"Frank Dresser" ) writes:
"Steven Swift" wrote in message
...
rar+p and rats:

Okay, here's the answer directly from the Radiotron Designer's
Handbook, fourth edition, pages 1226 and 1227:

"Section 3: The Synchrodyne"
[...]
"and hence the synchrodyne is likely to be most popular for
high-quality local-station reception."


I think the fact that the synchrodyne never became at all popular as an AM
radio detector in the tube era means something. If I recall correctly, I
read that synchrodyne detectors would howl until they sync'd. Also, the
phasing would have to be perfect to get good demodulation from normal double
sideband AM.


The quote is about the synchrodyne detector, not synchronous.

While Radiotron mentions "sync'ing" I'd not treat that as a basic of
the synchrodyne. At its basic, it's what we'd now call a direct conversion
receiver, ie beat the incoming signal down to audio. The "high fidelity"
derives from the fact that selectivity comes at audio, and one can build
good audio filters. Because one is translated the RF signal to audio,
any front end selectivity is there to prevent mixer overload. And the
translated signal goes from DC to daylight (a slight exageration), so
putting the filter there is not just a "tone control" but acts the same way
as a good filter further up.

The immediate problem is that such a receiver can do nothing of the audio
image (which is the same thing as the image in a superheterodyne receiver).
This is not a problem with AM, since the audio image (ie the signal on the
other side of the carrier) is the other sideband.

And of course, the lack of anything to sync the local oscillator
to the incomining carrier means that off-tuned receivers will provide
a beat note, and worse, a caucophony of sound as the two sidebands translate
to different audio frequencies and beat against each other.

While obviously there were schemes along these lines, to get better
AM reception, I don't think the synchronous detector was described until
1958 or so. At least, that's when it first hit CQ magazine, and if it wasn't
by Costas himself, it was by a guy named Webb who worked for GE (who were
the commercial proponent of DSBsc).

Michael


There you have it. Are we done. This is a 3-tube design for
local stations. One RF amp, 2 12au7s. Use an IC or two in the
oscillator loop and its perfect, almost a Costas Loop.

Build it; they will come.

Steve.


I suppose it's something worth experimenting with, but diode detectors
aren't bad. Even in the solid state era, sync detectors aren't particularly
popular. The add on sync detectors are expensive. It still doesn't seem to
be easy.

Frank Dresser