"OH1GTF" wrote in message
oups.com...
One thing I want to add. I have built tayloe detector polyphase
network receiver before and I know,
that we need I and Q to cancel out the image. But how the heck AM and
FM is done?

I and Q give you phase information; sqrt(I^2+Q^2) gets you the amplitude of
the signal. Now... AM is not phase-modulated, right? So **assuming you can
synchronize your carrier to the incoming carrier**, "I" will be the original
signal and Q will be zero, so sqrt(I^2+Q^2)=I -- poof, only I needed! If you
chose Q, you'd just get zero (ideally) out of the receiver... but since you're
presumably in control of the local oscillator, you can just advance or retard
it 90 degrees and now Q is in-phase with the signal and I is zero. Hence AM
can be made to work with either I or Q... although it's not really
recommended, since -- if you have an IQ demodulator anyway -- you can build
SSB receivers as well, which is useful.

The tricky part is that "synchronizing to the incoming carrier" bit: If the
receiver and transmitter have the exact same frequency but phase offsets of X
degrees, the result is that I receives the original signal mulitplied by
cos(X) and Q receives the original signal multiplied by sin(X). (This is just
the general case of what I described above where things were 90 degrees out of
phase.)

Notice that phase and amplituide, while connected by phase=arctan(Q/I) and
amplitude=sqrt(I^2+Q^2), are two separate, uniquely "identificable" "things"
that you can transmit. This is taken advantage of in, e.g., "compatble" AM
stereo broadcast standards: In Motorola C-QUAM, for instance, I is set to
1+L+R whereas Q is set to L-R. If you run through the math, the amplitude of
this is not 1+L+R, but it is "close enough" if L-R is relatively small, and
hence compatibility with traditional (envelope) receivers is maintained, while
allowing a synchronous receiver to dig out the full stereo information.

---Joel