"Joel Kolstad" wrote in message ...
With all this discussion of phasing fun... could someone answer the
following question for me?

Say I'm transmitting binaural audio, with I being L and Q being R. I
receive this signal and generate my own I' and Q' outputs. However, if the
RF carrier and my LO have a phase difference, the entire IQ (phasor) diagram
is rotated by that difference and, e.g., a 90 degree difference will result
in the left and right channels I receive being swapped.

How do IQ-binaural receivers recover a phase lock to present this?

For standard broadcast, don't they always put L+R on I and L-R on Q,
so standard receivers get L+R? All this is not my forte; I know only
enough to be dangerous with it. But I assume that in any transmission
standard, there is something transmitted that lets you recover the
right phase at the receiver. If you're sending symbols, presumably
there can be some symbol you transmit periodically to insure things
stay synched, a complex version of the old RS-232 start and stop bits
if you will. For analog signals, you can transmit some sort of pilot
tone, perhaps the carrier itself, which of course must be done for
compatible AM anyway.

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.

I went through the math and you end up with the magnitude of the original
signal. What's unclear to me is how to recover the phase offset between
your signal and the original -- although adding a DC component (or some
other unique frequency component) to either I or Q (or placed at some
strategic angle between them) would allow you to synchronize the phases.

Have any suggestions for a nice simple mixer (ala the NE602) that retains
both the I and Q signals at the output?

One easy way is with a "Tayloe mixer" -- you should be able to find
info on that on the web, but it's basically just a commutating switch
that switches the signal (through its source resistance) among four
different capacitors. The I and Q outputs are V(C1)-V(C3) and
V(C2)-v(C4) respectively. It must be driven with a "LO" at four times
the detected frequency: that is, the switch must rotate through all
four positions in one cycle of what would normally be considered the
LO frequency. If you use care in its construction, you should be able
to get very good balance. The size of the capacitors determines the
bandwidth. There are commercial quadrature active mixers, too, but
they typically cover a modest frequency range in the VHF region or
higher.

Cheers,
Tom

Tom Bruhns wrote:
"Joel Kolstad" wrote in message
...
For standard broadcast, don't they always put L+R on I and L-R on Q,
so standard receivers get L+R?

They may well -- I'm not sure which 'standard' we're talking about here
(there seem to be several out there...). :-) But note that with L+R on I
and L-R on Q, you only get L+R if you manage to synchronize with the phase
(the original problem)! If you use a quadrature detector and extract the
magnitude, you of course get sqrt(L^2+R^2) -- good, but not exactly L+R
either. The Motorola C-QUAM AM stereo system forces the magnitude of the
I-Q phasor to be L+R, which makes it compatible with both envelope detectors
and quadrature detectors. (I probably sound like a Motorola sales guy these
day... I just think it's clever and the end goal here is to build a direct
conversion receiver to decode it...)

But I assume that in any transmission
standard, there is something transmitted that lets you recover the
right phase at the receiver.

For AM, it would appear that the carrier itself is what people use for that
purpose.

If it very clear to me now why TV transmission need to send the colorburst
sequence these days -- otherwise they'd have no way to synchronize the
chroma decoder, which is taking in a DSB-SC modulated in both I and Q.

One easy way is with a "Tayloe mixer" -- you should be able to find
info on that on the web, but it's basically just a commutating switch
that switches the signal (through its source resistance) among four
different capacitors.

I was just thinking of doing something like this -- from a square wave, get
a 4 bit shift register (that always has one output active) and feed the
outputs to a 4066 such that I sample the 'I' channel during, say, clocks #1
and #4 and the 'Q' channel during clocks #1 and #2. Add some filtering
and -- presto chango! -- we've got I and Q.

Thanks for all the help -- this just might end up working after all.

---Joel Kolstad

Tom Bruhns wrote:
"Joel Kolstad" wrote in message
...
For standard broadcast, don't they always put L+R on I and L-R on Q,
so standard receivers get L+R?

They may well -- I'm not sure which 'standard' we're talking about here
(there seem to be several out there...). :-) But note that with L+R on I
and L-R on Q, you only get L+R if you manage to synchronize with the phase
(the original problem)! If you use a quadrature detector and extract the
magnitude, you of course get sqrt(L^2+R^2) -- good, but not exactly L+R
either. The Motorola C-QUAM AM stereo system forces the magnitude of the
I-Q phasor to be L+R, which makes it compatible with both envelope detectors
and quadrature detectors. (I probably sound like a Motorola sales guy these
day... I just think it's clever and the end goal here is to build a direct
conversion receiver to decode it...)

But I assume that in any transmission
standard, there is something transmitted that lets you recover the
right phase at the receiver.

For AM, it would appear that the carrier itself is what people use for that
purpose.

If it very clear to me now why TV transmission need to send the colorburst
sequence these days -- otherwise they'd have no way to synchronize the
chroma decoder, which is taking in a DSB-SC modulated in both I and Q.

One easy way is with a "Tayloe mixer" -- you should be able to find
info on that on the web, but it's basically just a commutating switch
that switches the signal (through its source resistance) among four
different capacitors.

I was just thinking of doing something like this -- from a square wave, get
a 4 bit shift register (that always has one output active) and feed the
outputs to a 4066 such that I sample the 'I' channel during, say, clocks #1
and #4 and the 'Q' channel during clocks #1 and #2. Add some filtering
and -- presto chango! -- we've got I and Q.

Thanks for all the help -- this just might end up working after all.

---Joel Kolstad