8/19/2019 Adc_unit One_FM_PM

1/28

ANALOG AND DIGITAL

COMMUNICATION

UNIT I - ANALOG COMMUNICATION• High Level AM Transmitter.• TRF AM Receiver.• Su erhetr!"#ne Receivers.

8/19/2019 Adc_unit One_FM_PM

2/28

M TR NSMITTERSHigh Level Transmitter

8/19/2019 Adc_unit One_FM_PM

3/28

High Level Transmitter O erati!n$

The modulating signal and the carrier signal is modulated to AMenvelope in the modulator.

The modulated signal in band limited by band pass flter. The Liner Intermediate & Linear power amplifer amplifes themodulated signal which would be su cient or transmission.

The coupling networ couples the modulating signal with antenna ortransmission.A"vantages !% High Level Transmitter$It is !on Linear."igh Level Modulator has good amplifcation actor with #lass #amplifer."igh Level Modulator has good potential to convert low re$uency tohigh re$uency signal using re$uency up%converter.

8/19/2019 Adc_unit One_FM_PM

4/28

M RECIEVERS:TRF –Tuned Radio Fre uen!" Re!eiver

The T M receiver has three stages'

( )tage.*etector )tage.Audio )tage.

8/19/2019 Adc_unit One_FM_PM

5/28

RF Stage$ The ( )tage contains two or three ( amplifer to flter and ampli y the receivedsignal su cient to the detector circuit.

&etect!r Stage The detector stage converts the ( signal directly to in ormation signal.

Au"i! Stage The Audio stage amplifes the in ormation signal and provides to the spea er.O erati!n

The received AM signal is coupled to the or ( stage. The ( stage allows only the re$uency that is f+ed by varying the L# tancircuits.

All the three L# tan circuits are couples so that all are tuned to samere$uency. The tuned re$uency is amplifed by the ( stage. The *etector circuit separated the in ormation signal rom the carrier signal. The in ormation signal is amplifed by the Audio amplifer and the output isprovided to the spea er.

&is-A"vantages !% TRF receiver$'an" (i"th$ The band width is not synchronous when the amplifer is tuned towide range o re$uency.Insta)ilit#$ The T ( receivers are instable due to the use o large number o (amplifers tuned to same re$uency.

*aria)le Gain$ In T ( receivers, the overall gain varies or a wide range ore$uency due to non%uni orm L-# tan circuits in ( amplifer.

8/19/2019 Adc_unit One_FM_PM

6/28

Su#er Heterod"ne Re!eiver

8/19/2019 Adc_unit One_FM_PM

7/28

Su#er Heterod"ne Re!eiver

"eterodyne means two mi+ two re$uencies together innonlinear device or to translate one re$uency to another

re$uency.A bloc diagram o non%coherent super heterodyne receiveris shown above.

There are fve sections to super heterodyne receiver.

( )ection.Mi+er-#onverter section.I( section.Audio detector section.

8/19/2019 Adc_unit One_FM_PM

8/28

Su#er Heterod"ne Re!eiverRF Secti!n.

The ( section consists o re%selector and an amplifer stage. The pre%selector is a board%tuned band pass flter with an ad/ustablecenter re$uency.

The purpose o pre selector is to limit the image re$uencies enteringin to the receiver circuit.

The pre%selector also reduces the overall receiver bandwidth. The ( amplifer also determines the sensitivity o the receiver. The ( receiver has ollowing advantages'

0reater gain & 1etter sensitivity.Improved image re$uency re/ection.1etter signal to noise ratio.1etter selectivity.

8/19/2019 Adc_unit One_FM_PM

9/28

Mi+er,C!nverter Secti!n. The mi+er- converter stage is the combination o ( oscillator andmi+er-converter.

The mi+er-converter is non linear device which converts radio re$uency

to Intermediate re$uency 2 ( to I(3."eterodyning ta es place in this stage, the radio re$uencies are downconverter to intermediate re$uencies.Although the re$uencies are down converter he envelope o the carrieris not changed.

The most common intermediate re$uency used in AM broadcast bandreceiver is 455 Khz .IF Secti!ns

The I( section o series o I( amplifers and 1and pass flters which iscalled as I( strip.Most o the receiver gain and selectivity is achieved in this section.I( re$uency and bandwidth are constant or all stations.

The I( re$uency is always less than the ( re$uency. The I( section amplifes and provides the signal to Audio *etector.

8/19/2019 Adc_unit One_FM_PM

10/28

&etect!r Secti!n$ The purpose o detector section is to convert the I(signals bac to original source in ormation.

The detector is generally called an audio detector. The detector can be as simple as diode or a LL or abalanced modulator.Au"i! Am li er Secti!ns$

The audio section comprises several cascaded audioamplifers and one or more spea ers. The number o amplifers used is depends on theaudio signal power determined.

8/19/2019 Adc_unit One_FM_PM

11/28

SIN$LE SI%E& N% M'%(L T'RS:

F T /ush- ull 'alance" M!"ulat!rs

8/19/2019 Adc_unit One_FM_PM

12/28

FET )ush*#ull &alan!ed ModulatorsCircuit ! erati!n$

The carrier signal are ed to two (4T amplifers 25 a & 5 b3

simultaneously which are in phase to each other. The carrier signal produces current in both top and bottom sideo trans ormer T6 which are e$ual in magnitude but 789: out ophase.As the current produced by the carrier signal are out o phasethey cancel each other so no carrier component is at theoutput.

The modulating signal is applied to the gates o the two (4Tswith 789: out o phase.

The modulating signal increases drain current in one (4T at thesame time decreases drain current in the other (4T.

8/19/2019 Adc_unit One_FM_PM

13/28

FET )ush*#ull &alan!ed Modulators The fgure shows that the increase in dc drain current I da and I db due to carrier signal which are e$ual but travels in opposite

direction so it cancels each other at the secondary o theoutput trans ormer T6.

The fgure below shows the phasor diagram when the carriersignal travels in opp direction produces the sum o the$uiescent and carrier currents which are same and in phase toeach other.

8/19/2019 Adc_unit One_FM_PM

14/28

FET )ush*#ull &alan!ed Modulators

The phasor diagram below shows the additional currentcomponent 2I ma & I mb 3due to modulating signal.

The total current or other side o winding is e$ual to sum o dc,carrier and modulating currents.

Thus the dc and carrier currents cancel each other. The continuously changing carrier and modulating signal

currents produces cross ; product re$uencies.

8/19/2019 Adc_unit One_FM_PM

15/28

SIN$LE SI%E& N% M'%(L T'RS:'alance" 'ri"ge M!"ulat!r$

The operation o the balanced bridge modulator depend on switchingaction o diodes *7, * condition o the diodes. The amplitude o the carrier signal is ept higher than modulatingsignal.

8/19/2019 Adc_unit One_FM_PM

16/28

&alan!ed &ridge Modulator:

?hen the carrier polarities are 2@-%3 the diodes are reverse biased, sothe modulating signal is trans erred to load resistor 2 L3.?hen the carrier polarities are 2%-@3 the diodes are orwarded biased,so the load resistor 2 L3 is short circuited.As the carrier voltage changes rom positive to negative and viceversa, the output wave orm is a series o pulses with upper and lowersideband.

8/19/2019 Adc_unit One_FM_PM

17/28

SIN$LE SI%E& N% TR NSMITTERS

The ollowing are the two types o))1 transmitters'

Single Sideband Transmitter: Filter Method.Independent sideband.

8/19/2019 Adc_unit One_FM_PM

18/28

SIN$LE SI%E& N% TR NSMITTER:FILTER METH'%

8/19/2019 Adc_unit One_FM_PM

19/28

SIN$LE SI%E& N% TR NSMITTER:FILTER METH'%

8/19/2019 Adc_unit One_FM_PM

20/28

IN%E)EN%ENT SI%E& N% TR NSMITTER

8/19/2019 Adc_unit One_FM_PM

21/28

SIN$LE SI%E& N% RECEIVER

The ollowing are the our ))1receivers'

!on%#oherent 1( eceiver.#oherent )ingle )ideband 1( eceiver.)ingle )ideband 4nvelope )ideband eceivers.Multichannel ilot #hannel )ingle sideband

eceiver.

8/19/2019 Adc_unit One_FM_PM

22/28

Non*Coherent &F' Re!eiver+ The selected carrier re$uency is amplifed and mi+ed downto intermediate re$uency or amplifcation and band

reduction. The output rom the I( amplifer is mi+ed with the output

rom the beat frequency oscillator 21( 3. The 1( re$uency is e$ual to I( carrier re$uency

A th i i % h t th ( ill t $ d

8/19/2019 Adc_unit One_FM_PM

23/28

As the receiver is non%coherent the ( oscillator re$uency andthe 1( oscillator re$uency are not synchroniBed.

The di>erence between the transmit and receive re$uency willproduce frequency o set error in the demodulated signal.

The ( mi+er and second detector are product detectors. The received signal which is suppressed or reduced ( carrier ismi+ed with one sideband to produce an intermediate re$uency.

The output rom the second product detector is the sum anddi>erence between the I( and beat re$uency.

The di erence signal in the original information signal .

C'HERENT SIN$LE SI%E& N% &F' RECEIVER

8/19/2019 Adc_unit One_FM_PM

24/28

C'HERENT SIN$LE – SI%E& N% &F' RECEIVER

The carrier recovery circuit consists o LL which trac s thereceived signal.

The synchroniBer circuit produces a coherent local oscillator

re$uency and 1( re$uency.

8/19/2019 Adc_unit One_FM_PM

25/28

SIN$LE SI%E& N% ENVEL')E %ETECTI'NRECEIVER

This type o receiver is also called as exalted carrierreceiver .

8/19/2019 Adc_unit One_FM_PM

26/28

C RRIER SIN$LE*SI%E & N%RECEIVER+

( ) $

8/19/2019 Adc_unit One_FM_PM

27/28

M(LTICH NNEL )IL'T C RRIER SIN$LE*SI%E & N% RECEIVER+ This receiver uses re$uency synthesiBer to produce coherent local andbeat re$uency oscillator re$uency.

The ( input signal ranges rom = M"B to 69 M"B. The C# centre re$uency is ad/usted with an e+ternal channelselector over a re$uency range o D M"B to 6< M"B.

The C# trac s above the incoming ( by

8/19/2019 Adc_unit One_FM_PM

28/28