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This mock test of Test: Communication Systems - 6 for Electronics and Communication Engineering (ECE) helps you for every Electronics and Communication Engineering (ECE) entrance exam.
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QUESTION: 1

A Discone antenna is

Solution:

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

QUESTION: 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 10^{6} rad/sec the frequencies of upper sidebands are

Solution:

.

QUESTION: 3

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

Solution:

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

QUESTION: 4

Dolby system of noise reduction divides the total audio spectrum into

Solution:

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

QUESTION: 5

In a system an input resistance of 20 kÎ© causes a noise voltage of 15Î¼V. If two such input resistances each of 20 kÎ© are used in parallel, the total noise voltage will be about

Solution:

Overall resistance = 10 kW. Noise voltage = (15) = 10.6 Î¼V.

QUESTION: 6

In the television system in India, the odd and even fields are scanned

Solution:

Each field is scanned 25 times but alternately.

QUESTION: 7

Consider the following

- A signal
*f*(*t*) and its Hilbert transform*f*(_{h}*t*) have the same energy density spectrum - A signal
*f*(*t*) and its Hilbert transform,*f*(_{h}*t*) have the same auto correlation function - A signal
*f*(*t*) and its Hilbert transform are mutually orthogonal

Solution:

All are correct.

QUESTION: 8

Vidicon and plumbicon operate on the principle of

Solution:

Photo conduction uses the fact that resistivity of some materials depends on intensity of light falling on their surface.

QUESTION: 9

The typical squelch circuit

Solution:

To suppress noise when carrier is absent, squelch circuit cuts off audio amplifier.

QUESTION: 10

A TV receiver has a width of 60 cm. The aspect ratio is 4/3. The required EHT supply is about

Solution:

EHT supply is 1 kV per diagonal inch.

Height = 60 x = 45 cm.

Diagonal = .

QUESTION: 11

A narrow band noise n(*t*) has symmetrical spectrum and has power density spectrum 0.2 x 10^{-6}. The power density of quadrature component is

Solution:

.

QUESTION: 12

The radio receivers mostly used now a days are

Solution:

Now-a-days only super heterodyne radio receivers are used.

QUESTION: 13

The reactance of coupling capacitors, for DC quantities, is

Solution:

For zero frequency capacitive reactance is infinite.

QUESTION: 14

An AM wave is given by *s*(*t*)_{AM} = 20 (1 + 0.5 cos 10^{3}*t* + 0.3 cos 10^{4} *t* cos 10^{6} *t*) the modulation index of the envelope is

Solution:

.

QUESTION: 15

The modulating frequency in frequency modulation is increased from 10 kHz to 30 kHz. The bandwidth is increased by

Solution:

BW = 2*f _{m}*(1 + Î²)

for Î² >> 1

B.W. a 2*f _{m}* .

QUESTION: 16

A sinusoidal signal with peak to peak amplitude of 1.536 volt is quantized into 128 levels using a mid-rise uniform quantizer. The quantization noise power is

Solution:

.

QUESTION: 17

An FM wave is made to pass through a frequency tripler. If original modulation index is *m _{f}*, the output of tripler has a modulation index equal to

Solution:

Frequency tripler makes modulation index three times.

QUESTION: 18

A balun provides an impedance transformation of

Solution:

Balun has turn ratio of 2 : 1. Therefore, impedance transformation is 4 : 1.

QUESTION: 19

A signal *m*(*t*) with bandwidth 500 Hz is first multiplied by a signal *g*(*t*) where . The resulting signal is then passes through an ideal low pass filter with bandwidth 1 kHz the output of the low pass filter would be

Solution:

Multiplication in time domain is equivalent to convolution. According to the property of impulse signal, convolution of a signal *m*(*t*) with Î´(*t*) is signal itself.

*m*(*t*) x *g*(*t*)

*m*(*t*).

QUESTION: 20

If Î³ = (S_{0}/N_{0})/(S_{i}/N_{m}) where S_{0} and S_{1} are signal power output and signal power input and N_{0} and N_{m} are noise power output and noise power input, the value of Î³ for SSB-SC and DSB-SC amplitude modulated system respectively are

Solution:

SSB - SC and SSB - DB have Î³ = 1 and are similar from noise point.

QUESTION: 21

Consider the draw backs

- Slope overload
- Serration noise
- Granular noise

Solution:

In delta modulation the, recovered waveform may be dc whereas the original signal is not dc. This noise is called granular noise.

QUESTION: 22

It is required to increase the maximum range of a radar by a factor of two. The peak power transmitted in the radar system has to be increased by a factor of

Solution:

Range Î¼ (Power)^{0.25}

QUESTION: 23

A source generates three symbols with probabilities 0.25, 0.25, 0.50 at a rate of 3000 symbols per second. Assuming independent generation of symbols, the most efficient source encoder would have average bit rate is as

Solution:

Use Shannon Fano coding.

(0.25) is encoded as 01

Number of bits/sec

= 0.25 x 3000 x 2 + 0.25 x 3000 x 2 + 0.5 x 3000

45000 bits/sec.

QUESTION: 24

Armstrong F.M. transmitter performs frequency multiplication in stages

Solution:

where (Î”*f*) is the frequency deviation.

QUESTION: 25

The filter circuit for feeding tweeter has

Solution:

Series capacitance and shunt inductance is high pass circuit.

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