Wow, excellent description, thanks.

Mark.
Auckland
New Zealand.

"Michael Black" wrote in message
...

"atomicthumbs" ) writes:
Just wondering, as no one explains what it does or how it works.

An AM signal has a carrier and two sidebands. The sidebands carry the
actual content (and they are identical, meaning only one is necessary)
while the carrier just goes along for the ride. But at the receiver end,
the carrier is needed to beat with the sidebands to translate the
modulation from RF back down to audio. (Imagine a 10MHz signal, with a
1KHz
tone modulating it. There's a sideband at 10.001MHz and one at 9.999MHz,
plus the carrier at 10MHz. Mix the 10.001MHz signal with the 10MHz
carrier at the receiver and you get the 1KHz signal out of the speaker
again.)

Due to propagation, there are times when the carrier can fade more than
the
sidebands, so it's too weak to properly demodulate the signal. You end up
with a distorted signal.

Synchronous detection uses a locally generated signal in the receiver to
beat
against the sidebands to translate them back to audio. Since this local
"carrier" is nice and strong, it can never be too weak in reference to the
incoming sidebands. The signal may be weak, but it won't be distorted.

The problem, though, is that this local "carrier" has to be right at
the spot halfway between the sidebands. An obvious reason is that if
it's mistuned, you will hear a beat note between the signal's carrier
and the local "carrier". A constant and obnoxious tone in your ear.

But more important, if that local "carrier" is not right in the middle,
the sidebands will not translate to the same audio frequency. WIth
that previous example, both the upper (10.001KHz ) and the lower
(9.999KHz)
sidebands both translate to 1KHz because the carrier is right in the
middle.
But if this locally generated "carrier" is not right in the middle,
one sideband would translate to 1.1KHz and 990Hz, for example. You've
not only got them translated to the wrong frequency, but you suddenly have
two frequencies instead of one at audio. It will sound horrible.

This is where the "synch" comes in. There is circuitry in the receiver
to lock this local "carrier" to the incoming signal. The incoming signal
includes enough information so that exact middle point between the
sidebands
can be determined, and this synchronization ensures that this local
"carrier" is set to that frequency.

The sidebands translate to the correct audio frequency, and everything
is well.

If there is no lock of this local "carrier" then the modulating signals
do not translate to the correct audio frequencies.

And as a side issue, once you have this circuitry in place, it doesn't
take all that much more to allow selectable sideband. Since both
sidebands carry identical information, only one is needed to recover
the modulation. With the right circuitry, the receiver can select which
of these redundant sidebands are used, so if one has some interference,
the other one may not and selecting that one will mean the listener
will not hear the interference.

Michael