Test: Analog Communication - 2
20 Questions MCQ Test GATE ECE (Electronics) 2022 Mock Test Series | Test: Analog Communication - 2
An AM signal is represented in the domain as S (-c) = 4 cos 1800T + 10 cos 20002 + 4cos2200T then the bandwidth of the AM signal is _____ Hz
A 100MHz carrier is frequency modulated by 10 KHz wave. For a frequency deviation of 50 KHz, calculate the modulation index of the FM signal.
Carrier frequency fc = 100MHz
Modulating frequency fm = 10 KHz
Frequency deviation Δf = 500 KHz
Modulation index of FM signal is given by
mf = Δf/fm
= 500 * 103/ 10 * 103
= 50
Consider a system shown in the figure-1. Let X(f) and Y(f) represent the Fourier transforms of x(t) and y(t) respectively. The ideal HPF has cutoff frequency of 10 KHZ. X(f) is shown in the figure-2. The positive frequency where Y(f) has spectral peaks are
2 KHz S. 24 KHz
Three message signals each band limited to 10 kHz, 10 kHz, and 5 KHz are transmitted through a channel using frequency division multiplexing. The modulation techniques used are any, D513-SC, SSB respectively. The guard band is 2 KHz. Band width of the channel is
BW after AM modulation = 2 x 10 = 20 KHz
BW after DSS-SC modulation = 2 x10 = 20 KHz BW after SSB modulation = 5 KHz
Guard band width = 2 x 2k=4 k
∴ Total BW after frequency multiplexing = 49 KHz
A signal '4 t is band limited to the frequency range 0 to fn. The signal frequency is translated by multiplying it by the signal V, t —cos2πfct . The value of fc such that the bandwidth of the translated signal is 1% of the carrier frequency fc will be
BW after multiplexing =
The peak amplitude of an AM signal for tone modulation with carrier having maximum amplitude as 5 V, varies from 2 to 8 V then the sideband power is _________________________ (W)
Consider a frequency multiplier shown in the figure below.ss
A NBFM signalX Mal t with f, = 200KHz and f, ranges 50 Hz to 150 KHz is given as an input to frequency multiplier. The Maximum Frequency deviation of the NBFM is 25 Hz. The maximum frequency deviation at the output is 75 KHz.
Q. The required frequency multiplication n is
Consider a frequency multiplier shown in the figure below.ss
A NBFM signalX Mal t with f, = 200KHz and f, ranges 50 Hz to 150 KHz is given as an input to frequency multiplier. The Maximum Frequency deviation of the NBFM is 25 Hz. The maximum frequency deviation at the output is 75 KHz.
Q. The carrier frequency at the output is
SSB modulation may be viewed as a hybrid form of amplitude modulation and frequency modulation. Assuming a message signal m(t) = sin t and a carrier frequency 'fc', the instantaneous frequency for the lower side band is
For SSB modulation, the modulated wave is
Given
In a certain experimental conducted with fm = 1 kHz 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.
The frequency sensitivity of the modulator is
When Am = 2V, carrier component = 0
ie. J0 (03) = 0 for the 1st time
From the curve, corresponding (3=2.44
If Ex and Ey represent the energies of m(t) and y(t) respectively, then 
_______________
Mh (ω) = -jM (6)) sgn(ω), (The given system is a Hilbert transformer) The π of a Hilbert transformer is
H (ω) = -jsgn (ω)
Determine the spectrum of y(t) in the system shown below.
Output of AM Modulator
Consider the scheme shown
If x (t) = m (t) cos (coct) then y (t) is
The above schematic generates FM signal as a wideband frequency modulator
using the indirect method. The output FM signal is characterized by fc = 100 MHz, Δf = 75kHz (min). If the phase modulator has a modulation index of β= 0.3 radians, then n1, n2 are respectively
The lowest modulation frequency i.e.,
Produces a frequency deviation
The highest modulation frequency ie.
Produces a frequency deviation of
Let us take the case of 100Hz 
as the worst case
Solving (1) & (2) we get,
n1=75 , n2 =50
What is the standard IF frequency for AM receivers?
In AM receivers, the standard intermediate frequency is approximately 455 KHz.
Consider a message signal m(t) = 2sin(10007t)
Phase sensitivity constant Kp=5rad/V
Frequency sensitivity constant Kf=75KHz/V
Q. If amplitude of message signal is doubled, then bandwidth for FM and PM will be
Δf = 75x 4 = 300KHz ,
BFM = 2(300 + 0.5) = 601KHz, Δ f= 2 x Kp x 2 x 500 = 10KHz,Bpm = 2(10 + 0.5) = 21KHz
A 20 KW and 1 MHz carrier is frequency modulated by a sinusoidal signal so that the maximum frequency deviation is 50 KHz and the message signal is band limited to 500 KHz. The generated FM signal is amplitude modulated using carrier of 1 GHz.
Q. The bandwidth of the generated signal is
It is a narrow band FM
A 20 KW and 1 MHz carrier is frequency modulated by a sinusoidal signal so that the maximum frequency deviation is 50 KHz and the message signal is band limited to 500 KHz. The generated FM signal is amplitude modulated using carrier of 1 GHz.
Q. The bandwidth of the generated AM signal is
Taking above spectrum as message signal and generate AM signal using carrier 1GHZ
BW = 2 1,5 MHz = 3MHz
The input x(t) and the output y(t) of a certain nonlinear channel are related as y(t) = x(t) + 0.001 x2(t)
If frequency spectrum of input signal is
Q. The bandwidth of the output signal is
The input x(t) and the output y(t) of a certain nonlinear channel are related as y(t) = x(t) + 0.001 x2(t)
If frequency spectrum of input signal is
Q. Spectrum of the received signal after appropriate low pass filtering is
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