Sunday, 3 August 2014

Mobile WSPR - Finally!

Well this took some flippin' doing! The audio isolation needed to make an effective interface was really quite simple, the trouble came about when trying to use the radios own Digital VOX circuits to control the transmitter. It would work, so long as the audio input was high enough, but that level was always far greater than the power output I wanted, and always driving up into the ALC action region! No matter what settings I used, adjusting the soundcard level, the interface level, the VOX level, or the Digital Gain control on the rig, I could not get the 1-2W I wanted AND have VOX PTT!

So, I had to work on a hardware solution.  This seemed simple - both channels of the soundcard carry the same audio signal at around 4Vpk-pk, it should have been a simple job to rectify this and drive a switching transistor. Er, no. The output of the diode doubler used only generated 0.3V, no where near enough to switch the PTT transistor.

So, some head scratching, and internet searching later, I added a single transistor audio amp to the detector circuit. Yeah! It switches!

Only trouble was, I had to use the 5V supply from the USB soundcard to power the amp, so I now no longer had the isolation between the radio and the computer! The photo above shows the interface in this state of development! I now had to add yet another little bit of veroboard, this time with a 4N25 optoisolator and a resistor on it.

That done, I got out to the car, connected everything up - and it wouldn't Tx! Arrgh! I'd forgotten to put the ground connection from the opto to the rig on! Back to the workshop, telling Julie that 'This is definately the last trip!' and then back to the car, and finally, a working audio driven PTT line!

So, the time now being around 19:00, and with a fully recharged laptop battery ready to give me its whole whopping 20mins capacity, I set up the Tx level to about 3-4W, sync'd the clock with an internet time server, and started WSPR, with one Rx period followed immediately by a Tx period. First spot, both heard and heard by, was HS0ZKM Thailand!

So what remains now is to rebuild the interface with everything, including the USB soundcard and the opto-isolator, all on the one compact board!

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