Computers are nasty RF bombs that can leave stray signals across a huge section of the radio spectrum and as we move further toward digital radio they have become increasingly necessary in any modern radio shack.

With analogue radio it’s always been a choice to use a computer or not, based on what your listening habits are. All this has changed now with software defined radio and going down the SDR route means you can’t do without a computer.

SDR computer interference

Nasty RF Junk Getting In The Way

Decoding via the sound card has never needed much in the way of computer performance and you can minimize interference by making use of older computers (which send out less in the way of RF junk). An example of this is the 500 MHz computer I use for a whole range of decoding including HF Fax, Slow Scan Television along with various other amateur and commercial data signals.

Unfortunately SDR needs much more processing power and coupled with the fact that a SDR Dongle demands a fast USB port, this automatically rules out computers of the more vintage variety.

In a normal shortwave radio/scanner to computer audio setup it’s easy to separate the two devices from each other by the use of a simple isolating transformer.

A Different Game

This separations a whole lot trickier when dealing with a more complex USB connection, but as with any radio receiver a certain amount of distance between computer and dongle can only make things better.

The Problem

The latest dongle I’ve setup is suffering badly with interference from the computer its running on. Turning down the dongles gain helps a little but things only get a lot better when the setting is so low it starts to impact how well the SDR works 🙂

Before I try more drastic (and time consuming) steps to get things back on track, moving the dongle far from the computer as possible seems like a quick fix.

Working Up a Solution

I thought there were some USB extension leads in my wire collection that were left over from old mobile broadband dongles and was a little confused after an hour of searching turned up nothing. Remembering a recent experiment in setting up a home computer network via USB instead of the traditional LAN connections, I found (half) of the cable hardwired into a USB hub.

Pretty sure I’d thrown away the other end of the USB lead and stuck for what to do about the bit that the dongle plugs into, I went through the “computer graveyard” and found a handy dual USB PCB (complete wire wires) that had been part of an old Packard Bell base unit (pictured below).

dual USB 2

A Handy Find, Dual USB Circuit Board

This saved a lot of messing around with chopping a USB connector from the back of a motherboard along with the soldering needed to attach the wires. The USB board was screwed to some standoffs which I just couldn’t budge from the old computers metal work but I’m sure there’s something I can use to mount the board properly on a stand.

Note : At the data transfer speeds needed to keep SDR happy, any poor quality cables or bad connection is going to badly eat into its performance and you’ll be left wondering what went wrong.

If you have to connect any USB lead together, take care to make sure they go the right way so you’ll get the dongle working first time.

Vast Improvement

The before and after screenshots from the SDR# software show the difference moving the dongle away from the computer has made. Not only is the dongle away from a wide band radio source it also means you can keep the antenna coax out of the way.

There’s been no change in the reception of transmissions and I’ve eliminated a whole mess of “radio hash” in the process.

reduced sdr noise level

Before and After Views, Showing a Massive Reduction in RF Noise

The other benefit for me is that I’ve managed to chop a few feet of the coax because it doesn’t have to come so far onto the desk now.

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