Monday, 26 May 2008

I don't want to fiddle with the Topband SSB transmitter again for a while; it's great for local work, and I have a bigger fish to fry...

SSB / cw / PSK31 from 1.8 - 7.2MHz
Quite a big project, but I'm really only adding sub-systems to an existing radio. I have a Racal Minical, a US-built 'green' USB packset, which I intend to extend to provide LSB, cw and PSK31/63 capability. I've already had it working on LSB, by shifting the BFO down 3kHz. PSK31 works, and cw experiments using mcw (modulated carrier wave) will be the first practical trials.

Doing morse code on a single-sideband
It's a common practice on VHF and UHF to modulate an FM radio with a sidetone to provide morse code transmission. I see no objection to doing the same with a single-sideband radio, provided the sidetone oscillator is as pure a sinewave as practicable, and the radio is not overdriven. I intend to 'key' the Minical by a phase-shift oscillator at 700Hz, and examine the output with DigiPan PSK31 software via a second radio.

Adding a VFO
The little Racal is crystal-controlled, offering six channels. I already provide an 80m VFO for it, using a variable crystal oscillator, and with the Racal's LO as a buffer. It works well, and all I need do now is extend this with a full mixer / VFO system to give coverage of all amateur bands from 1.8 to 7.2 MHz. Using a selection of crystals from my junk-box, and mixing them with a low-frequency VFO, the Racal will be transformed into a useful radio.

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