13-06-2013, 02:54 PM
PPM Modulation and Demodulation
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Aims of the Exercise
Learning about the functioning principle of pulse-phase
modulation/demodulation as a pulse-time modulation
procedure
Comparison with the pulse-width modulation
Presentation of the signal course at the output of the PPM
modulator
Demodulation of the PPM signal
Modulation
In pulse phase modulation the phase of the carrier pulse is varied by the
modulating signal. The amplitude of the carrier signal as well as its pulse
duration, remains unchanged. Thus, the phase shift of the pulses that is
proportional to the amplitude of the modulating signal, can be a maximum
of the same or half of, the pulse period of the carrier voltage.
Pulse phase modulation avoids two serious disadvantages of pulse width
modulation: Firstly in PPM, time-division multiplex can be used and
secondly, the harmonic content is shifted to the region above the
information band, as in pulse amplitude modulation. So these advantages
are combined with the low susceptibility to interference of PWM.
Demodulation
The demodulation of the PPM signal is achieved in two stages. First, the
signal is converted back to a PAM signal by comparison with an
amplitude-time reference. Then, by filtering this PAM signal with a lowpass
filter, the modulation signal can be recovered.
For a distinct separation in the frequency spectrums of modulation and
PPM signals and thus preventing alias-frequency effects, the lowpass filter
in the exercise assembly here has a limit frequency equal to half the
Sample frequency.
Displaying the signal at the output of the PPM modulator
Synchronise the oscilloscope with a timebase of 20μs/div. For this, switch the
oscilloscope to 2-channel operation and connect channel B to the output
SYNC. Connect the GND socket to ground. Switch the inputs of the
oscilloscope to DC and the synchronisation to DC, B. With the TIME switch,
select 20μs and set the trigger level slightly above the 0V line of channel B.
Then at the start and centre of the timebase, a SYNC pulse should be
displayed.
Connect the DC output of the internal DC voltage source on the PPM
modulator (SO 4201-9F) to the input LFin. Set the input amplification with the
"Gain" potentiometer to 1 (fully counter clockwise). Connect channel A to the
modulator output PPMout. Trigger on B.