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RE: [Paparazzi-devel] ppm encoder technical details

From: mark.griffin
Subject: RE: [Paparazzi-devel] ppm encoder technical details
Date: Fri, 25 Sep 2009 11:04:11 +0200

Hi Chris,
That's a useful improvement.
I have the encoder board and a 16 MHz crystal so I would like to try the
new code.
Cheers, Mark

-----Original Message-----
From: address@hidden
[mailto:address@hidden On
Behalf Of chris
Sent: Wednesday, 23 September 2009 12:39 PM
To: address@hidden
Subject: [Paparazzi-devel] ppm encoder technical details

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
If someone wants to try the new code just drop me an email.

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