Tom Bruhns wrote:
I guess this is further to the followup I just posted in another
thread!

Yep!

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.

This makes sense to me if you stick, e.g., (1+s(t)) -- where s(t) has not DC
component -- on your I modulator and, say, t(t) -- where t(t) has no DC
component -- on your Q modulator. DC filtering the I output, then, should
directly provide cos(phi) -- where phi is the phase offset phase the RF and
LO carriers -- and DC filtering the Q output provides sin(phi) (which is
probably the more useful signal for the sake of feedback). (For regular AM
transmissions, t(t)=0... I'm just thinking ahead to AM stereo here.)

The
problem? You'll need to be darned sure your mixer is really balanced!

A pair of NE602s sitting side by side doesn't really cut it? :-) I was
actually thinking that the solution here is that you let the AGC run off of
the quadrature detector output (sqrt(s(t)^2+t(t)^2), assuming you removed DC
from both channels) and only when the signal strength is above a certain
level do you allow the phase locking circuitry to kick in and attempt to
null out the phase differences.

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.

Good point!

Side note: In looking up quadrature detectors, I was amused to find that
someone has a patent on using one for standard commercial AM band reception.
Amazing...

For more on LO vs. RF carrier phasing differences, I'm going to follow up to
the other thread.

---Joel