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[Paparazzi-devel] ppm encoder technical details

From: chris
Subject: [Paparazzi-devel] ppm encoder technical details
Date: Wed, 23 Sep 2009 13:39:11 +0300
User-agent: Thunderbird (X11/20090817)

As i continue to work on the ppm encoder i discovered that the small jitter present
(~3 microseconds max)
is caused by the internal RC oscillator which is of course voltage dependent and it's
frequency vary from cycle to cycle.
It is not coming from the input measuring code as i was thinking.
While i don't think that this is important as the performance is more than adequate for our use If someone wants the jitter to fall to 0 (yes absolute zero) he will have to use a crystal oscillator at 16 Mhz. I also rewrote the code and now the firmware uses assembly for the critical measuring code
and a filter for removing any noise from the servo measurement.
I am flying with a ppm encoder at 16 Mhz like the one i described above
and even the digital servos are dead quiet.
In my oscilloscope the pulses do not have any jitter and the whole ppm output is very precise. The newer code also removes most of the jitter of the rc oscillator and it gives
a performance very very close to the performance of the crystal version.
If someone wants to try the new code just drop me an email.

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