RFD900 makes servos jitter

Hi all,I have 3 planes with APM 2.5 units and one set of RFD900 modules.In any plane I choose, I get slight jitter in the servos when I turn on the RFD900 radio.So far I have installed a ferrite bead at the radio, added 0.1uf and 100uf cap at the radio and shielded the wires going to the radio.I also tried running the radio from a different power source to no avail.If I disconnect the tx and rx wires from APM but still have power applied to the radio then the jitter disappears so the noise seems to be coming through these wires.Any other tricks to try? The radio is on a shielded lead about 30cm (12inch) from the APM2.5

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  • Hi, sorry not to read whole article and maybe not answering right, but this is my experience:

    I built my own OpenLRS with 868MHz/1W  modules. So it is somewhat similar to RFD900. Cheap lousy servos get violent jitter when I come by with Tx antenna. Better servos like analog Hitec have this issue reduced and digital servos are immune. Even 433Mhz/100mW modem caused me little jitter in my cheap lousy servos.

    Solution: separate modem from servos and their lines or use digital servos.

    Sometimes I get cheap servo and test by it's jitter if my Tx is working well.

  • I've also had the same issues with the RFD900 & servos; I always have to drop down the power on the air radio to around 20. I'll try the toroid as suggested!

  • Had the same problem recently with a 3DRadio causing jitter in a high speed tail servo.  It was definitely radiated, because if I cup my hand around the antenna, it stopped.  The antenna was near the servo and wiring.

    4-5 wraps of the servo wire through a toroid cured it.  You said you tried ferrite beads, do you mean the clamp over type?  I tried those, and they didn't help.  Only the toroid worked. 

    • I had this problem with a 3DR radio causing the tail servo on a tricopter to twitch (high speed coreless digital servo in an aluminum case).  I moved the 3DR radio from the tail boom to one of the front booms and the problem was fixed.

      Since the RFD900 is so much more powerful, I'm sure the problem is harder to avoid.

  • Hi - I had this same issue with the 3DR 900mhz radio.  Depends on the servo whether or not the servo twitches due to the 900mhz 3DR radio some do - some don't. 

    I found if I soldered a 33pf capacitor inside the servo between the incoming signal lead and ground I could filter out the glitch. 

    • Brian that's interesting. I will give that a go. Maybe time to pull out my oscilloscope. It's 100mhz so it won't see any 915mhz noise but it might give me a clue as to the PWM shape change with/without the capacitor.
      • Same problem in big rf power of rfd, same problem in orange 1w, my servo 2 Bluebird broke in 10 minutes of jittering. I try filter, shilding whith no luck.

        Change servo to analog Hitec and problem solved!

        • I have analog hitec, need to get mine in the air to see if it jitters, will report back in a few days. 

  • Sorry for OOT: any one also experiencing gps glitch when using rfd 900?
  • Are you saying you're shielding the wire between the APM and RFD900? If so, you don't need to really worry too much about that; what you're likely getting is currents induced in servo wires from radiation emitted from the radio's antenna; I've seen this myself when I had an RFD900, with 2 coaxial cables coming out of it to the antennas; those coaxial cables ran parallel near a servo wire (within <10cm) and caused severe servo juttering.

    To fix it, simply move the RFD900 away from any servo leads; 15cm's distance should suffice; also try and have the antenna perpendicular to wherever wires are running, so that the servo wires sit in the middle or at the top/bottom of the antenna's radiation pattern (assuming you're using omnidirectional antennas).

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