PULSE AMPLITUDE MODULATION

INTRODUCTION TO PULSE AMPLITUDE MODULATION

Pulse-amplitude modulation, acronym PAM, is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses.Example: A two bit modulator (PAM-4) will take two bits at a time and will map the signal amplitude to one of four possible levels, for example −3 volts, −1 volt, 1 volt, and 3 volts.Demodulation is performed by detecting the amplitude level of the carrier at every symbol period.Pulse-amplitude modulation is widely used in baseband transmission of digital data, with non-baseband applications having been largely superseded by pulse-code modulation, and, more recently, by pulse-position modulation.

Analog pulse modulation Digital pulse modulation

PAM PWM PPM PCM DM ADM

PULSE MODULATION

In the pulse amplitude modulation of pulsed carrier is changed in proportion with the instantaneous amplitude of the modulating signal x(t). So the information is continued in the amplitude variation of PAM signal.

the carrier is in the form of train of narrow pulses .

If you compare the PAM system with the sampling process ,you will find these two processes are identical.

The PAM signal is then sent by either wire or cable or it is used to modulate a carrier.

PULSE AMPLITUDE MODULATION

Natural PAM (Chopper PAM)Flat top PAM

TYPES OF PAM

Generation of natural PAM

NATURAL PAM OR CHOPPER PAM

Low pass filter Multiplier

Pulse train generator

Continuous modulating signal x(t)

PAM signal

Waveforms of natural PAM

1.Baseband signal

2.Sampled signal

Circuit for natural PAM

MOSFET acting as switch

Natural PAM Signal

RLx(t) c(t)

DETECTION OF NATURAL PAM

Low pass filter

PAM signal Demodulated PAM signal

The PAM signal can be detected (demodulated) by passing it through a low pass filter.

The low pass filter cutoff frequency is adjusted to fm so that all the high frequency ripple is removed and the original modulating signal is recovered back.

DETECTION OF NATURAL PAM

Waveform of detection of PAM

Flat Top PAM

Flat-Top PAM

Waveform of Flat-top PAM

Recovering the original signal x(t) from PAM signal

Due to the aperture effect an amplitude distortion introduces. This distortion can be corrected by equalizer

Advantages: Better SNR due to increased signal power. Generation is easy.

Disadvantage: Aperture effect introduces distortion.

Parameters Natural PAM Flat top PAM

Nature of sampling Train of finite duration pulses

Train of finite duration pulses

Circuit arrangement

Uses a chopper Uses a sample n hold circuit

Signal power Increases with increase in the pulse width

Increases with increase in the pulse width

Bandwidth requirement

Increases with the reduction in pulse width

Increases with reduction in the pulse width

Effect of noise Moderate Moderate

Waveforms

Effect of noise on PAMSince the information is contained in the

amplitude variations, the noise will contaminate the information

Noise superimposed on PAM signal

Q. Explain generation and demodulation of PAM signal with the help of suitable diagrams. (10 marks-May 2008)

EXAM QUESTIONS