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Design >> RF Design >> a basic question about the pll
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Message started by calven on Apr 21^st, 2007, 12:43am

Title: a basic question about the pll
Post by calven on Apr 21^st, 2007, 12:43am

hi!
i am a freshman in pll.i am reading best's book.i wonder if my understanding is right.
the book builds a linear model assuming the pll is in the locked state and the reference frequency is equal to the frequency of the divider's output signal(PD is a multiplier).but when the model is actually used,the reference frequency is not the same with the frequency of the divider's output signal(such as the referency frequency increases to another) while the pll is still in the locked model.why the model is still valid?we consider that the reference frequency does not change ,but the reference phase changes like a ramp function?
give me some advice,please
thanks a lot

Title: Re: a basic question about the pll
Post by calven on Apr 21^st, 2007, 7:55am

i find i do not understand the concepts clearly!my questions are as follows:
1.the pll is in the unlocked state when either the frequencies or the phases are different?
2.the linear model is strictly valid only when both frequencies and phases are equal?
3.but we can consider the circuits is approximately linear and use the transfer function to get some approximate results if the change of the frequency is small(so the phase change is small)?
4.if the frequency change is small,the circuit change quickly from the nonlinear state to the approxiately linear state when we can use the transfer function?
5.we can use the transfer function to get the setting time ,but we have to use the nonlinear model to get the acqusition time?
thanks a lot

Title: Re: a basic question about the pll
Post by adesign on Apr 25^th, 2007, 9:57pm

1. The PLL can be defined as locked when frequency and phase are in close proximity of reference. It depends on the application.
2,3. Can you elaborate more on the model you have?

Title: Re: a basic question about the pll
Post by mg777 on Apr 26^th, 2007, 6:24am

I don't know if you've played with those 3D picture puzzles - the ones you stare at until suddenly you can "see" the image? Then you can move your head this way and that & you still see it. PLL locking is like that. The mechanics of the locking is complicated, but one you have locked it's a fairly tame beast that even we EE's can analyze :-)

It's a pity that EE's are not exposed to non-linear dynamics the way aerospace engineers are.

M.G.Rajan
www.eecalc.com

Title: Re: a basic question about the pll
Post by Jess Chen on Apr 26^th, 2007, 8:05am

Think of the linear PLL model as a linearization about an operating point, something like the linearization SPICE does about a DC operating point. A better analogy is the linearization of switch mode power supplies. You can think of the linear PLL model as a state space averaged model. The linear model is good for small signal analysis. As for acquisition transients, AFTER you account for frequency slewing, if you are using a phase frequency detector, the near-linear response of the PLL dominates the remaining transient.

-Jess

Title: Re: a basic question about the pll
Post by calven on Apr 27^th, 2007, 9:08am

thanks experts!
i think i have understood it to some extend.pll using pfd usually have a larger linear range than that using the multiplier?