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Looking at the modulation of my old R/C transmitter: Rummaging round the attic, I found an old fashioned 4-channel Sanwa MK3 Radio Control Transmitter. Never got much into the radio aspects of the RC world before, but I recall reading in an old Practical Wireless mag that AM transmission was used, and the 4 analog values were lined up and sent serially using PWM. This makes sense, as at the receiver, all you need to do to recover the 4 analog values is to use a ramp waveform into a S/H circuit. At the end of the PWM slot, just latch the analog value, QED. To satisfy my curiosity, I set up the RC transmitter and demodulated its transmission using my IFR. I made a short movie clip to show what I found:
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Modulation and Demodulation |