Q I’m trying to become active in slow-scan television (SSTV) with software that sends and receives images using my computer’s sound card. I can receive images just fine, but I’ve been told that my transmitted images are very distorted. From the way my transmitted tones sound, I suspect that RF is getting into my sound card. In fact, if I reduce power substantially, the distortion disappears. I’ve placed ferrite cores on all of the leads going in and out of my PC, as well as on the cable going from the sound card output to the auxiliary audio input on my transceiver. They helped, but they didn’t suppress the RFI completely. Can you suggest something else I could try?
A My suggestion would be to try to isolate the sound card audio output line from the transceiver. You can do this by simply adding a 1:1 isolation transformer in the line (Figure 1). You may still need to use the ferrites, however, to get full suppression.
Figure 1—Adding a 1:1 isolation transformer between the sound card output and your transceiver may help alleviate some RFI woes. The transformer shown in this illustration is a RadioShack 273-1374.
From QST June 1999
