Friday, November 1, 2019
Direct Conversion (videos)
Here are a couple of videos from 2017 (never posted before). I built a little 40 meter Direct Conversion receiver for my nephew John Henry.
Whenever we work on circuitry like this, we should be be grateful for Wes Hayward W7ZOI who, in a 1968 QST article, reminded us of this important but until-then forgotten technique.
More information on this project appears in these links:
https://soldersmoke.blogspot.com/2017/10/a-direct-conversion-iphone.html
https://soldersmoke.blogspot.com/2017/11/iphone-direct-conversion-receiver-with.html
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Direct conversion wasn't so much forgotten in 1968. He made it solid state, and maybe gave it a new name. But in 1961 there was a QST article using tubes that gets referenced, maybe even in Hayward's article. Transistors make it easier.
ReplyDeleteAnd I'd argue that a regen set to oscillate isn't that far off in ultmiate results.
Direct conversion took over from regens in terms of simple receivers, and seemed to replace many simple superhet articles for some years. Endless discussion of how to improve DC receivers, mostly the mixer, never really making significant change. Bypassing the diodes in an AC supply to get rid of hum came later.
Simple superhets came back with ladder filters, simple than lattice filters and cheaper than commercial SSB filters.
DC ceivers became better when someone decided to complicate them. Gary Breed had an article in QST I think about a phasing receiver, but he properly terminated the mixer(s). That was the eighties or maybe early nineties. Terminating diode mixers had existed before, QST had an article in 1974 about VHF mixers, but I never saw it before in DC receivers. Unless Roy Llewelyn's DC transceiver with termination came first.
And of course Campbell made more complicated DC receivers in the nineties, the real comolexity coming in the audio chain, the large audio inductors being a deterent for some I think.
Of course, direct conversion has been used a !ot in SDRs in recent times.
Phasing DC gives of course single signal, and existed in the sixties, I was recently reminded of the one in the 1970 ARRL Sideband book. And phasing gives a foundation for DSP.