Just to set this straight… This is the greatest radio since Marconi asked “Can you hear me now?” Since the beautiful Hedy Lamarr said “Jam THIS!” This is an RC transmitter that has every switch and function user assignable, almost unlimited mixes, and plays music while some sexy fembot announces receiver signal strength, lipo voltage, and GPS coordinates !!! The audio hum was hardly there. I had to hold the radio within 12″ to hear it. It didn’t even bug me, but everyone was talking about it. It’s like you live somewhere for years and one day your wife points out the stain on the ceiling… Jeeez thanks! Well I had the tools and figured what the hell. I have the “B” model, and it already had the “Scott’s Special Speaker Mod”
I didn’t fall off the turnip truck yesterday….. That happened weeks ago, and as a paid RF noise chaser I had some clues. The hum changed on how you held the radio. It also changed with antenna position. When I opened the radio, the hum was VERY variable according to which wires and ribbons were moved. That led me to believe there is/was a lot of RFI noise inductive coupled into the whole wire harness. Usually I find that the noise is from DC/DC switching converters. Next in line is the processor clock. That’s what I found on the Taranis. The inter-board ribbons were pretty hot with EMI from 160MHz to 600MHz. The CPU clock frequency RFI has sub-harmonics like a big ugly comb generator response. These harmonics get into the DC bias lines, and pollute all the op-amps and audio amp chips. Even though they’re in the micro volt level, they get amplified up with the audio signals you want.
Since I don’t have a schematic, I didn’t want to do a lot of probing and testing to find where to install filter caps and inline induction. That would cost millions of dollars and government forms would have to be filled out. A simple, quick way of reducing the transfer of RFI between system boards seemed like an easy try. Noise is born on the processor mainboard, and I want to strangle it on it’s way to other boards.
For some fun I display a sample from the “big box o’ ferrites” EMI arsenal. Rings, clamp-ons, beads, plates, tape. Actually the plates are an idea I had in mind to epoxy on the bottom of the boards, and/or to fit and position between the boards when the case is closed. The smallest beads could work, but I’d have to disassemble the ribbon connectors to get them on. I decided on the ferrite tape. I fount the hottest ribbons and gave them a few wraps of ferrite. The probe is so sensitive that I could isolate the two noisiest wires from the ribbon and just wrap those. The ferrite tape seemed to absorb a good amount of RFI, and the noise reduction from one end to the other was about 15dB The audio hum was so reduced that I had to hold the radio at my ear to even acknowledge it.
Please click on the pics for the full view. I show the EMI response before and after, some pics of how I wrapped the ferrite tape around the ribbon cable, and a pic of my DIY near-field EMI probe. The description and diagram of the probe and be found here. http://www.compliance-club.com/archive/old_archive/010327.html
After a couple days of test, I finalized and improved . The tighter the ferrite tape is against the wires- the better. I re-wrapped them and tied it up with waxed lacing string.
Even though the tape mfg claims surface resistivity in the MegOhm range I also put a square of foam over the board so it wouldn’t rub against it when the case is closed.
I found out this also reduced the headphone hum.
Here’s the ferrite tape fitting. The upper ribbon cable gets two pieces of 2″ long tape wrapped around all wires. The lower ribbon in the pic- separate the last two wires from the ribbon, and wrap them with a piece of tape lengthwise.
Here are some links to ferrite tape manufacturers / distributors.
This EMI / noise mitigation was fueled by beer and rushed to press. Next time I do a mod I’ll have the video camera and whiteboard set up
IF you’re not a Taranis owner, check it out here- http://www.alofthobbies.com/frsky-taranis-x8r-combo.html