This is the interface that I use between my computer and my transceiver. It was originally published in Practical Wireless, (PW), in the February issue of 2009. It was in a regular column by Colin Redwood (G6MXL), entitled, "What's Next?" It's purpose is to electrically isolate the radio from the computer, but to allow audio signals to pass between them and to switch the radio from receive to transmit and vice-versa. The interface is essential, both to protect the computer and radio and also to stop "earth loops" from causing "hum" on the transmitted signal.
The received audio PSK signal is obtained, in my case, from the "Data" socket on my Kenwood TS-480SAT. It could just as easily be brought out from the external speaker socket. I use the "Data" socket because as well as received audio, also present are the transmitted audio input and the rig's PTT connections. On other rigs these latter two functions could be connected via the microphone socket. A shielded cable connects the "Data" socket of the rig to a 6-pin mini-DIN socket on the interface. From the mini-DIN socket the incoming audio is fed to the primary coil of an isolating transformer. This induces an exact copy of the audio signal in it's secondary coil which is totally electrically isolated from the radio's receive circuits.
In the original design, the audio signal was then led out of the interface and fed to the computer sound card's microphone or line input socket. However, a modification by Rodney Byne, G7OEL, was published in the April 2009 edition, which I felt merited incorporating into the circuit. This is basically a signal "clipper." If a sudden surge or spike appears on the audio output of the isolation transformer, the germanium diodes conduct and "shunt" excess voltage to ground, thus protecting the computer's sound card. The capacitor, C5, as well as isolating the diodes from the microphone input of the computer also prevents any DC which may be present at the microphone socket from being shorted by the diodes. This voltage is sometimes present to power electret microhones plugged into the socket.
The audio signal from the computer's speaker socket, which is used to transmit the PSK from the radio, enters the interface through the same 4-pin DIN socket that the received audio exits from. Like the received audio, it is fed into the primary of an isolation transformer. The induced signal passes through a 10K resistor to a 50K log potentiometer which is used to control the drive level to the transmitter. The signal then exits the interface through the 6-pin mini-DIN socket and through the shielded cable to the rig.
So far, so good. However, we require the radio to switch from receive to transmit when the PSK program requires it to. Some commercial interfaces use the serial port of the computer to switch the rig into transmit. However, many computers are no longer fitted with a serial port. This interface uses the computer's audio output itself to switch the rig's PTT circuit, simple heh! A sample of the transmit audio is taken by the capacitor C2 and is amplified by transistor TR1. This amplified output is fed through capacitor C3 which blocks the DC component of the signal and the AC signal is then rectified by diode D1. This DC voltage is then applied to the base of transistor TR2 which acts as an electronic switch. When this transistor turns on a current is allowed to flow through the LED inside the opto-coupler device, IC1. The LED in series with this chip illuminates to inform the user that the PTT has been activated. The light-activated transistor inside the device is turned on, which effectively shorts out pins 4 and 5 of the chip. As far as the PTT circuit of the radio is concerned, this is the same as a switch closing and that's enough for the radio to go into transmit. This PTT switching is also connected via the 6-pin mini-DIN socket and the shielded cable.
The amplifier obtains it's operating 5 volts from the USB of the computer. A lead from the 4-pin DIN socket connects with a spare USB socket on the computer so no battery is necessary. I also thought it prudent to include an on/off switch between the 4-pin DIN socket and the amplifier's smoothing/de-coupling capacitor, C1. This is to prevent the radio from being switched to transmit by ANY audio signal from the computer. After all, the computer can be used for other applications besides amateur radio ones! If I do inadvertently leave the switch in the on position, the LED illuminates and alerts me. This modification was also communicated to Practical Wireless by another amateur, G3VZM, his reason for adding the switch being the same as mine. I thought the P.W. columnist's response was a little dismissive, but I think our way is better than disconnecting leads from the computer soundcard, or disabling all the operating system's audio warnings, which was his approach.
I built the circuit on 0.1 inch matrix board with copper tracks on one side and fitted it inside an ASB box with the on/off switch, the "transmit" LED and the "drive" potentiometer on the front panel. On the rear panel are the 6-pin mini and the 4-pin DIN sockets.