Electronics and Communication Engineering - Communication Systems

1. 

A Discone antenna is

A. a combination of disk and cone in close approximity
B. a combination of disk and cone at λ/4 spacing
C. a combination of disk and cone far apart
D. a combination of disk and cone at λ/2 spacing

Answer: Option A

Explanation:

It is a combination of disc and cone in close approximity.


2. 

In a two tone AM system the two modulating frequencies are 2000 p and 4000 p rad/sec. If carrier frequency is 2p x 106 rad/sec the frequencies of upper sidebands are

A. 1.002 MHz and 1.004 MHz
B. 1.001 MHz and 1.002 MHz
C. 1.001 MHz and 1.004 MHz
D. 1.002 MHz and 1.008 MHz

Answer: Option B

Explanation:

.


3. 

For a system to be designated as hi-fi, the signal to noise ratio should be at least

A. 25 dB
B. 50 dB
C. 75 dB
D. 100 dB

Answer: Option B

Explanation:

Hi-Fi system has a ratio at least equal to 50 dB.


4. 

Dolby system of noise reduction divides the total audio spectrum into

A. 8 frequency bands
B. 4 frequency bands
C. 2 frequency bands
D. none of the above

Answer: Option B

Explanation:

Dolby A system divides the frequency in 4 bands viz.f < 80 Hz, 80 Hz < f < 2999 Hz, f > 3000 Hz, f > 9000 Hz.


5. 

In a system an input resistance of 20 kΩ causes a noise voltage of 15μV. If two such input resistances each of 20 kΩ are used in parallel, the total noise voltage will be about

A. 15 μV
B. 30 μV
C. 7.5 μV
D. 10.6 μV

Answer: Option D

Explanation:

Overall resistance = 10 kW. Noise voltage = (15) = 10.6 μV.


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