On the bench:

2012-Feb-15 - Building an Interface and Working PSK (and RTTY)

I use an old G4 Mac Laptop in the shack right now. It runs my logbook program and allows me to look up a few things on QRZ.COM. It's fairly slow but its nice to have a dedicated computer that just sits by the radio. It does not have a soundcard, so I ordered a really cheap USB soundcard interface <>. It plugs into the USB port and gives me a microphone input. I fired up fldigi and was able to copy PSK signals on 40m. Even though my radio is homebrew, it is pretty stable, the vfo barely drifts after it has been warmed up, and the fldigi program tracks any small changes in frequency. It seemed to decode the signals well.

I wanted to try transmitting back of course. Would that work with the home made radio?

I don't have a serial port from the computer, so I put together a simple PSK interface to key the radio when the audio signal is detected. It's based on a couple of circuits I found online. It was simple to construct and works well. I bought an isolation transformer for the audio signal, but I realized that the computer ground is still connected to the radio through the receive audio cable. So I thought I needed to buy another isolation transformer. That said, everything seemed to work fine, so I pressed on without audio isolation.

First of all, once the interface was built, I could not find any PSK signals. The band was jammed with RTTY (Radio Teletype) signals. I could not decode any of them. I tried every combination until I realized that RTTY is transmitted on Upper Sideband. I only have LSB on my home made radio...

Software to the rescue. I clicked a button in fldigi and it reverses the tones, so that LSB looks like USB. Suddenly I could receive the signals. It was the RTTY WPX contest.

Ok, so now I needed to setup for RTTY transmission. I fiddled with the potentiometer in the interface until the audio to the radio was producing a couple of watts output. I checked the signal on the scope and it looked nice and clean. I connected the radio to the amplifier and suddenly I had about 200 watts of RTTY signal on the air.

I picked a suitablly loud W2 station and sent my callsign a few times after his CQ TEST. He came back with

AC2CZ AC2CZ 599 - 1292 - 1292 k

WOW!  He can hear me.

I sent 599 001 001 001 TU

Enter Comments Here:

Comments on this post