I had two problems: the output level of my sound card was too high, and I was that thinking that with only 1W out, shielded cable and ferrite beads was enough isolation, but I was wrong. I compounded the problem by a lack of shielding when I test-clipped a pot into the line for level reduction.

When I got the scope on the AF input I realized that the distortion happened exactly when the TX kicked in, and not coincidentally!

So I wrapped my test leads in aluminum foil, put in the 600ohm 1:1 isolation transformer in the AF Input line, and put in a biggeer ferrite wrapped multiple times, and the problem almost went away. It comes back if I hook up the AF Out line, so I will isolate it as well.

One remaining issue is that I do still see some scope fuzz on the AF In jack when I hook up the antenna to the PSK-20 and that fuzz is not present when the antenna is unhooked (this is with the no DC power supplied) but I believe that is a minor isssue.

Setup: Sound card output fed to 2K ohm pot outer and output taken from wiper and one side, then fed to 600Ohm 1:1 transformer, then to PSK-20 and scope, with ferrite bead at connector on PSK-20 side. 10MOhm scope probe. Modulation is at 1000Hz.

1. I get a clean sine wave pair at 450mV when AF is not plugged in to PSK-20.
2. With the PSK-20 off, when I plug in AF In but no antenna, it loses about 10mv but remains clean.
3. When I hook up the antenna, the AF In on the scope picks up some high-frequency noise.
4. When I apply power, the XMIT keys and produces 1 Watt, and the AF gets very odd and doubles to about 900mV. This effect goes almost all the way away when I put in an isolation transformer from Buxcomm and a multi-turn ferrite on the AF In.

- posted by Leigh @ 00:23 z