# Electronics and Communication Engineering - Exam Questions Papers

21.
A uniform plane magnetic wave incident normally on a plane surface of a dielectric material is reflected and the percentage of reflected power is 75% What is VSWR?
14
1.4
15
13
Explanation:

22.
Silicon is doped with boron to a concentration of 4 x 1017 atoms/cm3. Assuming the intrinsic carrier concentration of silicon to be 1.5 x 1010/cm3 and the value of to be 25mV at 300K. Compared to undoped silicon, the Fermi level of doped silicon
goes down by 0.13 eV
goes up by 0.13 eV
goes down by 0.427 eV
goes up by 0.427 eV
Explanation:

Use EF - Ev =

Since it is doped with acceptor impurity, Fermi level will shift down.

23.
The Fourier series of a real periodic function has only
1. Cosine terms if it is even
2. Sine terms if it is even
3. Cosine terms if it is odd
4. Sine terms if it is odd
Which of the above statements are correct?
(1) and (4)
(1) and (2)
(2) and (4)
(2) and (3)
Explanation:

, then it has 0 and HZ frequency component.

24.
A communication channel with AWGN operating at a signal to noise ratio SNR >> 1 and bandwidth B has capacity C1. If the SNR is doubled keeping B constant, the resulting capacity C2 is given by
C2 ≈ 2C1
C2 ≈ C1 + B
C2 ≈ C1 + 2B
C2 ≈ C1 + 0.3B
Explanation:

C = B log2[1 + SNR]

C = B log2[SNR]

C' = B log2[2SNR] = B log2 SNR + B log22

C' = C1 + B.

25.
Power gain of antenna equals directive gain in VHF and UHF range if and only if efficiency of the antenna is __________ .
0.25
0.5
0.75
1