Adc 08a Apulsem

ANALOG & DIGITAL COMMUNICATION

By Engr. Hyder Bux MangrioInstitute of Information & Communication

TechnologiesMehran University of Engineering and Technology,

Jamshoro.

07TL-BATCH

Today's Lecture:24-26Analog

Pulse Modulation

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Analog modulation: input is continuous in time and value

Continuous-wave modulation AM, DB, SSB, VSB FM, PM Analog pulse modulation: Message value is continuous. Transmission

happens at discrete times. PAM, PWM, PPM

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Digital modulation: The message has discrete values

Sinusoidal carrier: uses a finite number of distinct signals to

represent digital data ASK, PSK, FSK Digital pulse modulation: use

quantization to get discrete message use coded pulse sequences: PCM, DPCM

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A series of regularly recurring pluses is made to vary in amplitude, duration, shape, or time as a function of the modulating signal.e.g. pulse-amplitude modulation (PAM)

pulse-width modulation (PWM) pulse-duration modulation (PDM) pulse-position modulation (PPM)

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Advantages of Pulse Modulation Noise immunity Inexpensive digital circuitry Can be time-division multiplexed with

other pulse modulated signals Transmission distance is increased

through the use of regulative repeaters Digital pulse streams can be stored Error detection and correction is easily

implemented

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Special encoding and decoding techniques may be necessary to increase transmission rates, thus making the pulse stream more difficult to recover

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Analog Pulse Modulation Digital Pulse Modulation

Pulse Amplitude (PAM)

Pulse Width (PWM)

Pulse Position (PPM)

Pulse Code (PCM)

Delta (DM)

Pulse Modulation

Nyquist Sampling Theorem :if a signal at a rate that is at least

twicethe highest frequency that it

containsthe original signal can be

completelyreconstructed

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Pulse Amplitude Modulation (PAM): The signal is sampled at regular intervals such that each

sample is proportional to the amplitude of the signal at that sampling instant. This technique is called “sampling”.

For minimum distortion, the sampling rate should be more than twice the signal frequency.

There are two classes of PAM signals: • PAM that uses

Natural Sampling (gating);

• PAM that uses Instantaneous Sampling to produce a flat-top pulse.

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Natural SamplingDEFINTION: If w(t) is an analog waveform

bandlimited to B hertz, the PAM signal that uses natural sampling (gating) is

ws(t) =w(t)s(t) Where

S(t) is a rectangular wave switching waveform and fs = 1/Ts ≥ 2B.

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Generating Natural Sampling:The PAM wave form with natural sampling can be generated using a CMOS circuit consisting of a clock and analog switch as shown.

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Instantaneous Sampling(flat-top )

• This type of PAM signal consists of instantaneous samples.

• w(t) is sampled at t = kTs

• The sample values w(kTs ) determine the amplitude of the flat-top rectangular pulses.

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DEFINITION: If w(t) is an analog waveform bandlimited to B Hertz, the instantaneous sampled PAM signal is given by

Where h(t) denotes the sampling-pulse shape and, for flat-top sampling, the pulse shape is,

( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( )s s s s s sk k k

w t w kT h t kT h t w kT t kT h t w t t kT

Pulse Width Modulation (PWM or PLM or PDM):

In this type, the amplitude is maintained constant but the duration or length or width of each pulse is varied in accordance with instantaneous value of the analog signal.

The negative side of the signal is brought to the positive side by adding a fixed d.c. voltage.

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Analog Signal

Width Modulated Pulses

In this type, the sampled waveform has fixed amplitude and width whereas the position of each pulse is varied as per instantaneous value of the analog signal.

PPM signal is further modification of a PWM signal. It has positive thin pulses (zero time or width) corresponding to the starting edge of a PWM pulse and negative thin pulses corresponding to the ending edge of a pulse.

This wave can be further amended by eliminating the whole positive narrow pulses. The remaining pulse is called clipped PPM.

PWM

PPM

Pulse Position Modulation (PPM):