Phase frequency detectors (starting with the legendary MC4044) being made out of flip flops, had metastability and/or race conditions. Motorola showed a block diagram made of gates, as if it were combinatorial logic, but because of the feedback, it is actually a state machine, as described in the MC4044 data sheet. It had a dead zone around zero phase that came to light when Fairchild introduced the competing 11C44 PFD using Eric Breeze's patent to fix the dead zone. The 11C44 data sheet showed their dead zone, vs Brand M. Even that improved chip still had a "funny" zone, it just never went to zero gain.
Fast forward to today, we are now seeing PFD's made with samplers. They too have a bunch of issues with phase noise floors. None of them come close to a mixer. In the 5071A, I used a mixer as a phase detector that had some flip flops only used for acquisition, so they were non players in terms of phase noise. I still think I would do that even if I had to do over 25 years later. Rick Karlquist N6RK _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
