Of course, if you were demodulating DSB suppressed carrier and you
injected the carrier at the wrong phase, you indeed would get the two
sidebands going through constructive and destructive phases. If
you're 90 degrees out with your LO, it looks a lot like narrowband FM,
though very slightly different as I posted in the thread on SSB-FM.
If you do the quadrature detector thing with DSB-suppressed carrier,
then when one of the two is just the wrong phase (and you get no
output from that one), the other will be just the right phase, and
vice-versa. When it's in between, does it work out right to just sum
the two? I suppose so, though it's worth going through the math to
make sure. And of course, with quadrature mixers, you can combine the
outputs with audio phase shifting to select just one of the two
sidebands (or just CW signals on one side of the LO). In fact, the
mixer LO inputs don't have to be exactly in quadratu it's possible
to apply a calibration to account for a phase error (and also an
amplitude error, where the gain through one mixer path is slightly
different from the gain through the other). That's all practical to
do digitally...we do that sort of thing at 100 megasamples per second
with some custom chips.

Cheers,
Tom


"Joel Kolstad" wrote in message ...
Dan Tayloe wrote:
This is indeed what happens only if the VFO and an incoming single are
at almost the same frequency ("zero beat"). However, in practice, if
the signal is a cw signal, we listen to a signal that is 600 Hz or so
away from the VFO so that we hear the 600 Hz tone difference.

...or at least, say, 595-605Hz is the local oscillator tends to drift +/-5Hz
over time, eh? Good enough.

With SSB, presumably you have the same 'problem' -- the entire voice signal
is shifted in pitch by the difference between the LO and the real carrier.
In fact, with SSB and direct conversion, how do you even decide you have the
correct LO frequency? Just when people sound 'most natural?'

Thanks,
---Joel Kolstad