Sunday, 9 June 2013

Phone !

After the frustrations of getting an analog oscillator under the closed-loop control of an Arduino, I had promised myself a rest. I decided a change would be just as good - so I took what is (for me) a very radical step. I thought about trying phone operation, rather than the habitual CW. Who says you can't teach an old dog new tricks?

The first challenge was finding a mic - such things have no place in the m0xpd shack! I ended up stealing one from the little FM bug transmitter I made (but NEVER operated - honestly Mr OFCOM) from Harry, SM0VPO's design back in the mists of time.

I decided that my restful change would see me revert to the comfort and convenience of DDS - so I planned a DSB transceiver using most of the bits already on the bench. I needed a pre-amplifier and modulator for my voice, so cooked up the following scheme, taking inspiration from the Wee Willy (amongst other sources)...


I found that too much Tx power was being wasted on low frequency content, so I added the 2nd order high-pass filter stage on the input. The output drives the ugly Sudden Tx, reported last week. For the receive side, I could have used the previous Rx stage, but decided to knock up a new SA612/LM386 combination, with oscillator input (to pin 6 of the 612) directly from the DDS.

The remainder of the expanded AF system from last week was still useful - so I made a common interface and just plugged 'n played! Rx/Tx switching is handled by a push-button DPDT switch, which operates the Tx "key" line and the Rx mute, as appropriate.

Here's the whole shooting match on the bench...

As you see, I didn't use the Arduino-based VFO (as its components were still hooked up to the Arduino-controlled Colpitts), using instead the pa0klt (Si570 based-) system from SDR-Kits...

I listened to the goings-on on 40m this morning with some trepidation; band conditions were reported as good by lots of ops with 400W to play with. How would my 1 Watt sneak past the big guns?

I needn't have worried - I answered the CQ from Chorley and District ARS' special event station mx0isn, operating the "Bunkers on the Air" event from Brinscall, in memory of the D-Day landings. Operator Dillon (m0ykb) made some generous remarks about my signal and gave me five and seven. Great!

 m0xpd operating phone - whatever next?
 ...-.- de "mike zero x-ray papa delta"

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