Phase Detection with Modulated Return Signals?

N8RDF

I have (2) applications which require generating an audio frequency reference signal (<=500hz, one is digital, the other sinusoidal), then measuring the phase shift on the returned signal after external processing (both return sinusiodal). I have successfully used Beau Schwabe's PhaseDetector object with an unmodulated reference. However, in actual use, the returning signals will include the processed reference mixed with voice, music or "weak RF" audio noise. Equivalent hardware-based solutions often include active op-amp and switched capacitor filters. Is there an easier way with the Propeller? I'm not familiar enough with filter theory to apply some of the software filters described in the forums (or to know if they're even appropriate). Any assistance would appreciated.

Mark_T

Phase linear (FIR) low/bandpass digital filter?

N8RDF

Can you elaborate? As I said, I'm not familiar with filter theory. Thanks.

Phil Pilgrim (PhiPi)

Use a quadrature detector, i.e. two mixers whose 500 Hz local oscillators are 90 degrees out of phase. The "I" mixer's LO should be in phase with the reference signal. The "Q" mixer's LO would then lag that by 90 degrees. In the Prop, the counters can be used as mixers when set to logic mode (either XOR or equality). The phase shift can be computed from the outputs of the two mixers (read from the counters' PHSx registers) as

atan2(phsQ - dt/2, phsI - dt/2), where

dt is the sampling interval in system clocks.

For more details about quadrature mixers, see this thread:

http://forums.parallax.com/showthread.php/105674

-Phil

N8RDF

Thanks Phil, I'll give it a try.
-Gary