# Communication Engineering (Test 1)

## Tancet Anna University : Ece Electronics And Communication Engineering

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Communication Engineering
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Q.1
A plane wave of wavelength λ is traveling in a direction making an angle 300 with positive x − axis and 900 with positive y − axis. The  field of the plane wave can be represented as (E is constant)
A.
B.
C.
D.
Explaination / Solution:

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Q.2
The field in a rectangular waveguide of inner dimension a × b is given by
Where H0 is a constant, and a and b are the dimensions along the x − axis and the y − axis respectively. The mode of propagation in the waveguide is
A. TE20
B. TM11
C. TM20
D. TE10
Explaination / Solution:

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Q.3
The circuit shown in the figure is used to charge the capacitor C alternately from two current sources as indicated. The switches S1 and S2 are mechanically coupled and connected as follows:

Assume that the capacitor has zero initial charge. Given that u(t) is a unit step function , the voltage vc(t) across the capacitor is given by
A.
B.
C.
D.
Explaination / Solution:

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Q.4
Consider the frequency modulated signal with carrier frequency of 105 Hz. The modulation index is
A. 12.5
B. 10
C. 7.5
D. 5
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Q.5
A transmission line has a characteristic impedance of 50 Ω and a resistance of 0.1Ω/m . If the line is distortion less, the attenuation constant(in Np/m) is
A. 500
B. 5
C. 0.014
D. 0.002
Explaination / Solution:

For distortion less transmission line characteristics impedance

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Q.6
In binary frequency shift keying (FSK), the given signal waveform are

u(t) = 5cos (20000 πt); 0 t T, and

u(t) = 5cos (22000 πt); 0 ≤ ≤ T,
Where T is the bit-duration interval and t is in seconds. Both u0 (t) and u1 (t) are zero outside the interval 0 ≤ ≤ T. With a matched filter (correlator) based receiver, the smallest positive value of T (in milliseconds) required to have u0 (t) and u1 (t) uncorrelated is

A. 0.25 ms
B. 0.5 ms
C. 0.75 ms
D. 1.0 ms
Explaination / Solution:

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Q.7
A transmission line terminates in two branches, each of length  as shown. The branches are terminated by 50W loads. The lines are lossless and have the characteristic impedances shown. Determine the impedance Zi as seen by the source.

A. 200

B. 100

C. 50

D. 25

Explaination / Solution:

The transmission line are as shown below. Length of all line is

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Q.8
x(t) is a stationary random process with auto-correlation function.   This process is passed through the system shown below. The power spectral density of the output process y(t) is

A. (4π2f2 + 1) exp (-πf2)
B. (4π2f2 - 1) exp (-πf2)
C. (4π2f2 + 1) exp (-πf)
D. (4π2f2 - 1) exp (-πf)
Explaination / Solution:

The given circuit can be simplified as

Power spectral density of output is

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Q.9
Two magnetically uncoupled inductive coils have Q factors q1 and q2 at the chosen operating frequency. Their respective resistances are R1 and R2. When connected in series, their effective Q factor at the same operating frequency is
A. q1 + q2
B. (1/q1) + (1/q2)
C. (q1R1 + q2R2) / (R1 + R2)
D. (q1R2 + q2R1) / (R1 + R2)
Explaination / Solution:

The quality factor of the inductances are given by

So, in series circuit, the effective quality factor is given by

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Q.10
The state variable description of an LTI system is given by
where y is the output and u is the input. The system is controllable for

A.
B.
C.
D.