PLL input filter vs. loop filter

Tom Becker · 2006-04-09T20:47:05+00:00

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.Tom

Apr 09, 2006 44 Replies

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Ken Smith 20 years ago

In article , Tom Becker wrote: [...]

I assume you are doing this to drop out part or all of the loop filter:

----/\\/\\/--+-----+--- ! ! --- --- --- --- ! ! Pin-------- GND

There can still be a bit of an issue with the capacitor trapping an error when you switch the bandwidth. Too bad the CA3080 is going away, there is a cute circuit to make the bandwidth smoothly sweep from one to the other using it. You may be able to do this however:

----/\\/\\/--+-----+--- ! ! ! --- ! --- ! ! ! GND ! ! Gain=1 +--!>--- ! ! ! \\ ! / ! \\ ! ! 1/3 O O CD4053 ^ ............. CPU pin ! --- --- ! GND

You start off with the amplifier charging the capacitor. If you just randomly switched to having the capacitor filtering, the ripple voltage would get trapped and cause a phase offset.

You can solve this by making the switch at just the point where the ripple crosses through the center line. You will still trap noise but not ripple.

If you really want to have some fun, you could try PWMing the switch slowly from 100% the amplifier to 100% filtering. I haven't really thought this through but I throw it out there.

-- kensmith@rahul.net forging knowledge

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Tom Becker 20 years ago

when you switch the bandwidth...

Yes. Your buffered VCO input voltage idea is very good, and PWMing a cap between it and the VCO input is intriguing; a PWM-variable cap might be a flexible design component.

I could, I suppose, avoid the issue by switching a parallel R (instead of switching caps of the loop filter RC) to change the loop bandwidth. Any surprises likely there?

Tom

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Ken Smith 20 years ago

You don't really avoid the issue that way. The capacitor will still trap the ripple voltage at the point of switching. You could use a JFET and slowly bias it off to ease the resistor out of the circuit.

-- kensmith@rahul.net forging knowledge

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Tom Becker 20 years ago

filtering...

Someone on another group says he's done this; with a sufficiently high PWM rate, he says it works well to effectively produce a continuously-variable capacitance. Very nice.

Tom

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