JMP_blackfoot wrote:takilara wrote:Switching from Negative shift SFTN (According to the manual, used by Hitec and Futaba) to Positive shift SFTP (used by JR, Airtronix and Multiplex) PPM resolved all my jittering, and choppy behaviour! Now the servos are as smooth as ever, and no buzzing at neutral...
To explain this, my theory is that the timing from coders is accurate and jitter-free on one edge of the channel separating pulses, while the actual width of the pulses is maybe not so consistant in some brands.
The FrSky module must be edge triggered (positive-going or negative-going, I don't know which). That is how it accomodates both positive and negative signals.
Depending on the polarity of the PPM signal from the coder, the FrSky module will react to the leading edge of the separating pulses or to the trailing edge.
If the pulse width is jittery (as distinct from the pulse position), polarity of the signal to the FrSky module will make a difference, as shown in this case.
It would be interesting to monitor the width of the PPM channel separating pulses fom the coder of this transmitter on a scope to confirm whether their width itself is jittery.
Then the problem would be the same with a 72 MHz autoshift receiver when changing polarity at the transmitter
It would show conclusively that it is a host transmitter problem, not a FrSky module problem.
JMP_blackfoot wrote:Has anyone had the opportunity to put a scope on the PPM signal from this transmitter - maybe at the teacher/student plug?
JMP_blackfoot wrote:1 - PCM/PPM Signal from Radio
2 - V+
3 - not connected/Rf detect
4 - GND
5 - RF Out
(In case the coder output is open collector, you may need a 10k resistor from PPM output to +V)
Source:
http://www.fpv-community.com/forum/show ... 254&page=2 (see post #16)
and:
http://www.rcgroups.com/forums/attachme ... tid=524885
http://static.rcgroups.com/forums/attac ... 1120039564
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