).

Question 3

A message signal given by m(t)=(1/2)cosω1t–(1/2)sinω1t is amplitude-modulated with a carrier of frequency ωc to generate s(t) = [1 + m(t)] cos ωct. What is the power efficiency achieved by this modulation scheme?

(PYQ || 2009 MCQ|| 2 MARK)




  • 8.33%

  • 11.11%

  • 20%

  • 25%

Question 4

Suppose that the modulating signal is m(t) = 2cos(2 πf m t) and the carrier signal is x c (t) = A c cos(2 πf c t). Which one of the following is a conventional AM signal without over-modulation?

(PYQ || 2010 MCQ|| 1 MARK)

  • x(t) = Acm(t) cos(2 πf c t)

  • x(t) = Ac [1 + m(t)] cos (2 πf c t)

  • x(t) =Accos(2πfc )+ Ac /4m(t) cos(2πf c t)

  • x(t) = CA cos(2 πf m t).cos(2 πf c t)+ CA sin(2πf m t).sin(2πfct)

Question 5

A message signal m(t) = cos2000 πt + 4cos4000 πt modulates the carrier c(t) = cos2 πf c t where f c = 1 MHz to produce an AM signal. For demodulating the generated AM signal using an envelope detector, the time constant RC of the detector circuit should satisfy

(PYQ || 2011 MCQ|| 2 MARK)

  • 0.5 ms< RC < 1 ms

  • 1 μs<<RC<0.5 ms

  • RC << 1 μs

  • RC >> 0.5ms

Question 6

Consider sinusoidal modulation in an AM system. Assuming no overmodulation, the modulation index (μ) when the maximum and minimum values of the envelope, respectively, are 3V and 1 V, is ______. (rounded off to two decimal places)

(PYQ || 2014 NAT|| 1 MARK)

  • 0.5

Question 7

In a double side-band (DSB) full carrier AM transmission system, if the modulation index is doubled, then the ratio of total sideband power to the carrier power increases by a factor of ________. (Rounded off to one decimal place)

( PYQ || 2014 NAT|| 1 MARK)

  • 4

Question 8

Consider the signal

s(t) = m(t)cos(2 πf c t) + ˆm (t)sin(2 πf c t)

where m (t) denotes the Hilbert transform of m(t) and the bandwidth of m(t) is very small compared to f c . The signal s(t) is a

(PYQ || 2015 MCQ|| 1 MARK)

  • high-pass signal

  • low-pass signal

  • band-pass signal

  • double sideband suppressed carrier signal

Question 9

In the system in Figure (a), m(t) is a low-pass signal with bandwidth W Hz. The frequency response of the band-pass filter H(f) is shown in Figure (b). If it is desired that the output signal z(t) = 10x(t), the maximum value of W(in Hz) should be strictly less than ______.

Screenshot-2025-09-30-015653

(PYQ || 2015 MCQ|| 2 MARK)

  • 350

Question 10

Consider the amplitude modulated (AM) signal A c cos ωCt+ 2cos ωMt.cos ωct. For demodulating the signal using envelope detector, the minimum value of AC should be

(PYQ || 2008 MCQ|| 1 MARK)

  • 2

  • 1

  • 0.5

  • 0

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