A Study of Pulse Modulation Techniques Nabil Hilman Jaafar Jaafar Communication Technology Technology Section, Uniersiti !uala "um#ur $ritish Malaysian %nstitute &Uni!"'$M%(, $atu ), Jalan Sungai Pusu, *+-- !uala "um#ur. nabilhilman/uni0l.org
This labo labora rato tory ry sess sessio ion n is cond conduc ucte ted d to Abstract- This describe the principles of the three pulse modulation techni technique quess which which are pulse width modulation (PWM), (PPM)) and pulse amplitude pulse position modulation (PPM (PAM) over a digital transmission system A modulation !igi !igita tall "ommu "ommuni nica cati tion on # circ circui uitt boar board d is use use on $A"%T base unit is used to simulate and obtain the results which is shown on the virtual oscilloscope The transmitter part of the board is the circuit that produces the pulse modulated of digital signal The message signal from the transmitted pulse&modulated or digital signal is then then recove recovered red and recons reconstru tructe cted d at the receiv receiver er section The $'T% circuit in the PAM circuit bloc* is also functioning for recovering signal from PTM signals !igital !igital signals signals are better because its noise immunity immunity characteristic compared to analog signal +ence, digital signals are much reliable for data transmission Index Terms— Terms— Pulse Pulse Modulation. %.
1%2%TA" C3MMUN%CAT%3N
As any other ther tra transm nsmissi ission on sys system tem, digital transmission is a #rocess of data transmission oer a communication system 4hich 4idely 0no4n consist of at least a #air of transmitter and receier or more. The signals can be either in binary or in some other form form of discre discrete' te'le leel el digita digitall #ulses #ulses.. The origin original al information could be in analog signal 4hich has been conerted into a series of digital #ulses and being conerted bac0 into analog form at receier section or originally in a digital'ready state. 5ith the technology of digita digitall transm transmiss ission ion system systems, s, commun communica icatio tion n system6s equi#ment6s such as a #air of 4ires, coa7ial cabl cable, e, or an o#ti o#tica call fibe fiberr cabl cablee is need needed ed to interconnect the arious #oints 4ithin that #articular syst system em.. 1igit igital al #uls #ulses es are are im#o im#oss ssib ible le to be #ro#agated through a 4ireless transmission system, for e7am#le, the 8arth6s atmos#here or in free s#ace.
9igure 9igure : A sim#le communica communication tion system bloc0 bloc0 diagram, diagram, sho4s the transmitter and receier section.
The first digital transmission system for the #ur#ose of carrying digitally encoded analog signals, such as
the human oice, oer metallic 4ire cable bet4een tele#hone offices 4as deelo#ed by AT;T. %n recent years, digital transmission systems are used to carry not only digitally encoded oice or ideo signals but also also source source inform informati ation on in digita digitall form form direct directly ly bet4een com#uters com#uters and net4or0. The most im#ortant of digital transmission oer analog transmission is because of its noise immunity . 1igital signals are less interru#ted by noise because it is not necessary to ealuate the #recise am#litude, frequency, or #hase to ascertain its logic condition. Moreoer, the #ulses are ealuated during a #recise time interal, interal, and a sim#le sim#le determina determination tion is made 4hether the #ulse is either belo4 or aboe certain reference leel. $esides, digital transmission systems are more resistant com#ared to analog transmission to additie noise because the uses signal regeneration , inst instea ead d of signal amplification 4hic 4hich h is used used for for analog analog signal signal transm transmiss ission ion.. Hence Hence,, transm transmiss ission ion error errorss can be detect detected ed and correc corrected ted easily easily than than analog signals. %%.
PU"S8 M31U"AT%3N
In pulse modulation, modulation, it generally consis consists ts essent essential ially ly analog analog infor informat mation ion sampling and transferring the pulses from a source to a specic destination over a physical transmission medium (ie, optical bre cable). There are four ma
Pulse Width Modulation
P5M is also 0no4n as pulse duration modulation &P1M &P1M(( or pulse length modulation &P"M(, &P"M(, as the 4idth 4idth &acti &actiee #art #art of the duty duty cycle cycle(( at consta constant nt am#litude #ulse is aried according to the am#litude of the analog signal at the time the signal is sam#led. P5M signal at 9igure = &c( sho4s that the am#litude of sam#le is lo4er than the am#litude of sam#le =. Thus, #ulse is narro4er than #ulse =. %t can be seen that, the ma7imum analog signal am#litude #roduces the 4ides 4idestt #ulse, #ulse, and the minim minimum um analog analog signal signal am#litude #roduces the narro4est #ulse.
B.
Pulse Position Modulation
%n #ulse #osition modulation &PPM(, the #osition of a constant>4idth #ulse 4ithin a #rescribed time slot is aried de#ends on the am#litude of the sam#le of the analog signal. $y referring to the PPM signal on 9igure = &d(, the higher the am#litude of the analog signal, the far to the right the #ulse is moed and the lo4est the am#litude, the far to left the #ulse is #osition. Note that the am#litude of PPM signals is constant.
9igure = &f(. PCM is the only digitally encoded modulation technique sho4n in 9igure = that is 4idely used for digital transmission. 5ith PCM, the #ulses are of fi7ed length and fi7ed am#litude. PCM is a binary system 4here a #ulse or lac0 of a #ulse 4ithin a #rescribed time slot re#resents either logic or a logic - condition.
C. Pulse Amplitude Modulation
5ith #ulse am#litude modulation &PAM(, the am#litude of a constant'4idth, constant'#osition #ulse is aried according to the am#litude of the sam#le of the analog signal. The PAM sho4n in 9igure = &e(, 4here it can be obsered that the am#litude of a #ulse is follo4ing the am#litude of the analog signal. %n other 4ords, PAM 4aeforms resemble the original analog signal more than the 4aeforms of P5M and PPM. 9igure =: 1ifferent form of modulated signal oer different modulation techniques &PAM, P5M and PPM(.
D. Pulse Code Modulation Pulse code modulation &PCM( on the other hand is different com#are to PAM, P5M and PPM. PCM analog signal is sam#led and then conerted into a series of n'bit binary code for transmission #ur#ose. 8ach code has the same number of bits and requires the same length of time for transmission as sho4n in
9igure +: Sim#lified bloc0 diagram of a single'channel, sim#le7 PCM transmission system.
9igure = sho4s a bloc0 diagram of a sim#lified single'channel, sim#le7 &one'4ay only( system. The
band#ass filter limits the frequency of the analog in#ut signal to the standard oice'band frequency 4hich has a range bet4een +-- H? to +--- H?. The sample-and-hold circuit #eriodically sam#les the in#ut of analog signal and conerts those sam#les to a multileel PAM signal. The digital-to-analog converter &1AC( conerts the PAM sam#les into #arallel PCM codes, 4hich are conerted to serial binary data in the parallel-to-serial converter and then the out#utted onto the transmission line as serial digital #ulses. @e#eaters are #laced to regenerate digital #ulses.
message signals are synchroni?ed 4ith the sam#le signals so that it can be easily obsere and analy?e the relationshi#s of the message, sam#le and #ulse signals 4ith the irtual oscillosco#e.
%B. A
@8SU"TS AN1 A NA"S%S
Pulse Amplitude Modulation
%n the receier, the serial-to-parallel converter conerts serial #ulses to #arallel PCM codes. Then the 1AC conerts the PCM codes to multileel PAM signals. A lo4 #ass filter then conerts the PAM signals bac0 to its original form. %%%.
M8TH313"32
This laboratory session is conducted by using the "ab'Bolt6s 1igital Communication Model -== circuit board on the 9AC8T base unit. The 1igital Communications course #roides com#rehensie, hands'on'instruction in the terminology #rinci#les and a##lications of digital circuits, including: Sam#le, Sam#leDHold, Adder, @am# 2enerator, Com#arator, "imiter, 9ilter C318C, P"", Com#ressor and others. $y follo4ing the designed instructional #rogram Unit module 4hich is the %ntroduction to 1igital Communications , a quic0 and #recise hands'on training 4as carried out.
9igure -: 9igure aboe re#resent the message signal &green sine 4ae( and the sam#le #ulse &blue(.
Theoretically, #ulse am#litude modulation changes in terms of its am#litude. The height of the modulated signal #ulse is de#ending on the message signal6s am#litude. %t has a constant'4idth and constant' #osition.
The 87ercise (Digital Communication Concepts is diided into three sections 4hich are #ulse am#litude &PAM( circuit bloc0 signals, #ulse time modulation &PTM( circuit bloc0 signals and the #ulse code modulation &PCM( circuit bloc0 signals.
9igure : Pulse a m#litude modulated signal
9igure E: "ab'Bolt6s 1igital Communications circuit bloc0 that is used in the laboratory session.
3n the circuit board, there are t4o internally generated message signals. There are M and M=. M is a sine 4ae about *B #0'#0 and 0H?. 5hile on the other hand, M= is a sine 4ae 4hich is about *B #0'#0 = 0H?. %n each circuit board, the M and M=
9igure re#resents the #ulse am#litude modulated signal &blue(. The result sho4s that the am#litude of the modulated signal is follo4ing the am#litude of the message signal. The higher the am#litude of the message signal, the higher the #ulse of the modulated signal, ice ersa. %n other 4ord, PAM 4aeforms resemble the original analog signal more than the 4aeforms for P5M and PPM. %t can be seen that some of the #ulses are affected to noises along the transmission #rocess.
9igure =: PAM signals after being regenerated bac0 to analog signal at 9%"T8@ section.
The PAM signals that 4e obtained from the 9igure is then being conerted bac0 to analog form.The #rocess is done at the 9%"T8@ section on PAM circuit bloc0. The M= message signal from the 9%"T8@ out#ut has the same frequency as the original M= message signal. This is because the ) 0H? SP frequency is greater than t4o times the M= frequency. !.
Pulse Width Modulation
%n #ulse 4idth modulation, the 4idth of the #ulse is the ariable 4hile the am#litude is constant. The higher the am#litude of message signal &analog(, the 4ider the modulated #ulse. The results are as sho4n belo4 in 9igure +.
9igure E: @econstructed message signal.
As can be obsered on the 9igure E, the message signal is constructed bac0 to its original state before it6s transmitted. The #rocess is done at the 9%"T8@ on PAM circuit bloc0. ".
Pulse Position Modulation
Pulse #osition modulation, is also categori?e under PTM because it6s arying in time. Technically, the #osition of the #ulse 4ill moe more to the right 4hen the message signal is high. 5hile the #ulse 4ill moe more to the left, 4hich indicates the lo4 leel signal. The 4aeform are as sho4n in 9igure *.
9igure *: Pulse #osition modulation &PPM( signal.
9igure +: Pulse 4idth modulation &P5M( signal.
The blue 4aeforms sho4n on the oscillosco#e is the #ulse 4idth modulate &P5M( signal. %t aries on its #ulse 4idth. The highest #ea0 of the message signal &green sine 4ae( is resembled by the 4idest P5M signal 4hile the narro4est #ulse resembled the lo4est message signal am#litude.
As can be seen on the 4aeforms sho4n in 9igure *, the #ulse remain in its am#litude and 4idth. The only aries is its #osition. The higher the message signal &M(, the more to the right the sam#led #ulse is moed and the lo4er the message signal, the more to the left the sam#led #ulse is moed. 9igure F sho4s the reconstructed signal of the #ulse #osition modulation &PPM(. %t recoer bac0 its form as the original signal.
circuit board is use on 9.A.C.8.T base unit is used to simulate and obtain the results 4hich is sho4n on the irtual oscillosco#e. 1ifferences on different #ulse modulation technique can be seen from the results obtained. PAM arying in am#litude, P5M changes in terms of its 4idth and PPM arying in #osition. "astly, #ulse modulation consists essentially of sam#ling analog information signals, and then conerting those sam#les into discrete #ulses and trans#orting the #ulses from a source to a destination oer #hysical transmission medium. Hence, digital signal is much better than analog signal, because of its #remium benefit, noise immunity.
. 9igure F: @egenerated PPM signal
B.
C3NC"US%3NS
%t can be conclude that this #a#er #resented a study of seeral different ty#e of #ulse modulation methods. They are pulse width modulation &P5M(, pulse position modulation &PPM( and pulse amplitude modulation &PAM( oer a digital transmission system. A 1igital Communication
@ 898@8NC8S G Tomasi, 5. &=--+( !lectronic Communications #ystems$ "undamentals %hrough Advanced. *th edn. U##er Saddle @ier, N.J: PearsonDPrentice Hall. G= S. . Suh, IPulse 5idth Modulation for Analog 9iber' 3#tic Communications., in )K %888 Journal of "ight4ae Technology, )K, ##. -=>=. G+ S0lar. $, &=--=( Digital Communications& "undamentals and Applications. =nd edn. Prentice Hall
