Saturday, April 17, 2010

Cell Phone Interference on the Softrock Series SDR

Ever since I built my first Softrock, I was plagued by the usual buzz sound caused by a cell phone in close proximity to the radio.  It was so bad with the rig, that the phone could be anywhere within about 15 feet (or more) and the interference was still quite noticeable.  I had some success reducing it slightly by putting ferrites on the cables going to the radio, but could never reduce it to the point where it would not be annoying - until today.

The following modification should work on any of the Softrock sets, although I tested it on a TX/RX 6.3.

On an old scrap circuit board in my junk box, I found some molded RF chokes.  The particular value I found were 4.7 microhenries.  I installed one in series with the B+ line at the power jack in my shielded metal chassis thats holds the radio.  This reduced the interference substantially.  I then bonded the B-, shield of the serial connector and the grounds of the 2 audio connectors as well as the ground of the USB cable in my 6.3, and that reduced the interference to the point where I no longer hear the interference with the phone in my pocket, about 3 feet away from the unit.

I tried the inductor on the B- side of the power supply also, but when I did so, the interference came back to about the same level as it was before I began.

As an added bonus, by making this modification, I was able to also reduce the center "hump" substantially as well.  My analyzer screen in PowerSDR is now almost totally flat across the passband of the sound card.

If you have this type of interference on your V9 or 6.3, I strongly recommend doing this mod.  I assume the inductor value isn't critical, so long as the value is relatively small.  It seems that there is not enough capacitance between windings on the inductor I used to pass the 800 MHz garbage.
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