Monday, May 25, 2009

dc-rx and/or super-het (for the qrss-trx?)

Did some research today. And again, I stumbled accross Tasić's superb weg-page full of information on all fronts of qrp and sdr:

The present idea is, to use one 74HC86 for both oscillators, the synthesizer-mixer and the phase-shifting. The resulting signals will be fed in a 74HC4066. Frequencie? Standard crystals, what else?!
4000kHz + 6144kHz = (tataaaa) 10144kHz.
First, very simple option: pull either one or crystals down a bit, as to reach, lets say, 10142kHz or so. Now the qrss-range will fall into the lower side-band. Not nice, at least not for a grabber, a start however.
Second, more sophisticated approach: have one crystal w/o pull, lets say the 4000kHz one. Now, the other has to cope with all the pull. Thus, why not just add a second crystal and have a super-VXO? I've never tried a super-VXO using digital ICs before. Anyone out there having experience in said field?

You may wonder why I do not consider a 10140kHz crystal in the first place. Very simple... the crystal-synthesizer approach opens the option of modifying the design into a super-het, without need for changing a lot. In fact, just the 4000kHz signal needs to be taken off the synthesizer-mixer and re-routed to a product-detector behind a side-band filter (e.g. ladder made from 4000kHz crystals).

The advantage of a super-het, to me at least, seems obvious. Depending on the super-VXO, substantial portions of the 30m-band are in reach, e.g. the Hell-band, whilst a dc-rx with a single crystal filter, binds to the frequency of the (qrss-) crystal used.

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