The clever geeks who have written software
also include ways for the software to control the transmitter, and
can tell it when to transmit, when to receive, and sometimes more!
Also in order to use either CW (computer
generated or recieved), FSK, or AFSK it is necessary to connect
the output from the computer to the input of the radio, and the
output from the radio to the computer. In order to do this an interface
This is for two reasons.
- Convert the output from the computer to a signal
that can control the radio.
- Isolate the computer from the radio to prevent
interference and other problems.
The first is usually achieved using an
electronic isolating switch such as an "Opto-isolator and the
second by using audio isolator transformers.
Commercial interfaces are manufactured, but can be quite expensive
(especially for "brand name" models, but cheaper independent
However a digital interface must be one
of the simplest projects that can be undertaken by a ham ( even
I made one, so it must be easy) and it sound very impressive when
you tell all your (none ham) friends that you have constructed a
"Digital Computer to Radio Interface for AFSK, and Remote TX/RX
control", and they will start to think you are now one of those
clever computer Geeks!!!
My first interface was a commercial one
made by G0TSG. It was very cheap, and introduced me to digimodes
very successfully. It basically was only an audio isolator, and
as my radio at the time did not have VOX control, transmitting/receiving
control was handled by a switch on the unit. This was fine, but
I occasionally forgot to operate, or release the switch, which became
very tiresome (and a little embarassing!!!). This was therefore
given away to a friend to try digimodes.
I replaced it with another commercial
unit from Watson, a WM-03. This now had automatic TX/RX control
by using one of the computers comports and software control.
However this also had to go, as it did not isolate the ground between
the radio and computer, and I was getting significant RF-Feedback
in the audio. Also I had two radios I wanted to use with the interface,
and both had completely different audio input and output requirements,
which meant that I had to constantly change the sound-card settings
when changing the radios over.
So I wanted an interface that could be
switched between the two radios, with a switch, and that could be
set up to work with the computer sound-card without changing sound-card
level settings. Only one commercial solution seemed to exist (though
not sur it did exactly what I wanted), but it was too expensive,
hence the need to make my own, which has been quite successful,
but with one big disaster (more about that later!!!)
Below is a schematic and some photos
of the one I made.