Analysis of servo chatter with FrSky 2.4GHz telemetry system

This video shows my analysis of the cause of digital servos 'chattering' (buzzing continuously when they should be stationary) when using the FrSky 2.4GHz TX/Rx telemetry combo on a JR x-3810 transmitter.

Prior to this video I started by examining the PPM signals at the input pin to the Tx module and at the servo outputs on the Rx, looking for any instability in the servo control pulses. Both proved to be rock solid, especially so on the FrSky Rx servo outputs, ruling out encoder problems (such as noise coupling through the power supply from the Tx module) or receiver instability as the cause of the servo chatter.

In the process of fiddling around however, I stumbled on the cause - RFI (Radio Frequency Intererence) from the Rx telemetry transmissions affecting the servos.

In the video I show that my JR DS811 digital servos are affected by RFI, while NES507 analogue servos are not (I call them linear servos in the video - same thing). I then do some testing with ferrites and extension leads to try and work out if the RFI is coming up the servo lead, or if it is being picked up by the servo itself. The results suggest there is a bit of both going on, but that direct RF coupling is the dominant issue.

The RFI is probably either: a) affecting the operation of the pot a/d, causing the pot position to be misread; or b) pushing the quartz oscillator frequency around, so the PPM signal timer miscounts; or c) both.

So far the only solution I have is not to use the DS811 servos with the telemetry system, but I'm going see if I can fix it with some extra bypass caps or ground planing on the servocontroller board.

Please visit
http://www.rcmodelreviews.com/forum/viewtopic.php?f=44&t=228&start=120
for more details and followups on this issue. Say Hi to Bruce and thank him for hosting such a great and impartial website for RC modellers

PS. Greetings to my loyal subscribers! You've waited a long time for this, my first video posting!
Sorry if it's a bit underwhelming - it's not the video I had hoped to make :-)

Видео Analysis of servo chatter with FrSky 2.4GHz telemetry system канала pkt42