Digital Interface

Waay back in 2004 (this is being written in 2013) I decided to try PSK. The Four State QRP Group's Wednesday Night Warble PSK Net was in full swing and I thought it would be fun to participate. I had tried RTTY many many years ago and didnt care for it, but the new digital modes were interesting.

My building buddy, WAMWW, had built an interface based the information on WM2U's web site. It looked easy to homebrew, so I followed his lead and began to assemble parts. I wanted the capability to switch the computer's microphone and speakers back and forth so I worked out that switching. But the rest of the circuit is from WM2U's information. His web page was, and still is, a treasure trove of digital communications information.

Another goal was to provide complete isolation between the rig and the computer's sound card, thereby preventing ground loops and providing solid reliable operation. The two isolation transformers and the opto isolator accomplish that. Complete control of both transmitted and received signal levels is attained through potentiometer adjustments.

A cable switch box serves as the home for the circuit. At the time they were available at most hamfests for about a buck. Labels applied to the front and back covered up the extra holes and dressed up the project. I added some LED's but they really arent needed and the only one that works reliably is the one on the PTT line. Thats the only one I would include if I were building it today. It has helped when trouble shooting COM port assignments.

Please refer to the pictures on the right, and the photopage here for more info. There are many commercial interfaces available today, but I would build again rather than buy. This interface performs very well, but the next one would be simpler, i.e. no switching and only one LED.

20 January 2013
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