Electronics and Communication Engineering - Exam Questions Papers

31. 

A transmitted using AM has in modulated carrier output power of 10 KW and can be modulated to a maximum depth of 90% by a sinusoidal modulating voltage without causing overloading. Find the value to which an modulated carrier power can be increased without resulting in overloading if the maximum permitted modulating index is 40%

A. 14 KW
B. 12.96 KW
C. 26.96 KW
D. 2.96 KW

Answer: Option B

Explanation:

Now PT is fixed, μ = 0.4

Then PC is PC =


32. 

The signal flow graph of a system is shown in figure. The transfer function of the system is :

A.
B.
C.
D.

Answer: Option D

Explanation:

P1 = 1 [Path R(s to C(s)]

Δ = 1 - (L1 - L2 + L3) + L1L2

P1 = 1; Δ1 = 1

[Non touching loop gain to forward path]

.


33. 

The Laplace transform of the given waveform is

A. (1 - e-s + e-2s)
B. (1 - e-s + e-2s)
C. (1 - 2e-s + e-2s)
D. (1 - 2e-s)

Answer: Option C

Explanation:

f(t) = f1(t) + f2(t) + f3(t)

= 2tu(t) - 2 x 2(t - 1) + 2(t - 2)u(t - 2)

f(s) = - e-s + e-2s

= (1 - 2e-s + e-2s).


34. 

With respect to the following 555 timer circuit what will be the value of RA and R such that the LED turns on for a duration of 10 ms every time it receives negative trigger pulse. LED operating current = 20 mA, Vf = 1.4 V, Vo on = 13.4 V and C = 0.22 nf

A. 41.4 KΩ, 600 Ω
B. 45 KΩ, 630 Ω
C. 43.5 KΩ, 580 Ω
D. 44.9 KΩ, 600 Ω

Answer: Option A

Explanation:

Timer operated in mono, mode

RAC =

RA = 41.4 KΩ

R = = 600 Ω.


35. 

The region between a pin of parallel perfectly conducting planes of infinite extent is y and z directions is partially filled with a dielectric as shown below.

A 30 GHz TE10 wave is incident on the air-dielectric interface. The value of reflection coefficient at the interface is

A.
B.
C.
D.

Answer: Option C

Explanation: