PLL input filter vs. loop filter

What's the actual signal frequency range? 252 Hz +- what?

If it's narrow, a bandpass filter is appropriate. Its bandwidth should be narrow to kill noise but wide enough to allow it to track changes in the signal frequency. It's possible that all you need is a good bpf, no pll at all.

A 4046 requires a logic-level input to any of its phase detectors, so I assume you have a comparator in the front-end somewhere. If so, that will make trouble at low s/n. A linear phase detector (a multiplier or a synchronous rectifier) is better at pulling a weak signal out of noise.

John

I'm expecting +/20% instantaneous error.

Maybe a tracking BPF, yes.

I'm using PC1 of the 74HC4046, the XOR comparator. I had no fortune with either of the edge detected phase comparators. Although I am using an opamp as a comparator at the moment, it has offered no advantage over the biased inputs of the modern IC, which permit 20mVPP AC-coupled inputs.

A tracking BPF. Hmmm. Thanks.

Tom

Even the XOR still needs a logic input, which tends to magnify noise when the SNR is low. John's suggestion of replacing the XOR with a multiplier is not to be ignored, but a tracking BPF may be the better thing to try first.

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Hello Tom,

The frequency is kind of low for this but it might be worth checking it out: The old technique of Q-multipliers. Basically these are resonant circuits where the connected stage is adjusted in feedback so that it just doesn't oscillate. The closeness to the point of oscillation determines the bandwidth, the closer the narrower.

This doesn't need to be LC, maybe a gyrator filter? Anyway, it'll take some serious studying and simulating to make sure it is stable and has the behavior you are after.

If by +/-20% you mean the deviation in rpm you'd have to start it pretty much wide open and then have it narrow down. It almost sounds like a job for a uC or low-end DSP.

A PLL could work as well. But I'd consider something better than the 4046.

Regards, Joerg

In article , John Larkin wrote: [....]

Check out the self biasing circuit on pin 14. You only need provide a coupling capacitor and the 4046 will work with a sine wave input.

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kensmith@rahul.net   forging knowledge

In article , Tim Wescott wrote: [....]

Also:

If you are making in multiplying PLL and have a lowpass before the comparitor, you can feed a small sawtooth, made from the VCO, into the comparitor to cause it to PWM. This allows you to multiply moderately well without a true multiplier.

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But it's still effectively a comparator, with its consequences, namely "capture effect" where big signals blow small signals away.

John

Once you quantify "pretty rapid", then you know the bandwidth of the loop filter when you have an input signal. Then "flywheeling" would be equivalent to switching in a low-bandwidth loop filter when the input signal is invalid. The problem, then, is detecting when it's invalid.

What about this:

signal --> phase detector --> high-bw lowpass just to convert phase detector pulses to voltage --> clamp to a voltage equivalent to 252 +/-20% at the VCO --> lowpass filter to "pretty rapid" rate --> vco --> back to phase detector

So now if you miss a pulse, the clamp after the first filter stage will clamp that to 252-20% and the whole PLL will start slewing toward that rate at the "pretty rapid" rate. If you continue to miss input signals, eventually "no input" will become 252-20% steady state.

If you expect to lose signal long enough that the "pretty rapid" rate will see an unacceptable change, you could take the output of the first filter stage, detect V < 252-20% with a comparator and use that to dramatically reduce the bandwidth of the second filter stage (or switch in a circuit that can quickly sample and hold the current VCO input voltage).

For a super slow speed like this, the idea of using a micro that someone else had is probably even better...

The problem with a filter in front of the PLL is that you can't "remove" missing pulses. A low-pass filter would work fine if you were seeing extra pulses...

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Ben Jackson

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Hello Ben,

True. 250Hz is easy to do. The MSP430F2013 contains a 16bit engine and a

16bit SAR converter. Seems almost ideal for this app. When there are too many unknowns in the signal characteristic a uC allows easy adaptation of algorithms, including a lot of stuff that simply cannot be done in the analog domain with any reasonable effort. That was hard to say for me as an analog guy but 'tis how it is.

If the F2013 aint't enough there is the F4xx family. These come with more RAM and hardware multipliers. Like the Camaro of the MSP430 series.

With a uC you probably could.

Regards, Joerg

Very interesting, if I understand. Can you find a link to an example?

Tom

Yes; my poor descriptor should have been "maximum brief" error, probably.

missing pulses.

No, but I believe the "missing pulse" - actually a missing zero-crossing, I think - is just an artifact of the summation of several signals. Envision 252Hz riding on an 18Hz narrow Gaussian pulse and you're close. I often see 13 pretty clean cycles of 252Hz, then a flat, then 13..., instead of 252/18=14.

Can't I just notch the 18Hz? Can it be done in the loop filter?

Tom

noise.

How about using a switched-capacitor bandpass filter? Design one filter section to give a 90 deg phase shift at filter cf (that stage would actually be a peaked lowpass, technically). Hang a phase detector across that stage's input and output, lowpass filter, and servo a v/f as the clock of the filter. That could be a tracking bpf with just a few ic's.

John

The way to do it: NARROW PLL restores the carrier, then multiply noisy signal with restored carrier. It's called correlation ;-)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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I love to cook with wine.      Sometimes I even put it in the food.

Did it. At one point I had a /8 in the loop; I clocked an eight-cap Tayloe filter with the VCO. Spectacular selectivity and fun to implement; it's like a _visual_ filter on a scope.

I had a lot of trouble with the loop, though; essentially nothing got through the Tayloe if it was unlocked - so it was hard to lock and hard to keep locked. It just wasn't very stable trying to follow a mechanical clock source; too sharp, probably.

The tracking BPF started me thinking about it again, though. I've learned much since then so maybe I'll visit this again.

Tom

Is that one of those Resistor/rotary switch/buncha caps things? Those are so cool. But I was thinking about a commercial IC switch-cap bandpass, an MF10 sort of thing, or one of the more modern versions.

Maybe most of the selectivity could be downstream of where the phase detection is done. That almost defies logic, a narrowband tracking filter with fast, wideband tracking ability. That wouldn't be real bad as long as the noise was fairly incoherent.

Next step, FFT?

John

I guess that's not out of the question. At Joerg's suggestion I took a look at the MSP430F2013. Unbelievable for $1.50. I'm hesitant to pick up another platform - I'm an old 8008 guy who's settled on AVRs for now

- but fast 16-bit mixed-signal processing is very inviting. [It's got a PGA on the ADC input, too. I dream of a processor that offers an on-the-fly programmable analog signal processing section; this is getting close.]

Suppose a sufficiently fast and resolute FFT was available, though. What subsequent function counts cycles in a narrow range of FFT bins? How do you get from an FFT to a regenerated clock?

Tom

finally a good answer by someone who knows. Take a venerable NE567 or XR215A, put a bit of amplification and maybe 200Hz highpass in front of it, so the signal is in the 20 to 100mV rms range. Set the BW to 1% and after

200cycles the output goes low and signalles your PLL locked. You can tap into the squarewave output to get a stable reference without missing pulses.

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ciao Ban
Apricale, Italy

In article , John Larkin wrote: [....]

Yes I agree. The input does act as a comparitor.

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kensmith@rahul.net   forging knowledge

No and it I told you where I saw it used ....

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