# Communication Engineering (Test 7)

## Tancet Anna University : Ece Electronics And Communication Engineering

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Q.1
During transmission over a certain binary communication channel, bit errors occur independently with probability p. The probability of AT MOST one bit in error in a block of n bits is given by
A. pn
B. 1 - pn
C. np( - p)n + ( + p)n
D. 1 - (1 - p)n
Answer : Option C
Explaination / Solution: Workspace
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Q.2
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. If these constellations are used for digital communications over an AWGN channel, then which of the following statements is true ?
A. Probability of symbol error for Constellation 1 is lower
B. Probability of symbol error for Constellation 1 is higher
C. Probability of symbol error is equal for both the constellations
D. The value of N will determine which of the constellations has a lower probability of symbol error
Answer : Option A
Explaination / Solution:
No Explaination.

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Q.3
At 20 GHz, the gain of a parabolic dish antenna of 1 meter and 70% efficiency is
A. 15 dB
B. 25 dB
C. 35 dB
D. 45 dB
Answer : Option D
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Q.4
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 expression and the region of convergence of the z -transform of the sampled signal are
A. B. C. D. Answer : Option C
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Q.5
Consider a baseband binary PAM receiver shown below. The additive channel noise n(t) is with power spectral density The lowpass filter is ideal with unity gain and cut-off frequency 1 MHz. Let Yk represent the random variable y(tk).
Yk = Nk , if transmitted bit bk = 0
Yk = a + Nk if transmitted bit bk = 1
Where Nk represents the noise sample value. The noise sample has a probability density function, (This has mean zero and variance 2/α2). Assume transmitted bits to be equiprobable and threshold z is set to a/2 = 10-6 V. The probability of bit error is
A. 0.5 × e-3.5
B. 0.5 × e-5
C. 0.5 × e-7
D. 0.5 × e-10
Answer : Option D
Explaination / Solution:

Probability of error is given by Workspace
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Q.6
The longitudinal component of the magnetic field inside an air-filled rectangular waveguide made of a perfect electric conductor is given by the following expression The cross-sectional dimensions of the waveguide are given as a = 0.08 m and b = 0.033 m. The mode of propagation inside the waveguide is
A.

TM12

B.

TM21

C.

TE21

D.

TE12

Answer : Option C
Explaination / Solution:

We have the expression, and the cross sectional dimension is
a = 0.08 m, b = 0.033m
Now, we compare the equation for TEm,n mode whose Hz is given as  Hence, given mode is TE21 mode

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Q.7
The modes in a rectangular waveguide are denoted by where m n and are the eigen numbers along the larger and smaller dimensions of the waveguide respectively. Which one of the following statements is TRUE?
A.  The TM10 mode of the wave does not exist
B.  The TE10 mode of the wave does not exist
C. The TM10 and TE10 the modes both exist and have the same cut-off frequencies
D.  The TM10 and TM01 modes both exist and have the same cut-off frequencies
Answer : Option A
Explaination / Solution:

TM11 is the lowest order mode of all the TMmn modes

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Q.8
The propagation constant of a lossy transmission line is (2 + 𝑗5) m-1 and its characteristic impedance is (50 + j0)𝜔 at 𝜔 = 106 rad s-1. The values of the line constants L, C, R, G are, respectively,
A. L = 200 μH/m, C = 0.1 μH/m, R = 50 Ω/m, G = 0.02 S/m
B. L = 250 μH/m, C = 0.1 μH/m, R = 100 Ω/m, G = 0.024S/m
C. L = 200 μH/m, C = 0.2 μH/m, R = 100 Ω/m, G = 0.02 S/m
D. L = 250 μH/m, C = 0.2 μH/m, R = 50 Ω/m, G = 0.04 S/m
Answer : Option B
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Q.9
A zero mean random signal is uniformly distributed between limits –a and +a and its mean square value is equal to its variance. Then the r.m.s value of the signal is
A. B. C. D. Answer : Option A
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Q.10
The impulse response of a continuous time system is given by h(t) = δ(t - 1) + δ(t - 3). The value of the step response at t = 2 is
A. 0
B. 1
C. 2
D. 3
Answer : Option B
Explaination / Solution:
No Explaination.

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