Michael Black wrote:
"Jim" ) writes:

There was an article in QST many years ago ( late 1960's ??) which was
one of the early direct conversion articles. This one used 88 mH filters,
and a three transistor audio amplifier. It was written by Doug Demaw, if
memory serves, and it is probably the simplest, but useful, direct
conversion design I have ever seen.


But one thing about direct conversion receivers is that it took years before
they became good.

The basic premise, beating a signal directly down to audio, is as old as
the regenerative receiver. ANd of course there were explicit mixer/oscillator
type receivers in the decades after that, that all seemed to end in "dyne".

IN 1961, QST ran an article, I can't remember if it mentioned "direct
conversion", that was direct conversion as we came to know it. A mixer
and an oscillator, and if I'm remembering they used two tubes in the mixer
for a balanced mixer.

Then there's the Hayward article, there was a co-author, in 1968, that
actually did make mention of "direct conversion". It used schottkey
diodes in a balanced mixer. And that's when they took off.

But in the articles afterwards used just about any possible mixer,
with varying results. Everyone was trying to get a good direct conversion
receiver, but I'm not sure they really understood them. So the focus
was on another variation of a well-balanced mixer.

It took till 1980, before there was a real change. That's when Roy
Lewallen came out with his "optimized QRP transceiver", which featured
a terminated mixer. I'm pretty certain that's the first time a DC
receiver had a terminated mixer. I can remember the 1974 QST article when
terminated mixers for VHF converters were introduced to the amateur world.

I really do say that 1980 article of Roy's was the dividing line. Before
that, no matter how good the mixer there always seemed to be problems. Then
we moved into an area where simplicity wasn't the first consideration for
direct conversion, but performance came first.

Michael VE2BVW


Granted, getting 88 mH coils is not as easy as it was back then, but all
in all, I think this fits the original request. The radio used NPN
transistors thoughout.



Jim
N6BIU






The direct conversion transceiver is what gave TenTec their start.
They had a few different models in kit, semi-kit, and assembled
formats. QST picked up the craze and there were a number of articles
on building these. Detectors were dual gate fets, CA2028's,
diode bridges, Mini-Circuit mixer modules, and later IC's with
built in vfo's.

Back in the 60's, there was an early direct conversion receiver that
used two 6SB7Y's in a balanced circuit with a B&W phase splitter run
bass-ackwards and an rf phase splitter in the vfo. This was actually
a single signal receiver using the idea of a phasing sideband modulator
in reverse. Today we'd call it an image-reject mixer.

What's old is new again.....