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GATE 2025 Communication Systems Foundation Quiz 31

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Question 1

A carrier wave is frequency modulated using a sinusoidal signal of frequency fm and amplitude Am. In a certain experiment conducted with fm = 1kHz and increasing Am (Starting from 0 volts), it is found that the carrier component of the FM signal is reduced to zero for the first time when Am = 2 volts. What is the frequency sensitivity of the modulator?

Question 2

Figure shows envelope of output of AM modulator. If message signal in waveform having zero dc value, find modulation index of AM signal.

Question 3

A 10MHz carrier is frequency modulated using modulated signal em = Fm sin 103 πt the resultant FM signal has deviation of 5kHz. Calculate the modulation index and capture range of PLL used for demodulation.

Question 4

Let 60% of modulation index is used in amplitude modulation. Now to same power, SSB modulation is used. Assume carrier power is 10W and sawtooth wave is used for modulation. What amount of power is used for AM modulation and how much % of power is saved while using SSB modulation?

Question 5

Consider the Armstrong FM modulator shown in the figure below

The narrow band FM signal has maximum angular deviation of 0.10 radians in order to keep distortion under control. The message signal m(t) has a bandwidth of 15 KHz, the oscillator frequency is 100 KHz and the mixer circuit is used for the up-conversion. If the wideband FM signal s(t) has a carrier frequency of fc = 104 MHz and a maximum frequency deviation of Δfmax = 75 KHz then the multiplication factor n2 will be

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