Michael Black wrote:
[...]

When Webb described the "synchronous detector" in CQ about 1958, the
intent was to demodulate DSB with no carrier. (The carrier doesn't
carry content, it just means you don't need a BFO at the receiver or a
means to know where it should be set, but the carrier does use power,
while the two sidebands provide information about where the carrier
should be, and allow for some level of diversity reception by selecting
one of the two sidebands.). It doesn't lock to the carrier, since no
carrier was expected. It uses information from outputs of the product
detectors (yes
two of them) to show where to place the reinserted carrier. It works
with any DSB signal, whether it has a carrier or not. It won't
demodulate SSB since there is no information on where to place the BFO
in reference to
the sideband, you need to unlock the loop since the PLL will otherwise
try to lock to what it can't find and thus not tune properly. This type
of detector is very similar to the "sideband slicer" and other such
products that used the phasing method of sideband reception for SSB,
allowing one to select upper or lower sideband by the proper combination
of phased signals, the addition being the circuitry to lock the BFO
to the proper place.

I gather that's the common type of sync detector in most receivers
nowadays, but I don't know for sure. One can have that sort of
arrangement, that does lock to the sidebands, without fully decoding
the two sidebands separately, and there seems to be receivers that don't
provide for that selectible sideband.

Michael



The type of sync detector that needs no carrier whatever in order to
lock on a DSB signal is called a Costas loop -- after John P.
Costas, who invented it. (Costas, by the way, did make a few rare
appearances on high-fidelity 40 and 75 meter amateur AM well into
the 2000s. He died in 2008.)

Sadly, no commercial receiver I am aware of uses a Costas loop. They
all use some form of carrier lock.

The last time I tuned a receiver with a Costas loop in it was in
1975, at the home of W3DUQ, using his National HRO-60 with his
homebrew 7360-based stereo sync system. It was amazing, and gave you
the ability to copy signals so deep in interference that you simply
couldn't believe it.

Interestingly, the (now out of production) Racal 6790 series used a
form of exalted carrier sync detection in which the incoming AM
signal was limited in the same way an FM detector limits, stripping
off all amplitude-modulated components.

Then the amplitude-limited carrier was applied to the carrier input
of the product detector to detect the AM signal.

This allowed for good sync detection (except perhaps for _extremely
deep_ selective fades) with the added benefit that _the signal was
always locked_. There were no heterodynes while tuning or if an
off-frequency station broke in on the QSO. It tuned just like a
regular diode detector AM receiver, with AM signals always
demodulated properly even as you tuned through them. If anyone wants
to give me one of those Racals, I'll take it.



With all good wishes,



Kevin, WB4AIO.
--
http://kevinalfredstrom.com/