Presented By:Niraj kumar14-24-208CSPEDept. of ECE

OutlineIntroductionTypes of CPMGeneration of PAM,PWM,PPMDetection of PAM,PWM,PPMApplicationConclusion

IntroductionIn Pulse-Amplitude Modulation (PAM) a pulse signal is used to sample an analog signal. The result is a train of constant-width pulses. The amplitude of each pulse is proportional to the amplitude of the message signal at the time of sampling.

Generation of PAM :–

PAMTo generate the PAM output signal, we

may choose to represent the input to the transmit filter hT(t) as a train of weighted impulse functions

k

ks kTtatx )()(

• Consequently, the filter output x(t) is a train of pulses, each with the required shape hT(t)

k

Tk kTthatx )()(

Filtering of impulse train in transmit filter

Transmit

Filter

k

Tk kTthatx )()(

k

ks kTtatx )()(

)(thT

)(txs )(tx

Hence the signal at the receiver filter output is

)()()( tvkTthatyk

k

Where h(t) is the inverse Fourier transform of H() and v(t) is the noise signal at the receive filter output

• Data detection is now performed by the Data Slicer

FilterPAMEqualizer

-Signal

PAM DetectionSampling y(t), usually at the optimum instant

t=nT+td when the pulse magnitude is the greatest yields

nk

dkdn vtTknhatnTyy

))(()(

Where vn=v(nT+td) is the sampled noise and td is the time delay required for optimum sampling

• yn is then compared with threshold(s) to determine the recovered data symbols

Nyquist Pulse Shaping

T

f

H(f)

T

f

Why?Sample h(t) with a train of pulses at times kT

k

s kTtthth )()()(

• Consequently the spectrum of hs(t) is

k

s TkHT

H )2(1)(

• Remember for zero ISI

0for 00for 1

)(nn

nTh

Consequently hs(t)=(t)The spectrum of (t)=1, therefore

1)2(1)( k

s TkHT

H

• Substituting f=/2 gives the Nyquist pulse shaping criterion

k

TTkfH )(

Nyquist Pulse ShapingNo pulse bandwidth less than 1/2T can

satisfy the criterion, e.g.,

T

f

Clearly, the repeated spectra do not sum to a constant value

Nyquist Pulse ShapingThe minimum bandwidth pulse spectrum

H(f), ie, a rectangular spectral shape, has a sinc pulse response in the time domain,

elsewhere 0

212T1-for )(

TfTfH

• The sinc pulse shape is very sensitive to errors in the sample timing, owing to its low rate of sidelobe decay

RC Pulse ShapingNormalised Spectrum H(f)/T Pulse Shape h(t)

x

x

x

f *T t/T

ApplicationUse in EthernetUse in photo biologyUse in electronic drivers for LED lighting

Thank you

PULSE WIDTH MODULATIONGENERATIONUSING A COMPARATOR OF TWO

INPUTS,SAWTOOTH WAVWFORM AND MESSAGE SIGNAL RESPECTIVELY.

AS THE NAME SUGGESTS,WIDTH OF CARRIER PULSES VARY WITH THE AMPLITUDE OF INPUT MESSAGE SIGNAL.

IS DONE BY CONVERTING IT FIRST INTO PAM,THEN USING COMBINATION OF LOW

PASS FILTER AND EQUALIZER.