Hi,
The AD9901 works very well as a stand alone phase detector.
It doesn't care a whit about feedback loops.
I use it as a phase detector, which drives an ADC.
My microcontroller can then do whatever it likes with that information. I've driven VCOs with DACs. I've fed this info to DDSes. Easy peasey.
Can you monitor the differential outputs to verify they are behaving?
I test my AD9901 by driving it with two signals: 80 MHz and 80.0000001 MHz. The you can plainly see the phase difference vs time on a scope.
Cheers,
David
