PULSE
MODULATION
CONTENTS
SAMPLING
T.D.M.
PULSE
MODULATION
P.A.M.
P.T.M.
SAMPLING

Process of converting a continuous time signal to an equivalent
discrete time signal.

Continuous time signal x(t) is applied
at input of multiplexer.

Other input of multiplexer is
train of impulse.

At the output of multiplexer
we get the sampled version of x(t)
SAMPLING cont.

One important property of sampling is that the signal can be fully
represented by their sampled value at some discrete time instant.

If the sampled values are known the signal can be reconstructed or
recovered as original signal.

The time interval can be used by other signals if a suitable transmission is
produced.


The concept of use of time interval by several signal is known as T.D.M. (Time
Division Multiplexing)
The sampling occurs at regular intervals of time Ts seconds apart. Thus
sampling frequency fs may be represented as
fs =1/Ts
SAMPLING cont.

Sampling theorem is defined for 2 signals

Low pass signal



A band limiting signal which has no frequency component above a
frequency fm Hz can be uniquely described by taking its sample at
uniform intervals less than equals to 1/2fm sec apart. So that the
sampling rate = 1/2fm seconds.
A band limiting signal which has no frequency component above a
frequency fm Hz can be uniquely recovered from the knowledge of
the sample taken at a rate of 2fm per sec. so that the sampling
frequency is greater than equal to 2fm .
Band pass signal



Band width is smaller compare with the highest frequency
components and its possible to use sampling rate i.e. less than the
twice the highest frequency component present in the signal.
Frequency interval
fc-fm<=fc<=fc+fm
Minimum sampling frequency = 2(fc+fm)/m
where m is integer
TIME DIVISION MULTIPLEXING


Sampling theorem makes it possible to
transmit the complete information of the
continuous signal by transmitting more
samples of f(t) at regular intervals.
This is done on time sharing basis.


All the signals to be transmitted are inter
laced in transmitter.


It is switched from channel to channel I
sequence to the sampling circuits by the
pulse generated by the timing circuit.
At the receiver, the samples of each signal
are separated.


The transmission of sample engage the
channel for only part of time.
Its sampling pulse is in synchronism.
The output of the sampling circuit is thus a
signal which consist of samples of all
signals interlaced
DIFFERENCE
PAM






Amplitude of pulse is
proportional to
amplitude of
modulating signal.
Band width of
transmitting channel
depends on width of
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.
Band width 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.
Band width of
transmitting channel
depends on rise time
of the pulse.
Instantaneous power
remains constant.
Noise interference is
minimum.
Simple to implement.
Similar to P.M.
PULSE AMPLITUDE
MODULATION (PAM)

PAM can be defined as a process in which the amplitude of regular
spaced rectangular pulse vary in direct proportion to the
instantaneous sample values of continuous signal.

Its is quite similar to Amplitude Modulation.


The difference is that here a Pulse Train acts as carrier rather than high
freq. sinusoidal wave
There are mainly 2 types of PAM signals


PAM with Natural Sampling
Pam with Flat Top
•PAM with Natural Sampling

Width of pulse do not have flat top.


With the Natural Sampling, a signal
sampled at NQUIST rate may be reconstructed
exactly by passing through LOW PASS
FILTER with cut off frequency fm .


The top of pulse varies in accordance
with the shape of modulating signal.
where fm is the highest frequency component
If N signals are to be multiplied, the max
sampled duration is
T=Ts/N
•Flat Top PAM

The top of pulses of this PAM is flat.



Noise interference at the top of transmitted pulse can be easily
removed.
Due to this it is widely used.
Better than Natural PAM


Because in case of Natural PAM, the varying top signal is when
received at receiver, it becomes quite difficult to determine
shape of top of pulse due to noise (which is always present).
Thus errors are introduced in the receiving signals due to wich
we prefer a flat top PAM.
•Flat Top PAM cont.


Sampled and hold circuit consist of 2
flat switches and a capacitor.
The sampling switch is closed for a
short duration by a short pulse applied
to gate G1 of transistor.



When the sampling switch is opened
the capacitor hold the charge.
The discharge switch is then closed
by a pulse applied to the gate G2 pf
the other transistor.


During this period the capacitor is
charged up to a voltage equal to the
instantaneous value of input signal x(t)
Due to this capacitor discharge to zero
volts.
Hence the output sampled and hold
circuit consist of Flat Top Samples.
Fig. shows sampled and hold circuit to
Produce FLAT TOP sampled P.A.M.
PULSE TIME
MODULATION(PTM)

There are two types of Pulse Time
Modulation



Pulse Width Modulation (PWM)
Pulse Phase Modulation (PPM)
In both PWM and PPM some time
parameters of the pulse is
modulated.

In PWM the width of pulse is
varied.
 In PPM position of the pulse is
varied.

Amplitude of pulses remain
constant for both PWM and PPM
DEMODULATION of P.W.M.


The transistor T1 acts as an inverter.
During the time interval when the signal is high, the input of transistor T2 is
low.

Thus during this interval T2 is in
cut-off stage.
 Thus capacitor C is charged through
RC combination.

During the time interval when the
signal is low, the input of transistor is
high.

Thus it get saturated during thus time.
 The capacitor gets discharge very rapidly through transistor T2.


Hence the waveform at the collector transistor T2 is more or less a saw-tooth
waveform whose envelop is modulating signal.
When this is passed through 2nd order Op-amp low pass filter, desired
demodulated signal is obtained.
DEMODULATION of P.P.M.


This circuit makes use of the fact that the gaps between the pulses of PPM
signal contains information regarding the Modulating signal.
During the gap between pulses

The transistor is in cut-off.
 The capacitor gets charged through
RC combination.

During the pulse duration

The transistor is in saturation.
 Capacitor is discharged through
transistor.
 Thus the collector voltage becomes low.


Hence the waveform at the collector is approximately a saw-tooth waveform
whose envelop is Modulating signal.
When this is passed through 2nd order low pass filter, the desired
demodulated output is obtained.