Currently I'm trying to build one UAV as my freetime project. But I have run into this problem which is blocking me to take the UAV for its maiden.

My configuration is as follow:
1 * dsPIC30F6014A for ppm decoding, stabilization and navigation control
1 * dsPIC30F4012 for servo pulse generation
1 * dsPIC30F4012 to extract the information from the GPS NMEA string.

The communication betweent the dsPIC is I2C.

The problem is if I power the RC receiver from the same battery as the autopilot, it will have a serious interference problem and shorten the range of the Rx. If I use a separate battery to power the Rx and opto-coupled the output from the Rx, the problem gets better, but still not completely gone.

I have tried to wrap the Rx and all the servo wires in the grounded aluminium foil but it doesn't help either. I can't move the Rx any further away from the autopilot as my compartment in the plane is limited.

Anybody have the similar experience that can advice me?

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Is your radio system FM (72MHz) or 2.4 GHz?
I'm using Futaba 6EX-AP with PCM receiver (72MHz)
Unfortunately you've already thought of the obvious things--RF shielding and distance--so I'm at a bit of a loss. I'd try one more thing before switching to 2.4GhZ. The thing you're not shielding is the rx antenna, so I'd wrap the autopilot boards in shielding foil, not the rx, and see if that changes things.
Things are getting better now...This is what I did:
- Wrap the autopilot and Rx with grounded aluminium foil.
- Use a separate battery for the Rx and opto-coupled the output.

Now I can achieve the range of 40 steps with antenna down.

My original target was to share the same power source for both the Rx and autopilot, but seems like the Rx really doesn't like to share things with the noisy autopilot :(
It sounds like you have a ground loop problem. Very common problem in everything from stereo systems, to cars, and to planes. I am including a link here to help you get started. You should probably start using sheilded cables for your data lines too. Electric motors, even brushless put out a lot of noise too.

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