Communication System IILecture 2

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Analog Pulse Modulation

“ In pulse modulation methods, the carrier is no longer a

continuous signal but consists of a pulse train. ”

2 Main Types

1. Pulse Amplitude Modulation (PAM)

2. Pulse Time Modulation (PTM)

a. Width Modulation (PWM)

b. Position Modulation (PPM)

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Advantages

1. Multiplexing of signal is possible.

2. As the information is not continuous, chances of

distortion is minimum.

3. Power saving.

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Pulse Amplitude Modulation (PAM)

1. Each sample equals to amplitude of modulating signal.

2. Flat top pulses are preferably used.

3. Two types of PAM

a. Single Polarity

b. Double polarity

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Generation of PAM (Technique 1)

Generation of PAM (Technique 1)

1. Sampling switch close for short duration by a short pulse applied to

gate G1.

2. Capacitor C gets quickly charged up to Voltage equal to

instantaneous sample value of the incoming signal x(t).

3. Sampling switch then make open, C holds the charge.

4. Discharge switch is close by pulse applied to gate G2.

5. Capacitor discharges to Zero.

6. Discharge switch is then make open.

7. In this way samples are produced.

8. C value plays an important role for Time of pulse.

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Generation of PAM (Technique 2)

AND PAM

AND GATE

PULSE SHAPING NETWORK

MODULATOR

TRANSMIT

1. AND gate is open during wanted time interval.

2. Output will be at Sampling rate fs.

3. Signal is finally modulated for transmission.

4. Modulation may be PAM-FM or PAM-AM.

Generation of PAM (Technique 2)

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Pulse Width Modulation (PWM)

1. Message modulates the width of pulse.

2. The randomness in pulse creates trouble for TDM.

3. Applications in Motor controlling, delivery of power which is

precisely regulated by PWM width.

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+

-

PWM

Generation of PWM

1. Input message and carrier frequency of saw tooth type is fed to

comparator.

2. The maximum amplitude of input signal must be less than that

of Saw tooth signal.

3. The rising edge of PWM is coincides with falling edge of Saw

tooth waveform.

4. When S/T I/P is at low potential which should be always higher

than I/P signal, O/P of comparator will be +ve.

5. When S/T signal rises with a fixed slope, crosses I/P signal value,

hence O/P turns to –ve.

6. The width of pulse depends on I/P signal magnitude i.e. +ve ON

time of comparator.10S.S.I.T.M, Bhilai, INDIA.

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Pulse Position Modulation (PPM)

1. Message modulates the position of pulse.

2. The randomness in position of pulse creates trouble for TDM.

3. PPM is the post processing of PWM.

4. For PPM output, PWM signal are widely used.

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Generation of PPM

INVERTER DIFFERENTIATOR+VE EDGE

TRIGGERED PULSE GENERATOR

PWM I/P

PPM O/P

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Generation of PPM

1. The method shown is known as Direct Method.

2. Inverter reverses the polarity.

3. Differentiator creates the spikes with respect to inverted pulses.

4. Spikes are of +ve type for HIGH to LOW PWM signal.

5. Spikes are of –ve type for LOW to HIGH PWM signal.

6. Positive edge triggered pulse generator produces a pulses of

fixed width.

7. Presence of +ve spike decide the position of these new pulses.

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Generation of PPM & PWM (Devices & Components)

1. Comparator IC710

2. Inverter OpAmp + RC components

3. Differentiator OpAmp + RC components

4. Pulse Gen. IC74121, IC555