8/18/2019 B.tech. Electronics (ECT-204) 2012

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NATIONAL

INSTITUTE

OF

TECHNOLOGY

KURUKSHETRA

THEORY

EXAMINATION

Question

Paper

Month

and

year

of

the

Exam

Programme:..........B.Tech.

Subject:.

. .

May-June

2012

Semester.........4th

Analog

Communication

Maximum

Marks...............S0

Time

allowed

......3

hours

Course

No............ECT-204

Number

of

questions

to

be

attempted

......0S

The

question

paper

has

......09.........

questions

l.(a)

(b)

(c)

2.(a)

(b)

3.(a)

(b)

4.(a)

(b)

5.(a)

Note:

Auempt

one questionfrom

each

unit.

euestion

oo.

I it

**pulsory.

Explain

how

time

constant

in

diode

detector

affects

demodulation

of

an

AM

signal?

Explain

the

relationship

between

frequency

modulation

and

phase

modulation.

Explain

the

effect

of

mixing

a

noise

signal

with

a

noise

signal?

T]NIT.I

Calculate

the

output

noise power

when

a

white

Gaussian

noise

of

power

spectral

density q

/2

is

qqsed

through

(i)

RC-low

pass

filter

(ii)

The

rectangular

(ideal)

Iow pass

filter (iii)

A

differentiating

filter (iv)

An integrator.

A

white

noise

u'(r)

of

power

spectral

density

No

/2

is

applied

to a

Butterworth

low-pass

filter

of

order

n,

whose

magnitude

response

is

defined

bv

lacll=flt.(/^J"]"'.

Determine

(i)

the

noise

equivalent

bandwidth

for

this

low-pass

filter.

(ii)

What

is

the

limiting

value

of

the

noise

equivalent

bandwidth

as

n

approaches

to

infinity?

A

coil

an

inductance

of 2H

and

an internal

resistance

of I

O

is

shunted

by

capacitor

of 2

F.

Determine

the

power density

spectrum

of

the

thermal

noise across

network tarminats.

Describe

the

mathematical

representation

of

noise

in

communication

systems.

Derive

the

expressions

for

power

spectral

density

and

noise

power.

UNIT.II

What

is

quadrature

null

effect?

How

it

can

be

reduced

in

case

of

DSB-SC

modulationd

",

consider

the

AM

signal

s(t)=4lt+pcos(znf;)fcos(2tf"t)

produced

by

a

sinusoidal

modulating

signal

of

frequencyf

.

Assume

that

the

modulation

factor

is p=2,and

the

carrier

frequency

f"

is

much greater

than

f..

The

AM

signal

s(l)

is

applied

to

an

ideal

envelope

detector, producing the

output

v(r) .

(i)

Find the

Fourier

series

representation

of

v(r)

(ii)

what

is

the

ratio

of

second-harmonic

amplitude

to fundamental

amplitude

in

v()

?

A modulated

signal

is

given

by

e(t)

=

I00cos[200rt+10 m@)dr),

where

m(t)isthe

periodic

message

signal

is

shown

below

Fig.

I

m(t)

(3+3+4)

(6)

(4\

(4\

(6)

(s)

(s)

(s)

8/18/2019 B.tech. Electronics (ECT-204) 2012

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(b)

(i)IsthisisaPMoranFMsignaI?(ii)Findthemodulationindexuna,n ffi

bandwidth'

(iii)

Sketch

carefully

the

frequency

deviation

with

respect

to time.

(iv)

Find

the

peak

frequency

deviation.

Show

that

the

demodulation

of

FM

signals

can

be

achieved

by

Foster-seeley

discriminator?

UNIT-III

Draw

the

block

diagram

and

explain

tlre

working

of

a

super

heterodyne

receiver.

List

out

the

advartages

and

disadvantages

ofa

super

heterodyrie

receivei.

Draw.the

block

diagram

and

describe

the

working

of

a

simple

FM

transmitter

using

reactance

modulator.

with

the

help

of

a

neat

block

diagram,

explain

Armstrong

F.M

transmiffer

method.

Explain

(i)

Image

Signal

Rejection.

(ii)

Frequency

Drift (iii)

Frequency

scintillation.

T'NIT-IV

calculate

the power

spectral

density

of

Noise

in

case

of DSB-SC

and

also

calculate

figure

of

merit.

A

baseband

signal

m(t)

is

transmitted

using

ssB.

The

received

signal

may

be

written

as

s,(r)=

m(t)cos

ztrTt+k@sin2rf t

where

kt|

isderived

from

m(r)

by

shifting

by

90.

phase

of

every

spectral

component

in

z(t).

Assume

that

the

power'rp our

d;,ty

of

m(t)

is

l't,

lrl

t t

G,,(f)=4;i'

Vl'f*.

Find

(i)

The

input

signat

power

(ii)

The

output

signal

power

(iii)

If

I

o,

lrl,

r,

white

Gaussian noise

with

power

spectral

density

r7

I

2

isadded

to the

SSB

signal,

find

the

output

SNR.

The

baseband

signal

cuts

off

atf

=

fu

.

Explain

how

can'

the

noise

performance

of

FM

communication

systems

be

improved

by

increasing

the

bandwidth?

6.(a)

(b)

7.(a)

(b)

8.(a)

(b)

(5)

(s)

(s)

(5)

9.

(s)

(10)