I know, it is just a little Direct Conversion receiver. Getting it going is no great technical achievement. But this little receiver gave me such trouble as a teenager, it has been in the back of my mind for a long time. Finally, yesterday afternoon it started picking up signals.
I felt a bit bad about insinuating (a few days ago -- see below) that QST may have made an error in the 1976 schematic. They didn't. So it was kind of spooky when I heard that first call-sign coming through the speaker: It was W1AW! It was as if they were saying: "See, the schematic was correct!"
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Isn't it a bit dated?
ReplyDeleteWhen "direct conversion" receivers came along in 1968 (the concept existed before, just not the name), it was to build simple receivers. They took over from regens (which of course for the purpose of CW and SSB, were "direct conversion"), and kind of bumped simple superheterodyne receivers out of the magazines.
And they were easy to build, so long as the meaning of the dots were standard, but good performance was elusive. Endless articles about better mixers or more front end selectivity, and still the same basic results The Heathkit HW-7 comes along, and endless mods to that, but still no perfection.
Slowly the move was back to simple superhets, especially with some of the early seventies ICs intended for radio, and then ladder filters came along (actually they came early at least by 1974 from the UK and/or France, but while they got mention in North America early-ish, it took some years before the KVG filters were pushed aside and ladder filters got the spotlight).
And then wham, in the mid-eighties someone caught on. The problem with direct conversion receivers wasn't the mixer (well not once it was a balanced mixer) or lack of front-end selectivity, it was the matter of properly terminating the mixer. The problems that had been there all along were gone. And direct conversion receivers started their climb to being complicated receivers.
I guess it was that receiver by Gary Breed in QST circa 1986 with diode balanced mixers and termination that changed things. A new concept, but not really, I remember an article in QST in 1974 where a DMB diode mixer for VHF was properly terminated, and yet the concept went no further until a decade later.
Actually, I think there is a tiny bit about mixer termination in "Solid State Design for the Radio Amateur" but it never went so far as to say "this is what we need".
Or perhaps that tiny transceiver by Roy Llewellyn in QST was the first, I cant' remember. It certainly used a diode mixer with termination for the receiver.
And that set the stage for Rick Campbell's various receiers, all counting on termination of the mixer.
The ideas can often be there, but not applied because technology doesn't allow it yet, or just not looking that far beyond this month's construction article.
Michael