PLL Terminology Question

If I ran across those terms in a report I'd remember that somewhere years ago I'd seen them, but I'd certainly have to Google for what they actually meant.

IMO this terminology is used only in Gardner's book; there is no universal meaning. It is about P, PI, or PII control loop. Remnants of old times, when they used to mix the details of implementation with the type of the transfer function.

Since nobody is going to read it anyway, why would that matter?

Vladimir Vassilevsky DSP and Mixed Signal Consultant

I think it has to do with the type of phase detection used.

You might look in a data book for a PLL chip.

tm

It certainly has universal meaning in control systems terms: it's the number of integrating stages that are cascaded in the loop, either from the compensator or the plant.

I had to sweat through it in my undergraduate days, then help undergrads sweat through it as a TA getting my Master's degree.

Pbpbpbpbpbtht. It will become a cult classic among my customer's customers, eagerly read and handed down from elder, competent and stately engineers to youngsters who are wet behind the ears and trying to make their systems match the performance of the existing ones in the field.

ooooh! zinger!

how do you know, Vlad? i am certainly interested in reading it.

personally, Tim, if the difference in "type" is what is in the PID controller in the loop, i would just stick with "P", "I", and/or "D".

like, for phase-linear FIR filters there are Type I, Type II, Type III, and Type IV. i know they have to do with all combinations of even-length vs. odd-length and even-symmetry vs. odd-symmetry. but i can't remember which Type goes with which combination, and i find the "Type" labeling useless because it is not descriptive. and i am not sure that all authors agree to the same convention, and that's when confusion really gets thick.

Type I is the old vanilla version, type II with the charge pump and pulsing. Usually. But where have you read about a type III?

Maybe this helps:

I only encountered this recently when using a very old JPL paper that was pertinent to a problem we were dealing with. I had to look up the meaning, and they were using it like you do, to indicate the number of integrators in the loop.

Most comm people that are familiar with PLLs would likely be more comfortable with describing a loop by its order, i.e., first order, second order, etc., which isn't exactly the same thing but seems to be the more widely understood terminology in my experience.

Eric Jacobsen Anchor Hill Communications

Take a look at the Fairchild datasheet for the MM74HC4046. It refers to Phase Comparators I, II, and III.

ve

thats something different..

Tim, I would put the term in and also clarify it....a Type one loop with one implicit integrator (the VCO) and a second order active loop filter ...or whatever the case may be... providing 0 Hz frequency error and TBD degress of phase error etc..

Mark

Mark

Dayum. Just to make sure I'm on the right page, is a 1st-order loop one with a single integrator in the loop filter, or one with no integrator in the loop filter (and the VCO supplying the integrator)?

That's what I did. If the customer wants revisions I may look up the meaning of "order" in a PLL and put that in there.

It'll be run by some PLL experts; I was specifically brought in to look at their problem with a fresh eye, which means that I may not get all the terms exactly right.

Do not know but my wild uneducated guess is that "P" stands for regular feedback as in a standard op-amp circuit, "PI" stands for first derivative (eg: "P dot") and "PII" stands for second derivative (eg: "P double dot").

'I' stands for an Integral term, not a derivative one.

I think that PLL designs should be classified by the number of significant poles and zeroes of their transfer functions. This 'type' business only introduces an extra layer of obscurity.

Jeroen Belleman

Hi Tim,

We used to use those terms in the 80s in our antenna control systems at GTE Government Systems. If I remember correctly, the "number" refers to the number of integrators in the loop.

I don't think they are used much today.

That is correct. From Gardner's 2nd Edition, page 11: "According to servo terminology, the _type_ of a loop is a number equal to the number of perfect integrators within the loop." ...Jim Thompson

The question then becomes whether you count the inherent VCO phase integration as one of those integrators.

I'll concur with what Doug and John posted, and I'll also concur with Doug that I tend to refer to Gardner's book as the "Gospel of Gardner" when it comes to PLLs.

So a first order loop has no integrator in the filter, i.e., merely a proportional loop, and a PI loop is then a second-order loop. In comm 2nd-order loops are the most common. If I understand the "Type" definitions correctly many (if not most) 2nd-order loops are implemented as Type-2 systems, with the loop integrator and the NCO as the two integrators.

Eric Jacobsen Anchor Hill Communications