After talking with a recent course graduate about operating using digital modes, etc., the topic of radio-to-computer (sound card) interfaces came up. For those who are unaware, a sound-card interface allows one to safely connect their radio’s audio input and output (and push-to-talk) to their computer, opening up the world of digital modes (such as PSK31, JT65, RTTY, SSTV, and many more).
After a brief discussion of the reason for isolating your computer from your radio (esp. true for desktop computers tethered to base radios) I explained a simple transformer-isolated circuit. It is true that capacitors can be used to DC isolate components, however, I firmly believe that using a transformer is a better practice.
During our conversation, I drew up an interface which I have very successfully used with my KG-UVD1P and FT-857D radios. The above schematic is what I currently use and can be built for roughly $20 (plus another 15 for a USB-to-serial converter).
The basic concept is that the two (600Ω:600Ω audio) transformers DC isolate the radio from the computer while still allowing audio frequencies to pass. Potentiometers then reduce the signal for systems as appropriate for low-level inputs (such as many microphone inputs). The PTT is handled by taking a RTS signal from an available serial port (USB-to-serial in this case) and passes the signal through an optoisolator (LTV817 in this case).
Add in a USB sound-card such as the Creative USB (for $30) and you have effectively made a $65 sound-card interface which would cost well over $100 if purchased from a commercial supplier.
Fast forward to my latest project.
I am currently designing a single-board sound-card interface for amateur use which should come in at roughly the same price (target is <$60), offering a kit-build project to introduce more radio amateurs to digital mode operation.
The image above shows an early prototype of my PCB .. there will be more changes as I explore different options, however, this is the rough skeleton. The PCB is 85 x 50mm (3 x 2″) at the moment which offers a nice compact foot-print. I will be making conceptual changes as I go along to make the project as user-friendly as possible. I may even integrate a micro-controller to handle mode settings, etc. to further stream-line the whole interface.
… more to follow!