Satellite communications

1. Pulse Modulation
Harish I
Sathiyan R
Srivenkatachalapathy M
Sugan G

2. Agenda Pulse Amplitude Modulation(PAM)
Pulse Code Modulation(PCM)
Pulse Width Modulation(PWM)
Pulse Position Modulation(PPM)
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3. Introduction
This type of modulation is based on
the “sampling principle” which states
that a continuous message waveform
that has a spectrum of finite width
could be recovered from a set of
discrete instantaneous samples
whose rate is higher than twice the
highest signal frequency.
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4. Pulse
Amplitude
Modulation
Pulse Amplitude Modulation (PAM) is an
analog modulating scheme in which the
amplitude of the pulse carrier varies
proportional to the instantaneous amplitude
of the message signal. The width and
position of the pulse remains unchanged.
The pulse amplitude modulated signal, will
follow the amplitude of the original signal,
as the signal traces out the path of the
whole wave. In natural PAM, a signal
sampled at the Nyquist rate is
reconstructed, by passing it through an
efficient Low Pass Frequency (LPF) with
exact cutoff frequency.
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Block Diagram
x(t)
c(t)
y(t) s(t)
Pulse
Shaping
Network

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Reconstruction
lowpass filter
PAM Signal
Reconstructed
PAM Signal
Block Diagram
The PAM signal is passed through a lowpass
reconstruction filter. The filter reconstructs the
analog signal from PAM pulses.

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In a pulse amplitude modulated (PAM) signal the pulse
duration ‘τ’ is considered to be very small in comparison
to time period (i.e., sampling period) Ts between any two
samples i.e.
<<𝑇𝑠 --------------(1)
Now, if the maximum frequency in the modulating signal
is fm, then according to sampling theorem, the sampling
frequency fs must be equal to or higher than the Nyquist
i.e.
𝑓𝑠 ≥ 2𝑓𝑚
1/ 𝑇𝑠 ≥ 𝑓𝑚
𝑇𝑠≤1/ 2𝑓𝑚
But according to equation (1), we have,
<< 𝑇𝑠
Therefore, << 𝑇𝑠≤1/ 2𝑓𝑚
Now, if the ‘ON’ and ‘OFF’ time of the pulse amplitude
modulated (PAM) pulse is same then maximum
frequency of the PAM pulse will be equal to,
fmax = 1/(+) = 1/2
Therefore, the bandwidth required for the transmission
of a PAM signal would be equal to the maximum
frequency fmax given by the above equation.
Thus, we have Transmission bandwidth,
BT ≥ fmax
BT ≥ 1/2
Since <<1/ 2𝑓𝑚 , B>> 𝑓𝑚

8. Advantages
1. PAM can be easily
Generated.
2. PAM forms the basis for
many other pulse modulation
techniques such as PCM.
Disadvantages
1. Bandwidth needed for
transmission is very large
when compared to maximum
frequency.
2. Interference noise is
maximum as the amplitude
keeps varying.
3. Peak power required by the
transmitter keeps varying.
Applications
1. PAM is used for transmitting
signals over a short distance
baseband channels and
simple communication.
2. It is used in analog-to-digital
converters for computer
interfacing.
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9. Pulse
Code
Modulation
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11. Signal Representation of PCM
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13. Advantages
1. Very high noise immunity.
2. It is possible to store the
PCM signal due to its digital
nature.
3. It is possible to use various
coding techniques so that
only the desired person can
decode the received signal.
4. Integration with other form of
digited data is possible.
Disadvantages
1. The encoding, decoding and
quantizing circuitry of PCM is
complex.
2. PCM requires a large
bandwidth as compared to
the other systems.
Applications
1. In telephony (with the advent
of fibre optic cables).
2. In the space communication,
space craft transmits signals
to earth. Here the transmitted
power is very low (10 to 15W)
and the distances are huge
(a few million km). Still due to
the high noise immunity, only
PCM systems can be used in
such applications.
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14. Pulse
Time
Modulation
In Pulse time modulation (PTM),
amplitude of the carrier is kept constant
and the Position or width of the carrier
varies with the amplitude of the
modulating signal at the time of
sampling.
There are two types of Pulse Time
Modulation(PTM):
1. 1)Pulse Width Modulation(PWM)
2. 2)Pulse Position Modulation(PPM)
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15. PULSE WIDTH MODULATION
PWM
1. Pulse width modulation reduces the average power delivered by an
electrical signal by converting the signal into discrete parts. In the
PWM technique, the signal’s energy is distributed through a series
of pulses rather than a continuously varying (analogue) signal.
2. The width of the pulse is directly proportional to amplitude of the
modulating signal at the sampling instant. The amplitude and
position of the pulse remains unchanged.
3. Types of PWM:
• Trail Edge Modulation – In this technique, the signal’s lead edge is
fixed, and the trailing edge is modulated.
• Lead Edge Modulation –In this technique, the signal’s lead edge is
modulated, and the trailing edge is kept fixed .
• Pulse Center Two Edge Modulation – In this technique, the pulse
center is fixed and both edges of the pulse are modulated.
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•A sawtooth generator generates a sawtooth signal of frequency fs, and this sawtooth signal in this
case is used as a sampling signal.
•It is applied to the inverting terminal of a comparator.
•The modulating signal x (t) is applied to the non-inverting terminal of the same comparator.
•The comparator output will remain high as long as the instantaneous amplitude of x (t) is higher
than that of the ramp signal.

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The PWM signal received at the input of the detection circuit is
contaminated with noise. This signal is applied to pulse
generator circuit which regenerates the PWM signal. Thus, some
of the noise is removed and the pulses are squared up. The
regenerated pulses are applied to a reference pulse generator. It
produces a train of constant amplitude, constant width pulses.
These pulses are synchronized to the leading edges of the regenerated PWM pulses but delayed by a fixed interval.
The regenerated PWM pulses are also applied to a ramp generator. At the output of it, we get a constant slope ramp for the
duration of the pulse. The height of the ramp is thus proportional to the width of the PWM pulses.
At the end of the pulse, a sample and hold amplifier retains the final ramp voltage until it is reset at the end of the pulse.
The constant amplitude pulses at the output of reference pulse gtenerator are then added to the ramp signal.
The output of the adder is then clipped off at a threshold level to generate a PAM signal at the output of the clipper.
A low pass filter is used to recover the original modulating signal back from the PAM signal.

18. Advantages
1. Unlike PAM, noise is less,
since in PWM, amplitude is
held constant.
2. Signal and noise separation
is very easy.
3. PWM communication does
not require synchronization
between transmitter and
receiver.
Disadvantages
1. In PWM, pulses are varying
in width and therefore their
power contents are variable.
This requires that the
transmitter must be able to
handle the power contents of
the pulse having maximum
pulse width.
2. Large bandwidth is required
for the PWM communication
as compared to PAM.
Applications
1. PWM is used for
asynchronous transmission
over noisy channel.
2. PWM is used for generating
PPM.
3. Motor control.
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19. PULSE POSITION MODULATION
PPM
1. In PPM, the amplitude and width of the pulses is
kept constant but the position of each pulse is
varied in accordance with the amplitudes of the
sampled values of the modulating signal.
2. The position of the pulses is changed with
respect to the position of reference pulses.
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20. GENERATION OF PPM
1. The sawtooth generator generates the sawtooth signal of frequency f, (i.e. period T).
2. The modulating signal x(t) is applied to the noninverting input of the comparator. The output
of the comparator is high only when instantaneous value of (1) is higher than that of sawtooth
waveform.
3. The trailing edge of output of comparator depends on the amplitude of signal x(t) When
sawtooth waveform voltage is greater than voltage of x(t) at that Instant, the output of
comparator remains zero. The trailing edge of the output of comparator (PWM) is modulated
by the signal x(t)If the sawtooth waveform is reversed, then trailing edge will be fixed and
leading edge will be modulated.
4. The Pulse Duration Modulation (PDM) or PWM signal is nothing but output of the comparator.
The amplitude of this PDM or PWM signal will be positive saturation of the comparator, which
is shown as 'A' in the waveforms. The amplitude is same for all pulses To generate Pulse
Position Modulation (PPM), PDM signal is used as the trigger input to one monostable
multivibrator. The monostable output remains zero untill it is triggered. The monostable is
triggered on the falling (trailing) edge of PDM. The width of the pulse can be determined by
monostable. The pulse is this delayed from sampling time KT, depending on the amplitude of
signal x(1) at KT,
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22. Detection of PPM
1. The detection of the pulse position modulation block diagram is shown below. In the following block
diagram, we can observe that it includes a pulse generator, SR FF, reference pulse generator & a
PWM demodulator.
2. The PPM signal which is transmitted from the modulation circuit will get distorted with the noise
throughout transmission. So this distorted signal will reach the demodulator circuit. The pulse
generator used in this circuit will produce a pulsed waveform with a fixed duration.
3. This waveform is given to the SR FF’s reset pin. The reference pulse generator produces a reference
pulse with a fixed period once a transmitted PPM signal is given to it. So this reference pulse is
utilized to set the SR FF. At the output of the FF, these set & reset signals will generate a PWM signal.
Further, this signal is processed to give the original message signal.
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24. Advantages
1. Like PWM ,in PPM amplitude is held
constant thus less noise interference.
2. Like PWM, signal and noise separation is
very easy.
3. Due to constant pulse widths and
amplitudes, transmission power for each
pulse is same.
Disadvantages
1. Synchronization between transmitter and
receiver is required
2. Large bandwidth is required as compared to
PAM
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25. Thank you
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