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Pulse Modulation

Presented byKetki Kulkarni-3764.Savitri Naik-3778.Dipali Patil-3792.Shweta Patil-3796.Ankita Sawant-3819.

IntroductionModulation is the process of frequency translation in which any

one parameter(Amplitude, frequency or phase) of high frequency carrier signal is varied in accordance with instantaneous value of low frequency modulating signal.

Modulation is either analog or digital.A digital signal is superior to an analog signal because it is more

robust to noise and can easily be recovered, corrected and amplified. For this reason, the tendency today is to change an analog signal to digital data.

The process of transmitting signals in the form of pulses (discontinuous signals) by using special techniques.

This process is called pulse modulation

Types of Pulse Modulation.

Analog Pulse Modulation

Digital Pulse Modulation

Pulse Modulation

Pulse Amplitude Modulation (PAM)Pulse Width Modulation (PWM)

Pulse Code Modulation (PPM)

Pulse Position Modulation (PPM)

Pulse Density Modulation (PPM)

Types of Pulse Modulation

Pulse Amplitude Modulation Pulse Width Modulation Pulse Position Modulation Pulse Code Modulation Pulse Denstity Modulation

Sampling The signal is sampled at regular intervals such that each

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

Sampling means breaking of continous waves into discrete set of points

Sampling is common in all pulse modulation techniques.Analog signal is sampled every TS secs.Ts is referred to as the sampling interval. fs = 1/Ts is called the sampling rate or sampling

frequency.

Sampling Methods

There are 3 sampling methods: Ideal - an impulse at each sampling instant Natural - a pulse of short width with varying

amplitude Flat top - sample and hold, like natural but with

single amplitude value

Sampling Methods

Pulse Amplitude Modulation

In 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 Signal

Modulating Signal

Low Pass Filter

MultiplierPAM signal

Pulse train generator

The carrier is in the form of narrow pulses having frequency fs.The uniform sampling takes place in multiplier to generate PAM signal.

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

Advantages & Disadvantages

Advantages:-Generation & detection is easy.

Disadvantages:-Added noise cannot be removed easily as it has

impact on amplitude which carries information.Transmission bandwidth is too long.

Pulse Width ModulationIn this type, the amplitude is maintained constant

but the width of each pulse is varied in accordance with instantaneous value of the analog signal.

In pulse width modulation (PWM), the width of each pulse is made directly proportional to the amplitude of the information signal.

Generation of PWM signal

Generation of PWM signalThe block diagram of next slide can be used for

generation of PWM as well as PPM. In this case a sawtooth signal of frequency fs is a sampling signal.

It is applied to inverting terminal of a comparator with modulating signal at non inverting terminal.

O/P remains high as long as modulating signal is higher than that of ramp signal.

Generation of PWM signal

Advantages & DisadvantagesAdvantages:-Very good noise immunity.Its possible to separate out signal from noise.

Disadvantages:-Bandwidth requirement is large as compared to

Pulse Position ModulationIn 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.

Generation of PPM signal

Generation of PPM signalThe PPM signal can be generated from PWM signal.The PWM pulses obtained at the comparator output are

applied to a monostable multivibrator which is –ve edge triggered.

Hence for each trailing edge of PWM signal, the monostable output goes high. It remains high for a fixed time decided by its own RC components. Thus as the trailing edges of the PWM signal keeps shifting in proportion with the modulating signal, the PPM pulses also keep shifting.

Therefore all the PPM pulses have the same amplitude and width. The information is conveyed via changing position of pulses.

Generation of PPM signal

Advantages & Disadvantages

Advantages:-High noise immunity.

Disadvantages:-Generation & detection is complex..

Pulse Code ModulationIn PCM, the available range of signal voltages is

divided into levels and each is assigned a binary number

The number of levels available depends upon the number of bits used to express the sample value

The number of levels is given by: N = 2mPCM consists of three steps to digitize an analog signal:I. SamplingII. QuantizationIII. Binary encoding

Generation of PCM Signal

Generation of PCM SignalSampling: The process of generating pulses of zero

width and of amplitude equal to the instantaneous amplitude of the analog signal. The no. of pulses per second is called “sampling rate”.

Quantization: The process of dividing the maximum value of the analog signal into a fixed no. of levels in order to convert the PAM into a Binary Code. The levels obtained are called “quanization levels”.

Generation of PCM Signal

Voltage

76543210

111 110 101 100 011 010 001 000

Levels

Binary

Voltage

0 1 0 1 0 1 1 1 0 1 1 1 1 1 0 1 0 1 0 1 0

Advantages & DisadvantagesAdvantages:-Secured.Encoding is possible.Very high noise immunity.Convenient for long distance communication.Good signal to noise ratio.Disadvantages:-Complex circuitry.Requires large bandwidth.Synchronization is required between Transmitter &

Reciever

Pulse Density ModulationPulse Density Modulation (PDM) involves changing

the density with which adjacent pulses are packed together.

It is a one-bit, high rate data stream that conveys a signal by modulating the density of the pulses

Generation of PDM SignalA PDM bitstream is encoded from an analog signal

through the process of delta-sigma modulation. This process uses a one bit quantizer that produces either

a 1 or 0 depending on the amplitude of the analog signal. A 1 or 0 corresponds to a signal that is all the way up or

all the way down, respectively

Generation of PDM Signal

Advantages & DisadvantagesAdvantages:-One bit code word for output.Low signaling rate.Low channel bandwidth.

Disadvantages:-Granular noise present.

Thank You

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