3DR radio causing speed change in motors (interference)

Hi There, 

Thanks for giving me the chance to join this community. I have recently built a 450 quadcopter with an APM 3.1 mini FC, FSi6 radio, simonk 30A ESCs and sunnysky 980kv motors. I have recently received a 433mhz 3DR radio telemetry and since then I have been having some issues.

The radio link works well. I can see the data sent from the quad and I can also send commands from mission planner. 

However, whenever the 3DR is plugged in, the motors have short speed changes as if the pwm values jumped and then back to where they were. Initially, I though this could be some errors received from the link so I disconnected the TX and RX wires from the 3DR to the APM and the problem still there. So it looks like the problem could be coming from the ground of the 3DR (kind of a noise), but I don't have and oscilloscope to see whats happening. I also tried running a cable from the GND of the 3DR to the GND of the ESCs (which is technically the same), just to prove that it was not a ground issue and it did not work.

So it looks like the APM does not like the 3DR(even without the TX and RX wires, just GND and +5v) and this causes changes on the motors.

I really do not understand what's going on and I would appreciate it if someone has come across something like this and can give me a hand



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very weird, I have seen plenty of knockoff flight controllers (no - not "clones" , but thown together crap) where manufacturer saves on every capacitor, component, resitor possible.   I am not saying yours is one of them, but it sounds like that.

This must be some sort of electric, noise problems.  - check it using an oscilloscope.

Thanks for your reply @Andre K.

Yeah, it looks like electronic noise. Unfortunately I do not have an oscilloscope at the moment so I cannot see what's going on.


Do you have a lot of logging activated? Could be cpu stall. 

now that I think of it.

it could also be something about the ESC's  - they are really only supposed to measure a digital pusle width , but try to connect only their PWM pin,  (not GND) 

try another radio as well, preferably a "3DR" - if you can just borrow one from somebody.

Hi Micha, to be honest I have not added anything else other than the 3DR.

As I mentioned before, the problem comes even if I just connect the GND and +5 wires which do not involve any processing. The voltage is not dropping below 5v when I connect the 3DR so the CPU has the right conditions to work

I can't relate a cpu stall with this issue...is there a way to measure it though?

Thanks a lot for your help

I've spoken to the supplier and they will send me another radio

Regarding the the PWM and GND pins, I don't think there's any issue with that. In fact, it is a good practice to have a good GND circuit throughout.

If the issue is noise coming from the GND pin on the 3DR, it has to be replaced



After many hours scratching my head I found out the problem.

Problem: For some reason, the power regulator in the APM mini is unstable. Which means that whatever you plug into it will cause noise or sudden voltage drops. This perhaps this is just a quality issue...

Solution: I decided to run the radio and telemetry straight from the power module which delivers 5.37v and up to 3A. 

Alternatively I can also use a separate UBEC to power the rest of peripherals (5V)


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