Frsky problem.

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Re: Frsky problem.

Postby mvvs35 » Mon Jul 26, 2010 1:58 pm

Hello Tommy I am currently flying my Trex 450 with FrSky. although not I like the movement of the servos. in the air do not notice the problem.

I made a video testing the module http://www.youtube.com/watch?v=LmWvge3OtEk&feature=channel

Greetings!
Last edited by mvvs35 on Mon Jul 26, 2010 10:53 pm, edited 1 time in total.
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Re: Frsky problem.

Postby takilara » Mon Jul 26, 2010 5:02 pm

I am planning on putting my V8 in my 450CX so i can get the feel for it in the air. I guess i'll get to test that in the air tomorrow, also the 600 ESP with the telemetry module should be airborne soon. (just waiting for a few connectors now)
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Re: Frsky problem.

Postby takilara » Mon Jul 26, 2010 9:35 pm

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...

Makes sense that a Futaba "formfactor" module uses Positive shift...

They should at least make a note of this in their manual.

Thanks for all the help though, hope that this might resolve the issues for other people as well.
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Re: Frsky problem.

Postby JMP_blackfoot » Mon Jul 26, 2010 11:19 pm

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.
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Re: Frsky problem.

Postby RCModelReviews » Tue Jul 27, 2010 12:06 am

Good thinking...

I've got an Optic 6 here with shift-select -- I'll compare +shift to -shift and see if I can spot a difference.

If there is a difference then a simple inverter should go a long way to solving the problems with some DIY installations.
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Re: Frsky problem.

Postby JMP_blackfoot » Tue Jul 27, 2010 12:22 am

RCModelReviews wrote:Good thinking...
I've got an Optic 6 here with shift-select -- I'll compare +shift to -shift and see if I can spot a difference.
If there is a difference then a simple inverter should go a long way to solving the problems with some DIY installations.

In a past life, I designed and sold a receiver (under the brand name JMP) which at first wasn't autoshift. Then I made it autoshift by simply reducing the value of the input coupling capacitor to the squaring stage after demodulation so that it would differentiate both edges. There would thus always be a pulse of the correct polarity. That's when I became aware this business of leading edge/trailing edge...
Could you by any chance have a look and check for jitter of the pulse width?
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Re: Frsky problem.

Postby RCModelReviews » Tue Jul 27, 2010 12:30 am

Okay, as soon as I get a chance to get to the workshop (which may not be for a day or so).
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Re: Frsky problem.

Postby mvvs35 » Tue Jul 27, 2010 2:43 am

if not ill understand .. reversing my JR PPM solves the problem??
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Re: Frsky problem.

Postby takilara » Tue Jul 27, 2010 4:16 am

mvvs35 wrote:if not ill understand .. reversing my JR PPM solves the problem??


Cannot say for sure, but it fixed my problem..
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Re: Frsky problem.

Postby erezra » Tue Jul 27, 2010 7:19 am

Might be that signal quality plays a part too.

I have an Airtronics Stylus. Played around with it and having positive or negative shift did not make any difference.
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