[RFI] Audio Interfacing to Sound Card and RFI

Jon Ogden jon.ogden@cain-forlaw.com
Mon, 19 Nov 2001 10:21:28 -0600

Hi folks,

OK another RFI problem here.

This weekend, I operated in the ARRL SSB sweepstakes.  I just recently
purchased Writelog as my logging program and wanted to interface the radio
and sound card to allow voice keying, etc.

After getting things hooked up and turning on the amp, WHAMO!  All sorts of
RFI.  UGH!  So I took and wrapped the big FT-77 material torroids around the
audio in and audio out lines right at the sound card.  I think I have about
25 turns on each one.  I also have about 10 turns or so on one of the
smaller torriods (also FT-77) for the microphone line.

I tested it on 20 meters Friday night and things seemed OK.  However, on
10M, I got some horrible RFI problems and even later in the contest on 20
meters (which originally seemed clean) it was really bad too.  It depended a
great deal on how my beam was pointed, etc.

Now I can't imagine the RFI getting in on any of the input or output cables
to the sound card.  Those are pretty well wrapped.  Could it be another line
on the computer that is causing this?  I have two radios connected to two
serial ports.  One port goes to a FIF-232 and then to my FT-1000D.  Both the
line going to the FIF-232 and the line to the FT-1000D from the FIF-232 are
wound as well with a few turns on torroids.  The other line is not.
Keyboard and mouse lines also are wound on torroids.

The power line and monitor line are not wound.

So where else could I be getting the problem?  If I disconnect the audio
lines from the back of the radio, all is fine (the line in on the sound card
is coming from the line out jack on the radio and the speaker output from
the sound card is going to the "patch in" jack on the back of the radio).

Is there some cable, perhaps inside the computer (like the one going from
the CD-Rom to the soundcard) that's getting RF on it?
Would it help to ground the case of the computer to RF ground (worked before
with another machine)??

Any help anyone has would be appreciated.  I can't believe I am the only one
to experience this.

Thanks es 73,


Jon Ogden
Sales Engineer
Cain-Forlaw Company
847-202-9898 (Voice)
847-202-9896 (Fax)