I guess this is further to the followup I just posted in another
thread!

Consider the quadrature mixers. If your LO is in phase with the
incoming carrier on one, as you say, there will be a high DC coming
out of that mixer. But out of the other mixer at that time, you'll
see zero DC. If you move the phase just a bit one way, the quadrature
detector output will go up; move it the other way and the quadrature
detector output will go down. So, if you appropriately filter that DC
signal, you in theory will be able to lock the phase of the LO. The
problem? You'll need to be darned sure your mixer is really balanced!
If you are receiving, let's say, a 10 microvolt carrier, then you
only get around +/-10uV of DC to work with, and if the mixer is
imbalanced enough with no signal at all into the RF input to give you
that much DC, you'll be in trouble trying to "see" the DC from the
small carrier.

But not all is lost. As noted in the other thread, you may be able to
combine the outputs of the two quadrature detectors and use that.
What you won't get is rejection of noise that's in quadrature with the
transmitted carrier, which is an advantage of synchronous detection
(properly phase locked).

I've thought that if you have two broadcast stations whose carriers
are phase locked, it should be possible to find an antenna location
for receiving that puts the two carriers 90 degrees out of phase, at
the receiver, and you could listen to the two stations independently.
I have a spectrum analyzer that can resolve (very) small fractions of
a Hz at RF, and it's interesting to put a few feet of wire on it as an
antenna and set it up on a standard MW broadcast frequency with a 10Hz
span. Even in the presence of a strong local carrier, you can see
typically several other carriers in there, at least during nighttime
propagation.

Cheers,
Tom

"Joel Kolstad" wrote in message ...
I had this crazy idea to attempt to build a direct conversion receiver for
regular old AM (i.e., two sidebands, carrier included). It eventually
occurred to me, however, that there's the very significant problem of
synchronizing the LO to the incoming RF carrier. In diagrams I've seen,
normally a PLL is used at the IF frequency (455kHz being common for AM
receivers, of course) to lock the two together. Is there anything
comparable one can do with direct conversion? It seems that if your LO is
sync'd with the incoming carrier, you'll have a very large DC component (the
AM carrier!)... if not, however, you still might get some DC that's been
aliased and while this will (should) always be less than the DC component
when you're locked, it gives no indication of which way your LO needs to
move to achieve lock.

So apparently what I'm really asking is... can one build a phase detector
that works at DC? Or does an AM direct conversion receiver necessarily
require an IF strip?

Thinking out loud,
---Joel Kolstad