YO3BN - Bitx80 PC Interface

Posted at - 27th September 2017

Bitx80 to PC Homemade Interface

I made this interface because I wanted to transform my bitx transceiver into a beacon primarily to test propagation conditions sometimes and to enjoy the other beautiful side of ham radio, working on digital QSOs.

The Schematic

Again, I didn't considered very important to draw this simple schematic in *CAD.

The interface mainly does nothing more than connecting audio outputs to audio inputs and switch between Rx/Tx. Therefore, audio output from transceiver is connected to microphone input from PC, and audio input (microphone) from transceiver is connected to audio output from PC. For receiving/transmitting command a pin from RS232 serial port is used.

The PTT Circuit

On the top side, the LTV817 photocoupler act as a "relay" with insulator role. Taking +3 to +5V from RS232 port, it will close the phototransistor between pins 3 and 4, thus putting PTT from transceiver down to GND, activating transmission. Next, the RF component is filtered out by 100nF capacitors and common mode choke. Another PTT line is provided after photocoupler just in case of using simple contact instead voltage controled commmand. Maybe is a good idea to add a 5V zener diode in front of LTV817 for future projects. The PTT command is taken from DTR pin from PC RS232 serial port at this moment.

The Audio Circuit

The audio lines are on the middle and the bottom of schematic. It can be seen that the audio output from transceiver is connected to the audio input of PC and audio input from transceiver is connected to audio output of PC. The 680pF capacitors and common mode chokes filters out the residual RF if any. The 1uF capacitors separates DC between PC and transceiver.

Construction Details

Other Notes

Depending of computer audio mixers settings or transceiver design, microphonics oscillation can occur. In my case, I think the bitx design creates this phenomenon because of common path in audio lines. The microphonics can be suppressed adjusting the audio volume from bitx below oscillation threshold, unfortunately the volume in computer speakers will be lower then.

I successfully used this interface in conjunction with bitx and fldigi on digital QSOs (RTTY, BPSK-31/BPSK-63). With 2xIRF510 50W amplifier and 20m long wire antenna from Bucharest Romania, I worked Spain, Germany, Finland and Russia on 3.5MHz, max. distance being ~2800Km.

73 de YO3BN