On 5 Mar 2007 03:53:20 -0800, "MadEngineer"
wrote:


I just want to correct a misunderstanding: I or Q alone are not
sufficient to demodulate AM or FM except in the special case of AM
mixed so the carrier amplitude and phase are exactly at DC 0 volts.
You need both I and Q together just as you do to reject unwanted
sidebands in other modes.

If you have frequency lock but not phase lock, the phase detector in a
phase locked loop will output a DC voltage proportional to the phase
difference. When using the Q multiplier as a phase detector, just
adjust the oscillator phase to bring the averaged Q output to zero and
you will get the demodulated signal from the I multiplier.

Costas loops have been used for AM detection, so you might find some
ideas from that direction too.

Paul OH3LWR