ANALOG & DIGITAL COMMUNICATION
By Engr. Hyder Bux MangrioInstitute of Information & Communication
TechnologiesMehran University of Engineering and Technology,
Jamshoro.
07TL-BATCH
Today's Lecture:24-26Analog
Pulse Modulation
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Analog modulation: input is continuous in time and value
Continuous-wave modulation AM, DB, SSB, VSB FM, PM Analog pulse modulation: Message value is continuous. Transmission
happens at discrete times. PAM, PWM, PPM
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Digital modulation: The message has discrete values
Sinusoidal carrier: uses a finite number of distinct signals to
represent digital data ASK, PSK, FSK Digital pulse modulation: use
quantization to get discrete message use coded pulse sequences: PCM, DPCM
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A series of regularly recurring pluses is made to vary in amplitude, duration, shape, or time as a function of the modulating signal.e.g. pulse-amplitude modulation (PAM)
pulse-width modulation (PWM) pulse-duration modulation (PDM) pulse-position modulation (PPM)
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Advantages of Pulse Modulation Noise immunity Inexpensive digital circuitry Can be time-division multiplexed with
other pulse modulated signals Transmission distance is increased
through the use of regulative repeaters Digital pulse streams can be stored Error detection and correction is easily
implemented
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Special encoding and decoding techniques may be necessary to increase transmission rates, thus making the pulse stream more difficult to recover
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Analog Pulse Modulation Digital Pulse Modulation
Pulse Amplitude (PAM)
Pulse Width (PWM)
Pulse Position (PPM)
Pulse Code (PCM)
Delta (DM)
Pulse Modulation
Nyquist Sampling Theorem :if a signal at a rate that is at least
twicethe highest frequency that it
containsthe original signal can be
completelyreconstructed
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Pulse Amplitude Modulation (PAM): The signal is sampled at regular intervals such that each
sample is proportional to the amplitude of the signal at that sampling instant. This technique is called “sampling”.
For minimum distortion, the sampling rate should be more than twice the signal frequency.
There are two classes of PAM signals: • PAM that uses
Natural Sampling (gating);
• PAM that uses Instantaneous Sampling to produce a flat-top pulse.
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Natural SamplingDEFINTION: If w(t) is an analog waveform
bandlimited to B hertz, the PAM signal that uses natural sampling (gating) is
ws(t) =w(t)s(t) Where
S(t) is a rectangular wave switching waveform and fs = 1/Ts ≥ 2B.
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Generating Natural Sampling:The PAM wave form with natural sampling can be generated using a CMOS circuit consisting of a clock and analog switch as shown.
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Instantaneous Sampling(flat-top )
• This type of PAM signal consists of instantaneous samples.
• w(t) is sampled at t = kTs
• The sample values w(kTs ) determine the amplitude of the flat-top rectangular pulses.
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DEFINITION: If w(t) is an analog waveform bandlimited to B Hertz, the instantaneous sampled PAM signal is given by
Where h(t) denotes the sampling-pulse shape and, for flat-top sampling, the pulse shape is,
( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( )s s s s s sk k k
w t w kT h t kT h t w kT t kT h t w t t kT
Pulse Width Modulation (PWM or PLM or PDM):
In this type, the amplitude is maintained constant but the duration or length or width of each pulse is varied in accordance with instantaneous value of the analog signal.
The negative side of the signal is brought to the positive side by adding a fixed d.c. voltage.
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In this type, the sampled waveform has fixed amplitude and width whereas the position of each pulse is varied as per instantaneous value of the analog signal.
PPM signal is further modification of a PWM signal. It has positive thin pulses (zero time or width) corresponding to the starting edge of a PWM pulse and negative thin pulses corresponding to the ending edge of a pulse.
This wave can be further amended by eliminating the whole positive narrow pulses. The remaining pulse is called clipped PPM.
PWM
PPM
Pulse Position Modulation (PPM):