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This mock test of Communication System for Electronics and Communication Engineering (ECE) helps you for every Electronics and Communication Engineering (ECE) entrance exam.
This contains 26 Multiple Choice Questions for Electronics and Communication Engineering (ECE) Communication System (mcq) to study with solutions a complete question bank.
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QUESTION: 1

The DSB-SC signal recevied at the output of a channel is given as S(t) = 2 cos2π(10^{3} - 0.5). cos2π(10^{6} -

1). Determine the group delay and phase delay introduced by the channel.

Solution:

For an amplitude modulated signal

the delay in carrier frequency

component gives phase delay and

delay in baseband (message)

signal gives the group delay.

{Reference: Page No 98, Simon

Haykin, common system 3rd

edition}

QUESTION: 2

Which of the following is incorrect about hilbert-transform?

Solution:

x_{h} (t) is delayed version by -90^{0} of

x(t)

QUESTION: 3

An amplitude modulated signal is given as s(t) = 14 cos8000πt + 20 cos10000πt + 14 cos12000πt.

Determine the modulation depth.

Solution:

QUESTION: 4

An amplitude modulated signal is given as S(t) = 10 (2 + cos2π x 10^{4}t) cos2π x 10^{7}t. If the carrier

frequency component is suppresed before transmission, then determine the percentage power

saved.

Solution:

QUESTION: 5

Suppose that non linear device (NLD) is available for which the output voltage V_{0} and input voltage V_{i} are related by V0(t) = a1 where a_{1} and a_{3} are constants. Let V_{i} = m (t) + cos2πf_{c}t and a_{1} = 1, a_{2} = 1. To generate a DSB-SC signal having carrier frequency is 5MHz, this NLD is followed by a BPF of appropriate centre frequency and band width. Determine f_{c}

Solution:

QUESTION: 6

Consider a square law device having input voltage V_{i} and output voltage V_{0} are related as

This SLD is used to generate an standard AM with a carrier frequency of 3MHz, as shown in the circuit below.

Determine the maximum bandwidth of message signal m(t) so that spectrum of s(t) is nonoverlapping.

Solution:

i.e the maximum bandwidth of

message signal may be 1MHz

QUESTION: 7

A message signal m(t) = 4 cos2πx 10^{4} t is used to generate an FM signal. Determine the minimum

value of (Δf)_{max} so that modulation efficiency is 100%.

Solution:

M(t) = 4cos2π x 10^{4}t

Here Am = 4, fm = 10 KHz,

Modulation efficiency is 100%

when β_{f} = 2.4, 5.5, 8.6-----

QUESTION: 8

A phase-modulated signal is given as S(t) = 30 cos[2πx10^{8}t + 100 cos2πx10^{4}t]. Determine the

transmission bandwidth required according to carson’s approximation.

Solution:

= 2 (100 + 1) x 10

= 20 x 101

= 2020 KHz

= 2.02 MHz

QUESTION: 9

Consider an FM signal, S(t) = 10 cos[2π x10^{7}t + 10 sin2π x10^{3}t]. Determine Kp/Kf if the above modulated signal is assumed phase-modulated signal.

Solution:

QUESTION: 10

The following message signal m(t) is given to both frequency modulator and phase modulator. If

the maximum phase deviation of both FM and PM is same, determine the Kf/Kp.

Solution:

and maximum phase-deviation of

FM signal

QUESTION: 11

Consider a sine wave of frequency 2 KHz and peak amplitude AM. This signal is applied to a delta modulator of step-size 0.4 Volt and sampling period 3.18 x 10^{-6} sec. What is the maximum power that

may be transmitted without slopeoverload distortion?

Solution:

QUESTION: 12

A PCM system uses an uniform quantizer followed by a 7-bit binary encoder. The bit rate of the system is equal to 50x10^{6} bits/ second. Determine the output signal to quantization noise ratio when a full-load sinusoidal modulating signal of frequency 1MHz is applied to the input.

Solution:

(SNR) = (6n + 1.8) dB

= 6 x 7 + 1.8

= 43.5 dB

QUESTION: 13

An analog signal is sampled, quantized and encoded into a binary PCM system. The specifications of the PCM system include the following: Sampling role = 8 KHz Number of representation levels = 64

Determine the minimum bandwidth required if PCM wave is transmitted over a baseband channel.

Solution:

QUESTION: 14

A binary PCM system uses pulse P(t) to transmit symbol 1 and - P(t) to transmit symbol 0. The additive noise at the reciever input is white and Gaussian, with zero mean and power spectral density

10^{-10} W/Hz. Assuming that symbols 1 and 0 occur with equal probability, determine the probability of error at the reciever output.

Solution:

For binary polar signal the

probability of error is given as

QUESTION: 15

A source emits one of four symbols S_{0}, S_{1}, S_{2} and S_{3} with probabilities 1/3, 1/6, 1/4 and 1/4 respectively.

The successive symbols emitted by the source are statistically independent. Calculate the entropy

of the source.

Solution:

QUESTION: 16

Consider a sinusoidal signal with random phase, defined by X(t) = where A and f_{c }are constants and θ is a random variable that is uniformly distributed over the interval Determine the autocorelation function of X (t)

Solution:

The probability function of random

variable is given as,

Now the auto-corelation function

of X(t)

QUESTION: 17

The probability density function of a random variable is shown in the figure. Determine P (X 0)

Solution:

The probability for X0 is exactly

1. (i.e whole area under density

function curve)

QUESTION: 18

A 1W video signal having bandwidth of 200 MHz is transmitted to a reciver through a cable that has 50 dB loss. If the efficitive two sided noise power spectred density at the receiver is 10^{-20} Watt/Hz, then the signal to noise ratio at the receiver is

Solution:

P_{S} = 1 W

Since cable loss is 50 dB (105), So

power at the input of reciever

P_{S} = 10^{-5} Watts

Noise power = 2 x200 x10^{6} x10^{-20}

2, Because of two sided PSD

= 4 x 10^{-12} Watts

Therefore

QUESTION: 19

A computer puts out binary data at the rate of 56 kbps. The computer output is transmitted using a

baseband binary PAM system that is designed to have a raised cosine spectrum. Determine band width

required if the roll-off factor is 0.5.

Solution:

Bit Rate (R_{b}) = 56 Kbps

Since it is transmitted by 2-array,

so minimum bandwidth without

raised-cosine pattern

and minimum bandwidth with

raised-cosine pattern

QUESTION: 20

A QPSK system transmits binary data at the rate of 2.5 x 10^{6} bit per second. Determine minimum

bandwidth required for the transmission.

Solution:

QUESTION: 21

For attenuation of high frequencies we should use

Solution:

Since X_{c} for high frequencies is low, the high frequencies are shunted to ground and are not transmitted.

QUESTION: 22

The pulse shown in the figure, is given as the input of a matched filter. Determine the peak value of

the output of matched filter.

Solution:

QUESTION: 23

A binary random variable X takes the value of 1 with probability 2/3. X is input to a binary symetric

channel with cross-over probability 1/2.

Determine the value of H(Y)

Solution:

QUESTION: 24

A voice-grade channel of the telephone network has a bandwidth of 3.4 KHz calculate the information capacity of the telephone channel for a signal to noise ratio of 30 dB.

Solution:

Capacity of a gaussian channel is

given as,

C = BW log_{2} (1 + SNR)

Here SNR = 30 dB (1000)

So,

QUESTION: 25

Determine the minimum sampling frequency of x(2t), where x(t) = sine (1000t)

Solution:

Minimum sampling frequency of

x(t) is 1000 Hz (1KHz)

So, minimum sampling frequency

of x(2t) is 2KHz

QUESTION: 26

The probability density function of random variable X is given as

Determine the variance (σ^{2}) of random variable X.

Solution:

Given

Comparing this equation with the

standard gaussian probability

density function

i.e

### Communication System GATE

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