I have a working (sofware and hardware wise) project that interfaces with a scanner radio. Everything was fine until I took the project mobile. Previously the radio had its antenna mounted outdoors and well away from the display so I never noticed any inteference problems. In the vehicle the display is on the dash and the antenna is a few feet away. I noticed broadband noise was generated in the VHF (108-150 MHz appx.) range. I was planning on embedding a small GPS module in the project behind the display as well. I couldn’t get it to work. Thinking it was a bad module or wiring I was at a loss then I realized if I moved the GPS about 18 inches from the display it blinked and received a GPS signal lock.
I then looked at what is emitted from these things using an SDR receiver from 25-200 MHz. It’s terrible. There is a massive broadband noise source emitted in the VHF range. I’m not seeing as much at 1545 MHz L band GPS but I’m thinking the huge broadband noise is swamping the preamp in the GPS. I tried numerous different types of GPS modules and it does it to them all. I tried wrapping the entire display in copper foil tape, still no improvement.
I’m not sure how these are part 15 compliant. Has anyone ran across this issue? Are there any known useful mods (filter caps, inductors) that mitigate it? I have tried adding. 1, .01 uF caps at the connector back pins to ground and tried a 47 uH inductor in line with power but it didn’t help.
At this point I have to abandon the project if I can’t figure it out. Putting my finger on the board seems to couple more energy out if it’s near the large flat memory chip and near the real time clock. I disabled the RTC chip by lifting the VCC pin but that didn’t help. I don’t know if it’s the memory reading and writing that’s doing it or maybe the FPGA or what but it’s frustrating as hell not being able to do anything about it. Obviously, many items like vehicle GPSs and other similar items use touch panels (cell phones) so it can’t be something that can’t be addressed.