Pulse Position Modulation

A subset of Pulse Width Modulation used to control the position of RC Servos

Paul B. Webster says:

Pulse Position Modulation:

... The actual transmitted version is a series of pulses of a given width (somewhat less than 1 ms), the *position* of each of which in respect to the previous defines the length of the pulse to a given channel.

More specifically, the 1 to 2 ms channel pulse is the time between the start of one pulse in the transmitted sequence and start of the next. The minimum pulse width of 1 ms corresponds to the transmitted signal going on for 0.5 ms, then off for 0.5 ms, then on again for the next pulse. Of course, where FM is used, these are two frequency states instead.

The length of the series of pulses is of course the sum of the individual output pulse lengths and may vary over a wide (2:1) range. The "rest" time before the next series is used for synchronisation.

What is so elegant is that a single 4017 CMOS chip, with a resistor and capacitor to reset for each series of pulses, neatly performs the decoding function to the individual servo outputs.

Comments:

• spamdean at reeceweb.com

  I'm looking for a schematic for a 4017-based PPM decoder as described above. I've found several schematics on the web, but they all require additional chips to handle the sync reset. I want a simple/tiny/cheap PPM decoder and using an RC network to handle the reset is very appealing; a reference would be much appreciated. Thanks!

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