Once completed, install the circuit in a suitable plastic or metal box. I added a couple of snap-on ferrites to my audio cables as a precaution against RF getting into the sound card and distorting the signal.
KR8L's Homebrew PSK Interface
KR8L PC Sound Card to Transceiver Interface Box
General Description
This interface consists of four separate circuits, some of
which can be considered optional depending on what functions you wish to
provide: (1) Receiver audio input to the PC sound card. If you only want to
listen to PSK-31, RTTY, SSTV, etc., this is the only circuit you will need. (2)
PC sound card output to the transmitter. (3) Push to Talk (PTT) control circuit
for keying the transmitter. (4) Frequency Shift Keying (FSK) circuit. This
circuit is needed only if you plan to run radio teletype using true FSK (as
opposed to audio FSK -- AFSK) using a program such as MMTTY.
In my implementation of this design the connections to the PC
sound card are made through two shielded audio cables terminated in 3.5mm stereo
plugs, which are plugged into the sound card input and output jacks. Only one
channel (left or right; your option) is used and the other is not connected. The
PTT and FSK circuits are connected to the computer's serial (EIA-232) port
through a shielded cable terminated in a nine-pin connector. If you do not have
a serial port, a USB-to-Serial adapter can be used. Most Ham Radio sound card
programs use the serial port's DTR and/or RTS signals to key the transmitter PTT
line. MMTTY uses the TXD signal for FSK.
For maximum flexibility my implementation uses a five-pin DIN jack for the
transceiver connection. This lets me use a different cable for each transceiver,
since each radio has a different type of audio input/output interface. (I use
this interface with my QRP rig for PSK-31, with my VHF/UHF rig for WSJT, and
with my MF/HF rig for PSK-31, RTTY, SSTV, Olivia, and other digital modes.)
Regarding connections on the transceiver, note that most
modern rigs have connections for low-level audio output which is independent of
the main audio volume control setting, and also a separate audio input to the
modulator that is independent of the microphone gain setting. If you have these
connections available, they are the ones you should use.
Circuit Details
This interface box provides two main functions: (1) complete
electrical isolation between the PC and the transceiver, and (2) rapid and easy
adjustment of audio levels between the two devices. The circuit can be
simplified by omitting the audio level components and using the sound card
controls provided by your computer's operating system. However, I find it much
simpler to twist a knob than to go through the click-click-slide-click-click
process ever time I want to adjust levels. (Depending on which program and mode
you are using, you may need to adjust levels more often than you might expect to
compensate for audio passband response, band conditions, etc. Also, other
programs that you use in between your Ham Radio sessions may make changes to
your sound card settings that you will need to compensate for.)
Starting at the top of the schematic, the two audio circuits
are almost identical except for the direction of signal flow. The 1-to-1 audio
transformers provide DC isolation while permitting audio to pass through. The
1kO linear taper potentiometers provide for adjustment of the audio level, and
the 68O resistors establish a minimum volume level. My laptop does not have a
line-in connection, so I added the 3.9kO resistor to adjust the audio from line
level to mic level. (Specific values were determined by trial-and-error.)
The PTT and FSK circuits are identical. The heart of each circuit is a 4N25
optoisolator. The 4N25 contains a phototransistor, which is switched into
conduction by an infrared light emitting diode. When the appropriate serial port
signal goes "high" the LED activates and turns on the transistor, so that the
connected circuit (PTT or FSK) is grounded. Because the signal is transferred by
infrared light, there is no DC connection across the optoisolator. The 1.2kO
resistor limits the current through the LED to an appropriate level, and the
1N914 diode provides a return current path for the serial port signal when it is
"low."
Components for this circuit can probably be obtained from
Radio Shack or Hughes Electronics. The audio transformers may be the least easy
to find, so I suggest having them on hand before starting to build. Layout is
not critical, and can be done on perf board, "dead bug" style, using glued pads,
with the hacksaw (or Dremel. Tool) method, with a custom etched board, or any
other method you choose. Probably the most important part of the layout is to
make sure that you provide two separate grounds, one for PC-connected
components, and one for radio-connected components. I used double-sided tape to
stick the audio transformers to the bottom of the level pots, and attached the
circuit board (Dremel. method) to the top of the box with the level pots'
control nuts.
Once completed, install the circuit in a suitable plastic or metal box. I added
a couple of snap-on ferrites to my audio cables as a precaution against RF
getting into the sound card and distorting the signal.