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PRINCIPLES OF ELECTRONIC COMMUNICATION

UNIT – I

FUNDAMENTALS OF ANALOG COMMUNICATION

2 MARKS

1. Draw the basi b!"#s "$ "%%&'iati"' s(ste%s.

2. De$i'e %")&!ati"' a') )e%")&!ati"'.

Modulation is the process of changing the parameters of a relatively high frequency

carrier signal in proportion with the instantaneous value of the modulating signal.

*. Is %")&!ati"' %&st $"r "%%&'iati"' s(ste%+ E,-!ai' wh(+

Modulation is necessary for distance communication system. The main reasons for need

of modulation are,

• The antenna needed for transmitting signals should have size at least λ/4, where, λ

is the wavelength. The information signal, also known as aseand signal is of

low frequency !and therefore the wavelength is high". #f we need to transmit such

a signal directly, the size of the antenna will e very large and impossile to uild.$ence direct transmission is not practical.

• The radiated power y an antenna is inversely proportional to the square of the

wavelength. %o, if we use high frequency signals, the power radiated will e

increased.

• #f we transmit the aseand signals directly, the signals from different transmitters

will get mi&ed up and the information will e lost.

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. C!assi$( the )i$$ere't t(-es "$ %")&!ati"'+

'%(, )%( and *%( also come under digital modulation techniques.

/. 0hat is %ea' b( the ter% a%-!it&)e %")&!ati"'+

'mplitude Modulation is the process of changing the amplitude of a relatively high

frequency carrier signal in proportion with the instantaneous value of the modulating

signal.

. "w %a'( i'-&ts are there t" a' AM %")&!at"r+ 0hat are the(+

Two inputs are there in 'M modulator. $igh frequency +arrier signal and low frequency

message signal are the inputs of 'M modulator.

. I' AM "%%&'iati"' s(ste%3 what is %ea' b( the ter%s %")&!ati'4 si4'a!3

arrier3 %")&!ate) wa5e a') AM e'5e!"-e+

Modulating signal- message signal !low frequency" which is to e transmitted.

+arrier signal high frequency signal which will carry the message signal.

Modulated signal output of modulation process.

'M envelope %hape of the 'M waveform.

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6. Draw the DS78FC 9AM: wa5e$"r%.

;. Draw the $re<&e'( s-etr&% "$ a' AM DS78FC wa5e.

1=. De$i'e &--er a') !"wer si)eba')s a') the &--er a') !"wer si)e $re<&e'ies "$ AM.

The signal components aove the carrier frequency constitute the &--er si)eba')

!US7", and those elow the carrier frequency constitute the !"wer si)eba') !LS7". The

frequencies within these ands are called as upper side frequencies and lower side

frequencies respectively.

. 0hat is the re!ati"'shi- betwee' the %")&!ati'4 si4'a! $re<&e'( a') the

ba')wi)th i' a "'5e'ti"'a! AM s(ste%+

The 0andwidth of an 'M 1%0)+ wave is equal to the difference etween the highest

upper side frequency and lowest lower side frequency. 0andwidth of the modulated

signal ecomes twice the frequency fm of modulating signal.

02 3 f c 5 f m!ma&"6 f c  f m!ma&"6 3 7f m!ma&" 

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12. De$i'e %")&!ati"' "e$$iie't 9%")&!ati"' i')e,: a') -ere't %")&!ati"'.

The ratio etween amplitude modulating signal and the carrier signal amplitude.

The modulation inde& !m" can vary etween 8 and .

9. 0hat is the si4'i$ia'e "$ -ere't %")&!ati"'+ 0hat ha--e's i$ -ere't

%")&!ati"' is ab"5e 1==>+

*ercent modulation gives the percentage change in the amplitude of the output wavewhen the carrier is acted on y a modulating signal. This provide the information aout,

to which e&tent the signal can e modulated. To avoid distortion, percent modulation

must not e&ceed 88 percent.

1. Draw the 5"!ta4e s-etr&% "$ DS78FC wa5e.

1/. 0rite the e<&ati"' $"r t"ta! -"wer "$ DS78FC wa5e$"r% a') )raw its -"wer

s-etr&%.

67

/

7

44

7

7

77

m P  P 

 P m

 P 

 P m

 P m

 P  P 

ct 

cc

ccct 

+=

+=

++=

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2here, *t is total power, *c is carrier power and m is modulation inde&.

1. 0hat are the )isa)5a'ta4es "$ AM DS78FC+

#n conventional 'M, carrier power constitutes two thirds or more of the total transmitted

 power. This is a ma:or drawack ecause the carrier contains no information; thesideands contain the information.

1?. 0hat are i')iret FM a') i')iret PM+

2hen the frequency of a wave is changed, the phase will also change indirectly and vice

versa. This leads to indirect *M when )M is performed and indirect )M when *M is

 performed.

16. 0hat ha--e's t" a%-!it&)e "$ the arrier wa5e i' $re<&e'( %")&!ati"'+

The +arrier amplitude remains the same after frequency modulation.

1;. Draw the FM wa5e$"r%.

2=. 0hat is $re<&e'( )e5iati"'+

The amount of change in carrier frequency produced y the modulating signal is known as

 frequency deviation (frequency shift).

<f -frequency deviation

m  fm E k   f    =∆

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( fm - deviation sensitivity

=m  - peak amplitude of message signal

21. 0hat is %")&!ati"' i')e, a') -ere't %")&!ati"' "$ FM+

The ratio of the frequency deviation to the modulating frequency is defined as

modulation inde& of )M.

.

*ercent modulation in )M

22. De$i'e the ba')wi)th "$ FM 9ars"'@s r&!e:.

2here f m is message signal frequency and ∆f is frequency deviation.

2*. 0hih is %"re i%%&'e t" '"ise AM "r FM+ 0h(+

)M is more immune to noise than 'M. >oise in channel can produce change in

amplitude and phase of )M, ut not frequency directly.

2. 0hat is FM '"ise tria'4!e+

 >oise triangle is a triangular noise distriution for )M. >oise triangle is the study of

effect of noise on the carrier signal of the )M wave. The noise triangle shows that noise

has a greater effect on the higher modulating frequencies than on the lower ones.

2/. 0hat is the -&r-"se "$ -re8e%-hasis a') )e8e%-hasis+

m

  f    f  

  f  

cynalfrequenmessagesig 

eviation  frequencyd m

  ∆==

/88?mod@!ma&"

"!

  f  

  f  ulation

  actual 

∆

∆=

"!7 m  f    f   BW    +∆=

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' high pass filter that arti$iia!!( e%-hasie the high‐frequency components !amplitude"

of the message signal prior to the modulation is called as pre emphasis.

The de‐emphasis filter !low pass filter" restores the original signal y )ee%-hasii'4 the

high‐frequency components !amplitude".

2. Draw PM wa5e$"r%.

1 MARKS

. 1raw the lock diagram of communication systems and e&plain functions of each

 lock.

7. 2hat is the principle of 'mplitude modulationA 1erive e&pression for the 'M

wave and draw its spectrum.

9. 1erive 'M modulated signal with relevant waveforms and notations and draw

the frequency spectrum of 'M and e&plain. *rove that the amplitude of carrier is

unaffected in modulation process.

4. 1erive the e&pression for total power in an 'M 1%0-)+ and draw the power

spectrum.

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B. =&plain the frequency modulation with neat diagram and derive the )M equation.

C. 2hat are the effects of noise on )MA =&plain a method to make )M wave

immune to noise.

. =&plain in detail aout pre-emphasis and de-emphasis.

D. =&plain the phase modulation with neat diagram and derive the *M equation.

E. +ompare 'M, )M and *M.

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UNIT – II

DIGITAL COMMUNICATION

2 MARKS

1. 0hat is )i4ita! %")&!ati"'+

#n digital modulation, an analog carrier signal is modulated y a digital it stream. 1igital

modulation methods can e considered as digital-to-analog conversion, and the

corresponding demodulation or detection as analog-to-digital conversion.

2. Gi5e the e,-ressi"' $"r Sha''"' !i%it $"r i'$"r%ati"' a-ait(.

The ma&imum rate at which data can e correctly communicated over a channel in

 presence of noise and distortion is known as its channel capacity.

+ 3 0 log7 ! 5 %/>"

where 0 is the andwidth of the channel and %/> is the ratio of received signal power to

received noise power.

*. Gi5e the e,-ressi"' $"r art!e(@s !aw.

# α   0 & t

where #3 information capacity !its per second"

 B3 andwidth !hertz"t = transmission time !seconds"

. Gi5e the N(<&ist $"r%&!ati"' $"r ha''e! a-ait(.

The minimum theoretical andwidth necessary to propagate a signal is called the

minimum >yquist andwidth or sometimes the minimum >yquist frequency. Thus,

$ b B 273 where f   is the it rate in ps and 0 is the ideal >yquist andwidth.

/. De$i'e bit rate a') ba&).

#n digital modulation, the rate of change at the input to the modulator is called the it rate

!f  " and has the unit of its per second !ps".The rate of change at the output of the modulator is called aud.

. 0hat )" ("& %ea' b( ASK 9Di4ita! a%-!it&)e %")&!ati"':+

A%-!it&)e8shi$t #e(i'4 !ASK " is a form of  amplitude modulation that represents digital 

data as variations in the amplitude of a carrier wave.

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?. 0h( ASK is a!!e) as "'8"$$ #e(i'4+

O'8"$$ #e(i'4  !OOK " the simplest form of amplitude-shift keying  !'%(" modulation 

that represents digital data as the presence or asence of a carrier wave.

6. De$i'e FSK with 'eat )ia4ra%.

)%(- The high frequency carrier signal frequency is changed with respect to the

amplitude of digital message signal.

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;. De$i'e -ea# $re<&e'( )e5iati"' 9shi$t: $"r FSK.

The peak shift in the carrier frequency !<f" is  proportional to the amplitude of the inary

input signal !vmt6", and the direction of the shift is determined y the polarity.

<f 3 Ff m  f sF / 7

2here, <f  = frequency deviation !hertz"

Ff m  f sF 3 asolute difference etween the mark and space frequencies !hertz"

1=. Draw the b!"# )ia4ra% $"r FSK )e%")&!at"r.

)%( non coherent receiverG

)%( coherent receiverG

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1 MARKS

. 2rite short notes on information capacity, its, it rate, aud and M-ary encoding.

7. =&plain on off keying !HH(" or '%(.

9. =&plain 0)%( !transmitter and receiver" and also discuss aout the andwidth.

4. =&plain 0*%( !transmitter and receiver" and also discuss aout the andwidth.

B. =&plain I*%( transmitter and receiver.

C. 2ith the help of lock diagram e&plain the concepts of I'M.

. 2rite short notes on

a" %quaring loop

 " +ostas loop

D. =&plain in detail aout the clock recovery.