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modulation

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PULSE MODULATION TECHNIQUES

Multimedia PresentationPulse ModulationPresented 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 ModulationDigital Pulse Modulation Pulse ModulationPulse 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 ModulationPulse Width ModulationPulse Position ModulationPulse Code ModulationPulse 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 pointsSampling 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 FilterMultiplierPAM signalPulse train generatorGeneration of PAM Signal

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.Generation of PAM Signal

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 PAM.

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 & DisadvantagesAdvantages:-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 numberThe number of levels available depends upon the number of bits used to express the sample valueThe number of levels is given by: N = 2mPCM consists of three steps to digitize an analog signal:SamplingQuantizationBinary 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 SignalTimeVoltage76543210111 110 101 100 011 010 001 000LevelsBinary CodesTimeTimeVoltage0 1 0 1 0 1 1 1 0 1 1 1 1 1 0 1 0 1 0 1 0Advantages & 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 & RecieverPulse 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 bymodulatingthedensityof thepulses

Generation of PDM SignalA PDM bitstream isencodedfrom an analog signal through the process ofdelta-sigma modulation. This process uses a one bitquantizerthat 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, respectivelyGeneration of PDM Signal

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

Disadvantages:-Granular noise present.Thank You