Department of Electronics & Instrumentation Engineering LAB MANUAL SUBJECT: Communication Systems [EEC-60!
B"#ec$ B"#ec$ III %e %ear 'I (emester (emes ter )Branc$* EIE+
,CE# ,reater Noi.a
E/E1IMEN# No" * 02 Aim* Study of Time Division Multiplexing System. Apparatus Use.*
1. TDM CM Modulation T!ansmitte! T!aine! ". C.#.$ %. Connecting leads
#$eor3*
Time Division is a tec&ni'ue of t!ansmitting mo!e t&an one info!mation on t&e same c&annel. T&e samples consists of s&o!t pulses follo(ed )y anot&e! pulse afte! a long time inte!vals* t&is is as s&o(n in fig.1
T&is no+activity time inte!vals can )e used to include samples f!om t&e ot&e! c&annels as (ell. T&is means t&at seve!al info!mation can )e t!ansmitted ove! a single c&annel )y sending samples f!om diffe!ent info!mation sou!ces at diffe!ent moments in time. T&is tec&ni'ue is ,no(n as Time Division Multiplexing o! TDM. TDM is (idely used in digital communication systems to inc!ease t&e efficiency of t&e t!ansmitting medium. TDM can )e ac&ieved )y elect!onically s(itc&ing t&e samples suc& t&at t&ey inte!leave se'uentially at co!!ect instant in time (it&out mutual inte!fe!ence. "
roce.ure* 1. -i!st set up t&e follo(ing initial conditions: a Mode s(itc& in -/ST position. ) DC1 0 DC" cont!ols in -unction ene!ato! Bloc, fully cloc,(ise. c 1 234 and " 234 signal cont!ol levels set to give 15 6pp. d E!!o! c&ec, code gene!ato! s(itc& / 0 B in /75 0 B75 position e /ll s(itc&ed faults off. ". 8o( connect only t&e 1 234 output to C& 5. Tu!n $8 t&e po(e! and c&ec, t&at t&e /M output of 1 234 sine (ave is availa)le at t.p.19 of t&e ,it. %. Connect c&annel of C#$ to tp. 15 and c&annel of C#$ to tp.19. 8o( o)se!ve t&e timing 0 p&ase !elation )et(een t&e sampling signal and t&e sampled (avefo!m. ;. 8o( also connect " 234 supply to C31. Connect c&annel 1 of t&e C#$ to tp.1" and c&annel of C#$ to tp.19. 9. $)se!ve t&e (avefo!ms and !elate t&e timing signals. 45seration*
1esult* Time Division Multiplexing System &as )een studied. recautions* 1. C&ec, t&e connections )efo!e s(itc&ing $n t&e ,it. ". Connections s&ould )e done p!ope!ly. %. $)se!vation s&ould )e ta,en p!ope!ly.
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E/E1IMEN# No" * 07 Aim* To study ulse Code Modulation and Demodulation tec&ni'ue. Apparatus Use.* 1. CM modulation < demodulation ST"15% t!aine!. ". C.#.$ %. Connecting leads #$eor3*
ulse Code Modulation tec&ni'ue involves follo(ing steps: =a Sampling: T&e analog signal is sampled acco!ding to t&e ny'uist c!ite!ia. T&e ny'uist c!ite!ia states t&at fo! fait&ful !ep!oduction of a )and limited signal* t&e sampling !ate must )e at least t(ice t&e &ig&est f!e'uency component p!esent in t&e signal. So sampling f!e'uency ≥ " fm* (&e!e fm is maximum f!e'uency component p!esent in t&e signal !actically t&e sampling f!e'uency is ,ept slig&tly mo!e t&an t&e !e'ui!ed !ate. =) /llocation of )ina!y codes: Eac& )ina!y (o!d defines a pa!ticula! na!!o( !ange of amplitude level. T&e sampled value is t&en app!oximated to t&e nea!est amplitude level. T&e sample is t&en assigned a code co!!esponding to t&e amplitude level* (&ic& is t&en t!ansmitted. T&is p!ocess is called 'uanti4ation and it is gene!ally ca!!ied out )y t&e /
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roce.ure* 1. Ensu!e t&at t&e M$DE s(itc& s&ould )e in -/ST mode. ". Connect C3 5 0 C3 1 to DC1 /8D DC". %. Ensu!e t&at t&e DC1 and DC" cont!ols in -unction ene!ato! Bloc, s&ould )e in fully cloc,(ise di!ection and >1234 and " 234 signal cont!ols set a!t 156pp. ;. 8o( tu!n $8 t&e ,it and see t&at t&e ?ED glo(s. 9. @it& t&e &elp of Digital 6oltmete!* adAust t&e DC1 amplitude cont!ol until t&e DC1 output measu!es 56. . $)se!ve t&e output on t&e /
D 1
D9 1
D; 1
D% 1
D" 1
D1 1
D5 1
@&e!e fo! t&e negative values it is less t&an 1555555.-o! F96 t&e output is as follo(s: D 5
D9 5
D; 5
D% 5
D" 5
D1 5
D5 5
9
T&is is o)tained at t&e app!oximately full anti cloc,(ise position of t&e DC Cont!ol. H. Tu!n t&e DC1 cont!ol fully anticloc,(ise and !epeat t&e a)ove p!ocedu!e )y va!ying t&e DC" cont!ol. 15. T!igge! t&e dual t!ace oscilloscope exte!nally )y t&e C3.1 signal availa)le at t.p.1" and o)se!ve t&e signal at C3.5 and C3.1 at t.p.9 (it& !efe!ence to t&e signal at t.p.. 11. 8o( connect t&e oscilloscope c&annel 1 to C31 sample at t.p. and s,etc& t&e t&!ee (avefo!ms.
45serations*
1esult* T&e CM Modulation < Demodulation studied. recautions*
1. Connections s&ould )e c&ec,ed )efo!e s(itc&ing $8 t&e ,it. ". $)se!vations s&ould )e ta,en p!ope!ly.
E/E1IMEN# No" * 06 Aim* To study delta modulation 0 demodulation and o)se!ve t&e effect of slope ove!load. Apparatus Use.* 1. Delta modulation < demodulation t!aine!. ". C.#.$ %. Connecting leads #$eor3 * Delta Modulation is a system of Digital Modulation sc&eme in (&ic& t&e diffe!ence )et(een t&e sample value at sampling time 2 and sample value at t&e p!evious sampling time = , F 1 is encoded into Aust a single )it.
$ne (ay in (&ic& delta modulato! and demodulato! is assem)led is as s&o(n in fig.1 and fig.".
T&e )ase)and signal m=t and its 'uanti4ed app!oximation m=t a!e applied as inputs to a compa!ato!. / compa!ato! simply ma,es a compa!ison )et(een inputs. f signal amplitude &as inc!eased* t&en modulato!s output is at logic level 1. f t&e signal amplitude &as dec!eased* t&e modulato! output is at logic level 5. T&us t&e output f!om t&e G
modulato! is a se!ies of 5s and 1s to indicate !ise and fall of t&e (avefo!m since t&e p!evious value. T&e compa!ato! output is t&en latc&ed into a D flip+flop (&ic& is cloc,ed )y t&e t!ansmitte! cloc,. T&us t&e output of t&e flip+flop is a latc&ed 1 o! 5 sync&!onous (it& t&e t!ansmitte! cloc, edge. T&e )ina!y se'uence is t!ansmitted to !eceive and is also fed to t&e unipola! to )ipola! conve!te!. T&is )loc, conve!ts logic 5 to voltage level of ;6 and 1 to voltage level of F ;6. T&e )ipola! output ia applied to t&e integ!ato! (&ose output is : a #ising linea! !amp signal (&en F ;6 is applied to it ) -alling linea! !amp signal (&en ;6 is applied to it. T&e integ!ato! output is t&en connected to t&e F ve te!minal of voltage compa!ato!. roce.ure*
1. Ma,e connections as pe! diag!am. ". Ensu!e t&at t&e cloc, f!e'uency selecto! )loc, s(itc&es / 0 B a!e in /75 and B75 position. %. 8o( tu!n $n t&e ,it and see t&at ?ED glo(s. ;. n o!de! to ensu!e fo! t&e co!!ect ope!ation of t&e system* (e fi!st connect 5 volts to t&e ve input of t&e compa!ato!. 8o( o)se!ve t&e output of t&e integ!ato! 1 =i.e. tp 1 and t&e output of t!ansmitte!s level c&ange! =i.e. tp 19. @&en t&e positive and negative output levels of t&e level c&ange! (ill )e e'ual t&e output (ill )e a t!iangula! (avefo!m as s&o(n in fig% =Case /. @&en t&e negative level is g!eate! t&an positive level* t&e integ!ato!s output level (ill )e as s&o(n in fig% =Case B. /nd (&en t&e positive output level is g!eate!* t&en t&e integ!ato!s output (ill )e as s&o(n in fig% =Case C. T&e levels can )e adAusted )y tu!ning t&e potentiomete! f!om one ext!eme to anot&e!. 9. /dAust t&e t!ansmitte!s level c&ange! p!eset until t&e output of integ!ato! is a t!iangula! (ave cente!ed at 5 volts. T&e pea, to pea, amplitude of t&e (ave s&ould )e 5.9 volts =app!ox.* t&is amplitude is ,no(n as t&e integ!ato! Step Si4e. . 8o( o)se!ve t&e output of t&e t!ansmitte!s )ista)le ci!cuit =i.e. tp 1;. t is no( a st!eam of alte!nate I1 and I5. T&is is t&e output of a delta modulato! and t&e Delta modulato! is no( said to )e )alanced fo! co!!ect ope!ation. . 8o( examine t&e output of integ!ato! at t&e !eceive! =i.e. tp ;. t s&ould )e a t!iangula! (ave (it& step si4e e'ual to t&at of integ!ato! in t!ansmitte! and ideally cente!ed a!ound 5 volts. G. 8o( o)se!ve t&e output of lo( pass filte!. t (ill )e a DC level cente!ed a!ound 5 volts. T&is is t&e output of Delta demodulato! and it is )alanced fo! co!!ect ope!ation. H. 8o( disconnect t&e 5 volts f!om t&e ve input of t&e compa!ato! and !econnect it to "95 34 signal of t&e function gene!ato! )loc,. 8o( o)se!ve t&e output of voltage compa!ato! =tp H* integ!ato! =tp 1. /lso o)se!ve t&e delta modulated output at t&e output of )ista)le ci!cuit. t &as )een encoded into st!eam of I5 and I1. 15. /lso o)se!ve t&e output of lo( pass filte! in t&e !eceive! =tp 91* (&ic& is t&e output of demodulato!. 11. 8o( disconnect "95 34 f!om t&e ve input of compa!ato! and !econnect it to 955 34 * 1 2&4* and " 2&4 outputs in tu!n. 8o( note t&e f!e'uency of t&e analog signal inc!eases* so t&e lo( pass filte!s output )ecomes mo!e disto!ted and !educed in amplitude. T&is effect is ,no(n as ISlope $ve!loading.
H
45serations*
1esult* T&e Delta modulation < demodulation and Slope $ve!loading effect &as )een studied. recautions* 1. T&e connections s&ould )e made p!ope!ly and tig&tly. ". C&ec, all t&e connections )efo!e s(itc&ing $8 t&e ,it.
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