Folks, I've been working with a 4046-based shaft rotation tracking PLL circuit. The source is a weak, noisy and pretty sloppy inductively-generated shaft speed signal. I want to regenerate and track it's principal signal of 252Hz, but that is punctuated with occasional large and sometimes periodic low-frequency resonances and gear noise.
The physical device is subjected to random accelerations from any direction so while tracking it must remain responsive to pretty rapid input phase and frequency changes - yet be able to flywheel though the missing cycles caused by the resonances.
That seemed a challenge, but I have found reasonable success with simple passive filters for some input conditioning, and for the loop filter. While experimenting with active filters, though, I've found that any low-pass on the input reduces responsiveness, thus tracking. As I write this, I think I see that's due to any integrator eating time.
That suggests that whatever low-pass filtering I need should be done in the PLL loop, not ahead of it. That leaves me with the low-frequency resonance effect inside the loop, though, and that can ding the VCO pretty well - producing a cycle-slip or breaking lock.
I'm tempted to try a twin-T notch or a high-pass to remove the low-F stuff ahead of the loop, but I know I'm still playing with time. In general - intending to optimize for tracking - should I try to clean up the signal ahead of the PLL or be more selective in the loop filter?
Or am I off track? TIA.