MAYURBHANJ SCHOOL OF ENGINEERING � LAXMIPOSI ,BARIPADA,757107

• DEPARTMENT- E&TC ENGG.
• SEMISTAR- 5^TH
• SUBJECT-A & D Communication
• TOPIC – 5 – ANALOG TO DIGITAL CONVERSION & PULSE MODULATION
• NAME OF TOPIC –PAM,PPM,PTM,PWM
• PREPARED BY - U S Panda (Sr. Lect. E & TC Engineering)
• AY – 2021-2022, WINTER-2021

Pulse Amplitude Modulation (PAM)

• Amplitude of the pulse carrier varies proportional to the instantaneous amplitude of the message

signal.

• The width and positions of the pulses are constant in this modulation.
• PAM could be:
1. Single polarity PAM: A suitable fixed DC bias is added to the signal to ensure that all the pulses are positive.
2. Double polarity PAM: In this the pulses are both positive and negative.

• Depending on type of sampling PAM can be:
1. Ideal Sampling PAM, (ii) Natural sampling PAM and (iii) Flat top PAM.

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• The advantage of this modulation is the generation and detection is easy in this modulation and

also allows multiplexing.

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• The disadvantage is large band width of transmitted signal.

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BPF characteristics

• For a PAM signal produced with natural sampling, the sampled signal follows the waveform of

the input signal during the time that each sample is taken.

• A PAM signal is generated by using a pulse train, called the sampling signal (or clock signal) to operate an electronic switch or "chopper". This produces samples of the analog message signal.
• The switch is closed for the duration of each pulse, allowing the message signal at that sampling time to become part of the output.
• The switch is open for the remainder of each sampling period making the output zero. This is

known as Natural PAM.

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In simplest form PAM can be visualized as o/p of an AND gate whose two inputs are message signal x(t) and pulses at sampling rate

.

• For flat-top sampling, a sample-and-hold circuit is used in conjunction with the chopper to hold

the amplitude of each pulse at a constant level during the sampling time,

• Flat-top sampling, produces pulses whose amplitude remains fixed during the sampling time. The amplitude value of the pulse depends on the amplitude of the input signal at the time of sampling.
• Aperture Effect seen in this type of PAM. Equalizers used at receiver end

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Transmission Bandwidth in PAM

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𝝉 ≪ T_s

f ≥ 𝟐𝒇𝒎 ; 𝑻𝒔 ≤

𝟏

s

𝝉≪ Ts≤

𝟐𝒇𝒎

𝟏

𝟐𝒇𝒎

max

₌ 𝟏

𝟐𝝉

BW≥ 𝒇𝒎𝒂𝒙; 𝑩𝑾 ≥

If on and off time of PAM pulse is same then f

𝟏

𝟐𝝉

𝟐𝝉

𝑩𝑾 ≥ ^𝟏 ≫ fm

Transmission of PAM signals

​

• For PAM signals to be transmitted through space using antennas, they must be amplitude/ frequency/ phase modulated by a high frequency carrier and only then they can be transmitted. Thus the overall system is PAM-AM. PAM-FM or PAM-PM and at receiving end, AM/ FM/PM detection is first employed to get the PAM signal and then message signal is recovered.

​

Drawbacks of PAM

• Bandwidth required for transmission of PAM signal is very large in comparison to maximum frequency present in modulating signal.
• Since amplitude of PAM pulses varies in accordance with modulating signal so interference of noise is maximum in PAM
• Variation of the peak power required by transmitter

Demodulation of PAM

• PAM signal sampled at Nyquist rate can be reconstructed at the receiver end , by passing it through an efficient Low Pass Filter (LPF) with exact cut off frequency of fs/2. This is known as Reconstruction or Interpolation 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

• For a flat topped PAM, a holding circuit followed by a LPF gives demodulated signal

Holding circuit

Received PAM signal

C

Zero order Holding Circuit

• Switch S closes after the arrival of pulse and opens at the end of pulse.
• Capacitor C charges to pulse amplitude value and holds this value during interval between two pulses.
• The sampled values are shown in fig.
• Holding circuit o/p smoothened in LPF.
• Known as zero order holding circuit, which considers only the previous sample to decide value between two

pulses

• First order holding circuit considers previous two samples, second order holding circuit considers previous three samples.

PAM

signal

Pulse Time modulation

• In PTM, amplitude of pulse is constant while position or width of pulse is made proportional to the amplitude of the signal at the sampling instant.
• It can be PWM and PPM
• In both the cases amplitude constant and does not carry information so amplitude limiters can be used ( like in FM) providing good noise immunity

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Generation of PTM signals can be either by:

1. Indirect Method: Firstly PAM signals are generated, Synchronized is generated during each pulse interval. These two signals are added and the sum is applied to a comparator whose reference level is suitably chosen. The second crossing of comparator level used for PPM
2. Direct method: PTM waveforms generated without using PAM waveforms

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Pulse Width modulation

The pulse width modulation is the modulation of signals by varying the width of pulses. The amplitude and

positions of the pulses are constant in this modulation

Generation of PWM and PPM by Direct Method

• The non inverting input of the comparator is fed by the input message or modulating signal x(t)

and the other input by a saw-tooth signal which operates at carrier frequency.

• The comparator compares the two signals together to generate the PWM signal at its output. Its o/p is high only when the instantaneous value of x(t) is higher than sawtooth waveform.
• The rising edges of the PWM signal occurs at the fixed time period (kT_s) while trailing edge depends on amplitude of message signal x(t).
• When saw-tooth voltage waveform greater than x(t), o/p of comparator is zero, trailing edge is modulated
• If saw-tooth. waveform is reversed, trailing edge is fixed while leading edge is modulated.
• Replacing saw-tooth waveform by triangular, both leading and trailing edge modulated. (symmetrical PWM)
• The amplitude of PDM/PWM will be positive saturation of the comparator shown as ‘A’, being same

for all pulses,

Three types of pulse-width modulation (PWM) are possible:

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• The leading edge of the pulse being constant, the trailing edge varies according to the message signal.
• The trailing edge of the pulse being constant, the leading edge varies according to the message signal
• The center of the pulse being constant, the leading edge and the trailing edge varies according

to the message signal (Symmetrical PWM)

Indirect Method:

Modulating signal (A) applied to i/p of PAM circuit [s(t) pulse train] and PAM signal generated(B). S(t) also is i/p to Ramp generator(Integrator circuit), all having equal slopes, amplitude and generation(D). These ramp pulses added to PAM pulses to produce varying height samples. These varying height ramp gates a S.T ckt to generate varying width rectangular pulses of PWM.

PWM

​

Summer

Schmitt Trigger

Ramp Generator

x(t)

A

PAM

generator

B

C

D

E

F

PWM detector

Schmitt

Trigger

Ramp Generator

Synchronization

Pulse generator

Adder

Level Shifter

Rectifier

LPF

1

2

3

4

5

6

• Received PWM signal applied to ST circuit to remove noise
• Regenerated PWM applied to Ramp generator and synchronization pulse.
• Heights of Ramp proportional to width of pulses.
• Pulse generator produces reference pulses with constant

amplitude and width but delayed by specific amount.

• Delayed reference pulses added to o/p of ramp generator
• The o/p given to level shifter, negative offset shifts waveform. Then clipped by rectifier followed by LPF to give message signal.

Noisy PWM

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.
• PPM is done in accordance with the PWM signal.
• PWM signal is used as the trigger input to a monostable multivibrator.
• Its o/p remains zero until it is triggered on the trailing edge of PWM
• O/P of monostable MV switches to positive saturation value A and remains high for fixed period then goes low
• Hence, the position of these pulses is proportional

to the width of the PWM pulses.

Advantage As the amplitude and width are constant

the power handled is constant

Disadvantage: Synchronization between Transmitter and receiver is a necessity

• The PDM is differentiated, and then rectified and shaped.
• PPM carries exactly the same information as long as the position of the clock pulses (leading edge) is well defined in the received signal.
• PPM is superior to PDM for message transmission, since the wide pulses of PDM require more energy than PPM when transmitted
• PPM is suited for communication in the presence of noise.
• Very high peak narrow pulses can be transmitted and the pulse position can be determined even when the noise level is high,
• However, transmitting very narrow pulses requires a large band

width

• When light is used as the media for transmitting analog signals, PPM or PCM are the most suitable types of modulation because the maximum power output in the modulated light source, such as LED or LASER is achieved when it is pulsed at a very low duty cycle.
• In PPM, necessary to transmit a series of sync pulses at a much lower repetition rate than the sampling pulses, to avoid interference with original signal and/or minimise the number of pulses transmitted in order to conserve transmission power

Transmission BW of PWM and PPM

• Both PWM and PPM have DC value.
• Both need a shrp rise time and fall time to preserve the message information
• Rise time be very less than Ts i.e. t_r≪ Ts

T

• Transmission BW: B ≥

𝟏

𝟐𝒕𝒓

• BW higher than PAM

Application of PWM

• Although PWM is also used in communications, its main purpose is actually to control the power that is supplied to various types of electrical devices, most especially to inertial loads such as AC/DC motors.
• Pulse-width modulation (PWM) is used for controlling the amplitude of digital signals in order to control devices and applications requiring power or electricity. It essentially controls the amount of power, in the perspective of the voltage component, that is given to a device by cycling the on- and-off phases of a digital signal quickly and varying the width of the "on" phase or duty cycle. To the device, this would appear as a steady power input with an average voltage value, which is the result of the percentage of the on time. The duty cycle is expressed as the percentage of being fully (100%) on.

 A very powerful benefit of PWM is that power loss is very minimal. Compared to regulating power levels using an analog potentiometer to limit the power output by essentially choking the electrical pathway, thereby resulting in power loss as heat, PWM actually turns off the power output rather than limits it. Applications range from controlling DC motors and light dimming to heating elements.

This simple circuit based around the familiar NE555 or 7555 timer chip is used to produced the required pulse width modulation signal at a fixed frequency output. The timing capacitor C is charged and discharged by current flowing through the timing networks RA and RB as we looked at in the 555 Timer tutorial.

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The output signal at pin 3 of the 555 is equal to the supply voltage switching the transistors fully “ON”. The time taken for C to charge or discharge depends upon the values of RA, RB.

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The capacitor charges up through the network RA but is diverted around the resistive network RB and through diode D1. As soon as the capacitor is charged, it is immediately discharged through diode D2 and network RB into pin 7. During the discharging process the output at pin 3 is at 0 V and the transistor is switched “OFF”.

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Then the time taken for capacitor, C to go through one complete charge- discharge cycle depends on the values of RA, RB and C with the time T for one complete cycle being given as:

PAM

• The amplitude of the pulse is proportional to the amplitude of modulating the signal.
• Band width of transmitting channel depends on the width of the pulse
• Instantaneous power of transmitter varies. Noise interference is high
• Complex system. Similar to A.M.

PWM

• Width of pulse is proportional to amplitude of modulating signal.
• The Bandwidth of transmitting channel depends on rise time of the pulse.
• Instantaneous power of transmitter varies. Noise interference is minimum.
• Simple to implement Similar to F.M.

PPM

• Relative position of pulse is proportional to amplitude of modulating signal.
• The bandwidth of transmitting channel depends on the rise time of the pulse.
• Instantaneous power remains constant. Noise interference is minimum.
• Simple to implement. Similar to P.M.

 Difference Between PAM, PWM, and PPM

Parameter

• Type of Carrier:

PAM

Train of Pulses

Amplitude

Low

Low

PWM

Train of Pulses Width

High

High

• Variable Characteristic :
• Bandwidth Requirement:
• Noise Immunity :
• Information Contained in: Amplitude Variations

• Power efficiency (SNR)
• Transmitted Power

Low Varies

• Need to transmit synchronizing pulses Not needed

Width Variations Moderate Varies

Not needed

rise time of the pulse

PPM

Train of Pulses Position

High

High

Position Variations High

Remains Constant Necessary

rise time of the pulse

Instantaneous power varies with Constant width of the pulses

• Bandwidth depends on width of the pulse
• Transmitter power Inst. power varies

with amplitude of pulses

• Complexity of generation and detection

Complex Easy

 12 Similarity with other Modulation Systems Similar to AM

Similar to FM

Complex Similar to PM

THANK YOU