captcurt2000 wrote: > Good evening: > > What is the recommendation for bypassing or otherwise protecting the > sound card ins and outs from coupling in stray RF? I'm having some > distortion issues running with PSK31 and SSTV using a AWE64 and my > Drake 4 Line. All works as it should but when keyed up I get things > in the sound card that shouldn't be there--ends up creating a hi IMD > figure on the signal. > > Also, I'm using all transformer isolated in and out on the interface > and the line-level I/O on the card--no mic. All unused I/O are muted. > > How does one effectively shield the cabling with everything floating > from ground? The old instrumentation method is to leave the grounds > float from DC but bypass the ends bigtime.(shields) > > Thanks > > Curt > KU8L > > > Hi Curt,
Are you using PTT or VOX to key the rig? If you are using PTT, is it opto-isolated? From my own experience, transformer coupling of the audio will not solve the problem of ground loops that can significantly degrade your PSK or SSTV signal, if you still have the rig and computer ground tied together for the PTT. I use an opto-isolator in my home brew interface to completely isolate the rig and soundcard/computer grounds. When I first built my interface, I had the opto-isolator, but mistakenly left the sound card ground connected to the rig ground. This resulted in a very poor PSK31 signal. The signal was cleaned up considerably by simply removing the common ground connection. With regard to RF isolation, I insert small RF chokes in each of the audio lines in my interface, and use .001uF bypass capacitors from each audio line to the computer ground. This keeps the RF out of the soundcard. 73, Milt W8NUE
