GATE Questions & Answers of Analog communications: amplitude modulation and demodulation, angle modulation and demodulation, spectra of AM and FM, superheterodyne receivers, circuits for analog communications

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Total 23 Questions have been asked from Analog communications: amplitude modulation and demodulation, angle modulation and demodulation, spectra of AM and FM, superheterodyne receivers, circuits for analog communications topic of Communications subject in previous GATE papers. Average marks 1.43.

The unmodulated carrier power in an AM transmitter is 5 kW. This carrier is modulate by a sinusoidal modulating signal. The maximum percentage of modulation is 50%. If it is reduced to 40%, then the maximum unmodulated carrier power (in kW) that can be used without overloading the transmitter is__________

A modulating signal given by $ x\left(t\right)=5\;\sin\left(4\pi10^3t-10\pi\;\cos\;2\pi\;10^3t\right) $V is fed to a phase modulator with phase deviation constant kp = 5 rad/V. If the carrier frequency is 20 kHz, the instantaneous frequency (in kHz) at t = 0.5 ms is___________

The block diagram of a frequency synthesizer consisting of a Phase Locked Loop (PLL) and a divide-by-N counter (comprising ÷2 ,÷4, ÷8, ÷16 outputs) is sketched below. The synthesizer is excited with a 5 kHz signal (Input 1). The free-running frequency of the PLL is set to 20 kHz. Assume that the commutator switch makes contacts repeatedly in the order 1-2-3-4.

The corresponding frequencies synthesized are:

A superheterodyne receiver operates in the frequency range of 58 MHz − 68 MHz. The intermediate frequency fIF and local oscillator frequency fLO are chosen such that fIFfLO. It is required that the image frequencies fall outside the 58 MHz − 68 MHz band. The minimum required fIF(in MHz) is ________

The amplitude of a sinusoidal carrier is modulated by a single st=5 cos 1600πt + 20 cos 1800πt +5 cos 2000πt sinusoid to obtain the amplitude modulated signal. The value of the modulation index is __________

For a superheterodyne receiver, the intermediate frequency is 15 MHz and the local oscillator frequency is 3.5 GHz. If the frequency of the received signal is greater than the local oscillator frequency, then the image frequency (in MHz) is __________

Consider the signal s(t)=m(t)cos2πfct+m^(t)sin2πfct where m^t denotes the Hilbert transform of m(t) and band width of m(t) is very small compared to fc. The signal s(t) is a.

In the system shown 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 ________.

A message signal mt=Amsin2πfmt is used to modulate the phase of a carrier Accos2πfct to get the modulated signal yt=Accos2πfct+mt. The bandwidth of $y\left(t\right)$

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 3 V and 1 V, is ________.

In the figure, Mf is the Fourier transform of the message signal mt where A = 100 Hz and B = 40 Hz. Given vt=cos2πfct and wt=cos2πfc+At,, where fc>A. The cutoff frequencies of both the filters are fc.

The bandwidth of the signal at the output of the modulator (in Hz) is _____.

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 ______.

The signal m(t) as shown is applied both to a phase modulator (with kp as the phase constant) and a frequency modulator (with kf as the frequency constant) having the same carrier frequency.

The ratio kp/kf (in rad/Hz) for the same maximum phase deviation is

The Column-1 lists the attributes and the Column-2 lists the modulation systems. Match the attribute to the modulation system that best meets it

Column-1 Column-2
P.  Power efficient transmission of signals I.   Conventional AM
Q. Most bandwidth efficient transmission of voice signals II.   FM
R. Simplest receiver structure III.  VSB
S.  Bandwidth efficient transmission of signals with Significant dc component IV.  SSB-SC
   

A message signal mt=cos2000πt+4cos4000πt modulates the carrier ct=cos2πt fct where fc=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

Suppose that the modulating signal is m(t) = 2cos (2π fmt) and the carrier signal is xC(t) = AC cos(2πfct), which one of the following is a conventional AM signal without over-modulation?

Consider an angle modulated signal x(t) = 6cos[2πx106t+2sin(8000πt) + 4cos(8000pt)] V. The average power of x(t) is.

For a message signal mt=cos2πfmt and carrier of frequency fc, which of the following represents a single side band (SSB) signal?

Consider the amplitude modulated (AM) signal AC cosωct+2cosωmtcosωct .For demodulating the signal using envelope detector, the minimum value of Ac should be

Consider the frequency modulated signal 10cos2π×105t+5sin2π×1500t+7.5sin2π×1000t with carrier frequency of 105 Hz. The modulation index is

The signal cosωct+0.5cosωmtsinωct is

A Hilbert transformer is a

In the following scheme, if the spectrum M(f) of m(t) is as shown, then the spectrum Y(f) of y(t) will be

(A)

(B)

(C)

(D)