Sunday, 17 November 2013

Rising to the challenge

Bill Meara, n2cqr, has been hugely supportive of my recent fun-and-games with digital systems - but he's also been goading me...

"I have encouraged Paul to take a break from the microcontrollers and build something discrete and analog... like a BITX!"

Well - I've taken Bill's advice...

Above you see the (IRF510-based) PA, the LPF and BPF filters (both arranged as plug-ins - see below) and some other lovely analog, discrete stuff. 

However, try as I might to please Bill, I couldn't resist sprinkling a little "digital" into the project...

Whilst I've taken direct inspiration from Farhan's words and music for most of the transceiver, I've put together a new digital "RF generator" system, which uses two DDS modules to generate BFO and VFO signals...

This approach - I argue - mixes the best of both worlds... 

You get all the fun of "conventional" radio homebrewing and you supplement it with the stability, controllability and simplicity of Direct Digital Synthesis for generating the RF signals. Exactly the "Occam's Microcontroller" thesis - but taken to the next level with SSB. 

Here's the main board in close-up...

The band-pass filter is the ordinary G-QRP circuit (SPRAT 154, p 25), which I had knocking around on the bench as a plug-in module I'd developed for the "Occam's" rigs. This was a 40m filter and it was this - more than anything else - which dictated that this will be a BITX 40. I preserved the "plug-in" format in the output low-pass filter, so now I just need to knock up different filters and adjust the "RF Generator" to change band (try doing that with analog RF generation).

I'm calling it an "RF Generator" for want of a better name - it is far more than a VFO, because it generates the BFO signal and automatically handles the "interlocking" between VFO, BFO and "dial frequency". This makes aligning the whole system a different kettle of fish than would be the case with old-fashioned (sorry!) analog oscillators.

The whole shebang reached operational status today - so I had some nice QSOs with Steve, m5stc, Brian, gw1uju and Ellis, who was operating the gm100rsgb station.

Brian put his finger on the button - "your audio sounds a bit stifled". He's right - my crystal filter (which was thrown together with zero thought) is too narrow. I'll re-work it and try to get my audio quality up where it ought to be.

I'm going to be publishing the Double DDS "RF Generator" code when I work it up into something worthy for your attention. Now, at least, I have an SSB platform with which to work - all thanks to Farhan and (of course) to Bill.


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