RFD900x Interference with Orange Cube

Hi everyone,

We have been experiencing an intermittent connection issue when using Orange Cubes and RFD900x radio modems. When the problem occurs the telemetry conection appears to be a one-way link allowng the normal flow of data from the aircraft, but only allows the occasional packet from the ground staion to be recieved by the flight controller. The ground station (typically Mission Planner) will be unable to download parameters, load missions, or request changes to the data stream rate. After much testing we believe the problem is direct interference between the RFD900x radio and the Orange Cube.

The problem has occured on three different aircraft. Two of which had significant flight time on a Black or Blue Cube prior to upgrading to an Orange Cube. Swapping the Black/Blue Cube back into each aircraft elimnated the problem. ArduCopter firmware version does not seem to be a factor as we have tested with 3.6.12 & 4.0-4.0.3. The problem occurs on the Telem ports, GPS ports, and CONS port. When an FTDI cable is connected directly to one of the serial ports the problem does not occur.

All of the modems were tested to be fully operational outside of this issue. The modems are powered by an external Castle BEC in all of the aircraft and we have produced dozens of aircraft of this configuration using Black Cubes without issue. Two sets of RFD modems were replaced for new radio modems during troubleshooting. A choke placed on the telemetry serial wire in the aircraft had little effect. However, reducing power on the RFD to 20db, increasing the air data rate to 125k, or increasing the number of hopping channels from 21 to 40 resolves the problem.

Because of this we believe it is direct electromagnetic interference between the Orange Cube and the RFD900x modems. We speculate this may be due to smaller trace size, smaller feature size in the new processor, or different layout of the PCBs. We are continuing to investigate the problem. For now we will increase the number of channels on the radio modems to mitigate the issue.

Has anyone else experienced this issue or have any other ideas on how to reduce it?