This is a simple interface box to allow connecting a radio to a PC with some improvements with respect to similar projects available almost everywhere. Interfaces like this one are used to manage via PC the transmission and reception of digital signals (RTTY, BPSK and other digital modes) and of CW (morse) signals (using skimmers to decode them). I also use this interface to record on the PC my QSOs in particular during contests.

The main features of this interface board are the following:

The reson why I realised this board and I did not choose one of those available on the market is mainly for the two above mentioned reasons. Having a goog galvanic isolation between PC and radio is fundamental, to avoid ground-loops which may introduce noise in the receiver and at the same time this will protect both the radio and the PC. Just to make an example, I had a PC sound-card damaged probably by a too high current on its output line (probably the impedance on the radio side was too low).
The possibility to adjust the audio levels to fit with the radio and PC is important. In this way I can avoid distorsions that may compromise the transmission and reception, in particular when using AFSK.

Obviously, you can buy a digital interface like one of those produced by microHAM. This kind of interfaces will give you much more functionalities, but the category of price is much different and from my point of view, a good PC sound-card works fine (or at least you can get reasonable performances).


The connection between the PC and the radio is performed via one serial port which manage the PTT and RTTY/CW signals and via "line in" and "line out" ports of the PC sound-card. Parallel port can be also used as an alternative to the serial port, in case needed. Probably all SW for contesting and for digital modes handle both ports, but the newest PC will provide only serial port and in some cases only USB ports are now available. In the last case a USB-to-RS232 adapter can be bought at very low cost.

This is the electrical diagram of the circuit. The insulation of the PTT and RTTY signals are obtained via two optocoupler 4N35 while the two audio signals (in and out) are insulated with two audio 1:1 transformers. Two op-amp (LM358) are used to amplifiy the in and out signals and to provide the constant impedance at the input and output ports of about 1kOhm. The audio level can be adjusted by mean of the two potentiomenters, but I preferred to set a fixed gain on the two op-amp. This gain can be adjusted changing the value of R1 and R3 to fit with the radio in/out levels (it should be set to have a maximum signal level just a bit higher than the value expected by the sound-card and by the radio).
The circuit also include a voltage regulator to allow supply it directly with a 12V AC transformer. I preferred to use this solution to avoid ground-loops with the radio 12V supply line. In case you want you can also supply the circuit using the radio 12V stabilised supply.

I have realised a small single-layer PCB of this interface. You can see it in the following two pictures.

The assembled board and my interface box can be seen below. I put on the front panel the ON/OFF switch, three leds (ON/OFF, RTTY and PTT) and a DIN connector for the radio interface (this will allow to connect the cable of each radio I have).
On the back-side I used a CANON 9-PIN connector to interface with the PC. I used a signle shielded cable for all signals. RS-232 and audio lines are split inside the RS-232 connector on the PC side, as you can see in picture below).


In case yuo want to realise your interface I have still some PCBs available (as shown in picture above). The transformer I used can be bought on-line on the RS Components site. Its order code is .You can find it directly here. I have also few available.