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PLL input filter vs. loop filter

to PWM...

This was super-easy to do, Ken (just three resistors and two caps on an existing comparator), and it instantly tripled, in my case, the ripple frequency. I also used an active notch and highpass ahead of the PWM.

Very nice - and, by directly tying the top of the triangle-generating RC cap to a processor pin, I can turn the PWM on and off by floating or grounding the pin in code; if the cap isn't permitted to charge, the comparator switches at Vcc/2 as it normally would. I can do the same with two loop filter caps, too, so I think I've got the wide capture and narrow tracking modes I sought, very easily, under program control.

It's interesting how a long learning path can bring one close to where one started, but with insight. I think I grok it. Thanks again, folks.

Tom

Reply to
Tom Becker
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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.

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

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

Reply to
Tom Becker

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.

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

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

Reply to
Tom Becker

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