Tackling USB RFI.

These days, the use of various USB-based power cables and supplies is pretty much universal.

You might think that using battery-powered USB supplies doesn't produce RFI that harms amateur radio reception.  Sadly, that isn't always true.

The most common problem is that where a small transformer steps down 12VDC to USB's standard of 5VDC.

In field tests with my mobile setup, I've found that connecting a USB cable to the outlet of such a transformer - even when the other end is not connected to a load, produces very considerable levels of RFI - on my TS480's lazy 'S' meter, it reaches about S7.  WSPR reception is not an option with that kind of interference!

Yesterday, I had forgotten to bring my lithium power bank with me, which I normally use to power my Raspberry Pi, because it doesn't lead to any RFI at all.  Having come a fairly long way, I looked around to see what I had in the car that would let me use the car's USB outlet without RFI.

It turns out I had two good quality TDK split ferrites around the sound card interface.  That was probably redundant, so I used it to put three rounds of the USB power cable around the core, immediately where the cable left the transformer outlet.

Immediately, the RFI dropped dramatically, with the S-meter now not registering, though there was a small amount of weak residual hiss.  Using the second, smaller ferrite at the computer end, the RFI was eliminated altogether, provided one eliminated any stray capacitive paths by careful positioning of the Raspberry Pi itself.

On a later outing, I found the solution to the residual RFI was to wrap the USB connector from the Raspberry Pi to the soundcard interface around a large ferrite (see below for details).  Now I had absolutely no RFI at all.

With these measures, and with the results obtained on WSPR as reported in a previous post, I was able to turn a day spoiled by RFI into a successful mobile outing.

The specific ferrites I used were a large TDK ferrite, type ZCAT 3035-1330 (13mm inner diameter), originally supplied with my TS480SAT for interface RFI suppression, and the smaller one, a TDK ZCAT 2032-0930, 9mm inner diameter.  Both are widely available online, albeit at about £5 or more each, but be careful to choose a reputable supplier, to avoid cores made of river mud, rather than the proper mixes!