5.1: introduction to pulse modulation

Chapter Five Pulse Modulation

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5.1: Introduction to Pulse Modulation

In pulse modulation some parameter of a pulse train is varied in accordance

with the massage signal.

Analog pulse modulation is classified as

Pulse Amplitude Modulation (PAM)

Pulse Width Modulation (PWM)

Pulse Position Modulation (PPM)

Digital modulation is classified as

Pulse Code Modulation

Delta Modulation

5.2: Pulse Amplitude Modulation (PAM)

PAM is an analog modulating scheme in which the amplitude of the pulse

carrier varies proportional to the instantaneous amplitude of the message

signal.

Fig 1: Pulse Amplitude Modulation

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There are two types of sampling techniques for transmitting a signal using

PAM. They are:

1. Flat Top PAM

2. Natural PAM

Flat Top PAM: The amplitude of each pulse is directly proportional to

modulating signal amplitude at the time of pulse occurrence. The amplitude

of the signal cannot be changed with respect to the analog signal to be

sampled. The tops of the amplitude remain flat.

Fig.2: Flat Top PAM

Natural PAM: The amplitude of each pulse is directly proportional to

modulating signal amplitude at the time of pulse occurrence. Then follows

the amplitude of the pulse for the rest of the half cycle.

Fig.3: Natural PAM


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Circuit Design of Pulse Amplitude Modulation

A PAM is generated from a pure sine wave modulating signal and a square

wave generator which produces the carrier pulse and a PAM modulator

circuit.

Figure 4: Pulse amplitude modulator, natural sampling

-Op-amp2 is a high impedance voltage follower capable of deriving low

impedance load

-The resister R is used to limits the output current of op-amp1 when FET is

“on”

Figure 5: Sample-and-hold circuit and flat-top sampling.


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Demodulation of PAM

For demodulation of PAM signal, the PAM signal is fed to the low pass

filter. The low pass filter eliminates the high-frequency ripples and generates

the demodulated signal. This signal is then applied to the inverting amplifier

to amplify its signal level to have the demodulated output with almost equal

amplitude with the modulating signal.

Figure.6: PAM Demodulation

fo for the LPF must within the range of fm < fo < fs - fm

Applications of PAM

It is used in Ethernet communication.

It is used in many micro-controllers for generating the control signals.

It is used in Photo-biology.

It is used as an electronic driver for LED lighting.

Advantages

It is the simple process for both modulation and demodulation.

Transmitter and receiver circuits are simple and easy to construct.

PAM can generate other pulse modulation signals and can carry the

message at the same time.

Disadvantages

Bandwidth should be large for transmission PAM modulation.

Noise will be great.

Pulse amplitude signal varies so power required for transmission will be

more.

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5.2: Pulse Width Modulation

Pulse Width Modulation (PWM) or Pulse Duration Modulation

(PDM) or Pulse Time Modulation (PTM) is an analog modulating scheme

in which the duration or width or time of the pulse carrier varies

proportional to the instantaneous amplitude of the message signal.

The width of the pulse varies in this method, but the amplitude of the signal

remains constant. Amplitude limiters are used to make the amplitude of the

signal constant. These circuits clip off the amplitude, to a desired level and

hence the noise is limited.

Modulation of PWM:

Figure 7, shows a PWM modulator. This circuit is simply a high-gain

comparator that is switched on and off by the sawtooth waveform

derived from a very stable-frequency oscillator.

Notice that the output will go to +Vcc if the analog signal exceeds the

sawtooth voltage.

The output will go to -Vcc if the analog signal is less than the sawtooth

voltage.

Figure 7. Pulse width modulator.


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Demodulation of PWM:

To recover the original analog signal we can convert the PWM to PAM

then demodulate the PAM with a LPF

Fig. 8: Demodulation of PWM

Advantages of Pulse Width Modulation (PWM):

As like pulse position modulation, noise interference is less due to

amplitude has been made constant.

Signal can be separated very easily at demodulation and noise can also

be separated easily.

Synchronization between transmitter and receiver is not required unlike

pulse position modulation.

Disadvantages of Pulse Width Modulation (PWM):

Power will be variable because of varying in width of pulse.

Transmitter can handle the power even for maximum width of the

pulse.

Bandwidth should be large to use in communication, should be huge

even when compared to the pulse amplitude modulation.

PWM Integrator

and hold PAM LPF


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Applications of Pulse Width Modulation (PWM):

PWM is used in telecommunication systems.

PWM can be used to control the amount of power delivered to a load

without incurring the losses. So, this can be used in power delivering

systems.

Audio effects and amplifications purposes also used.

PWM signals are used to control the speed of the robot by controlling

the motors.

PWM is also used in robotics.

Embedded applications.

Analog and digital applications etc

5.3: Pulse Position Modulation

Pulse Position Modulation (PPM) is an analog modulating scheme in

which the amplitude and width of the pulses are kept constant, while the

position of each pulse, with reference to the position of a reference pulse

varies according to the instantaneous sampled value of the message signal.

The transmitter has to send synchronizing pulses (or simply sync pulses) to

keep the transmitter and receiver in synchronism. These sync pulses help

maintain the position of the pulses. The following figures explain the Pulse

Position Modulation.


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Fig. 9: PPM modulation

Pulse position modulation is done in accordance with the pulse width

modulated signal. Each trailing of the pulse width modulated signal

becomes the starting point for pulses in PPM signal. Hence, the position of

these pulses is proportional to the width of the PWM pulses.


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Modulation of PPM:

Fig.10 : Modulation of PWM and PPM


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Demodulation of PPM:

Figure 11: PPM demodulator.

This is achieved by full-wave rectifying the PPM pulses of Figure 11,

which has the effect of reversing the polarity of the negative (clock-

rate) pulses.

Then an edge-triggered flipflop (J-K or D-type) can be used to

accomplish the same function as the RS flip-flop of Figure 11, using

the clock input.

The penalty is: more pulses/second will require greater bandwidth,

and the pulse width limit the pulse deviations for a given pulse period.


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Advantages of Pulse Position Modulation (PPM):

Pulse position modulation has low noise interference when compared

to PAM because amplitude and width of the pulses are made constant

during modulation.

Noise removal and separation is very easy in pulse position

modulation.

Power usage is also very low when compared to other modulations due

to constant pulse amplitude and width.

Disadvantages of Pulse Position Modulation (PPM):

The synchronization between transmitter and receiver is required,

which is not possible for every time and we need dedicated channel for

it.

Large bandwidth is required for transmission same as pulse amplitude

modulation.

Special equipments are required in this type of modulations.


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Applications of Pulse Position Modulation (PPM):

Used in non coherent detection where a receiver does not need any

Phase lock loop for tracking the phase of the carrier.

Used in radio frequency (RF) communication.

Also used in contactless smart card, high frequency, RFID (radio

frequency ID) tags and etc.

5.4: Comparison between PAM, PWM, and PPM

The comparison between the above modulation processes is presented in a

single table.

PAM PWM PPM

Amplitude is varied Width is varied Position is varied

Bandwidth depends on

the width of the pulse


the rise time of the pulse


the rise time of the pulse

Instantaneous

transmitter power

varies with the

amplitude of the pulses

Instantaneous transmitter

power varies with the

amplitude and width of

the pulses

Instantaneous

transmitter power

remains constant with

the width of the pulses

System complexity is

high


low


low

Noise interference is

high Noise interference is low

Noise interference is

low

It is similar to

amplitude modulation

It is similar to frequency

modulation

It is similar to phase

modulation

5.1: introduction to pulse modulation

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