Download PULSE MODULATION

PULSE
MODULATION
Three different sampling methods of PAM
4.2
Pulse Amplitude Modulation (PAM)
Modulation in which the amplitude of pulses is
varied in accordance with the modulating signal
Pulse Width Modulation (PWM)
Modulation in which the duration of pulses is varied
accordance with the modulating signal
in
Pulse Position Modulation (PPM)
Modulation in which the temporal positions of the
pulses are varied in accordance with some characteristic of the
modulating signal.
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)
DIFFERENCE
PAM


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
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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





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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.
•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