Topic: Communications (Test 1)



Topic: Communications
Q.1
Consider the signal  where  denotes the Hilbert transform of m(t) and the bandwidth of m(t) is very small compared to fcThe signal s(t) is a
A. high-pass signal
B. low-pass signal
C. band-pass signal
D. double side-band suppressed carrier signal
Answer : Option C
Explaination / Solution:

We have the signal,

Here, s(t) represents SSB - Lower side band, and thus a band pass signal.

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Q.2
In a GSM system, 8 channels can co-exist in 200 kHz bandwidth using TDMA. A GSM based cellular operator is allocated 5 MHz bandwidth. Assuming a frequency reuse factor of 1/5, i.e. a five-cell repeat pattern, the maximum number of simultaneous channels that can exist in one cell is
A. 200
B. 40
C. 25
D. 5
Answer : Option B
Explaination / Solution:



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Q.3
The time domain behavior of an RL circuit is represented by  For an initial current of
For an initial current of the steady state value of the current is given by 
A.
B.
C.
D.
Answer : Option A
Explaination / Solution:

Steady state all transient effect die out and inductor act as short circuits and forced response acts only. It doesn’t depend on initial current state. From the given time domain behavior we get that circuit has only R and L in series with vThus at steady state 

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Q.4
Two 4-array signal constellations are shown. It is given that φ1 and φ2 constitute an orthonormal basis for the two constellation. Assume that the four symbols in both the constellations are equiprobable. Let N0/2 denote the power spectral density of white Gaussian noise.

The if ratio or the average energy of Constellation 1 to the average energy of Constellation 2 is
A. 4a2
B. 4
C. 2
D. 8
Answer : Option B
Explaination / Solution:



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Q.5
The driving point impedance of the following network is given by

The component values are
A. L = 5H, R = 0.5Ω, C = 0.1F
B. L = 0.1H, R = 0.5Ω, C = 5F
C. L = 5H, R = 2Ω, C = 0.1F
D. L = 0.1H, R = 2Ω, C = 5F
Answer : Option D
Explaination / Solution:



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Q.6
For a parallel plate transmission line, let v be the speed of propagation and Z be the characteristic impedance. Neglecting fringe effects, a reduction of the spacing between the plates by a factor of two results in
A. halving of v and no change in Z
B. no changes in v and halving of Z
C. no change in both v and Z
D. halving of both v and Z
Answer : Option B
Explaination / Solution:
No Explaination.


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Q.7
One end of a loss-less transmission line having the characteristic impedance of 75Ω and length of 1 cm is short-circuited. At 3 GHz, the input impedance at the other end of transmission line is
A. 0
B. Resistive
C. Capacitive
D. Inductive
Answer : Option D
Explaination / Solution:



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Q.8
The bit rate of a digital communication system is R kbits/s. The modulation used is 32-QAM. The minimum bandwidth required for ISI free transmission is
A. R/10 Hz
B. R/10 Hz
C. R/10 Hz
D. R/5 kHz
Answer : Option B
Explaination / Solution:

In ideal Nyquist Channel, bandwidth required for ISI (Inter Symbol reference) free transmission is
 
Here, the used modulation is 32 - QAM (Quantum Amplitude modulation
 


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Q.9
A white noise process X(t) with two-sided power spectral density 1 X 10-10  W/HZ is input to a filter whose magnitude squared response is shown below.


A.

5 X 10-10  W

B.

1 X 10-6  W

C.

2 X 10-6  W

D.

1 X 10-5  W

Answer : Option C
Explaination / Solution:
No Explaination.


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Q.10
In the following network, the switch is closed at t = 0- and the sampling starts from t = 0. The sampling frequency is 10 Hz.

The samples x(n), n = (0, 1, 2, ...) are given by
A.

5(1 - e-0.05n)

B. 5e-0.05n
C. 5(1 e-5n)
D. 5e-5n
Answer : Option B
Explaination / Solution:
No Explaination.


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