kooller said:Hi,
every one.
I'm now designing a pll with reference clock frequency only 6 times to the bandwidth of the pll loop, and in this case the s-domain model is no longer suitable, and z-domain model must be used to model the pll loop. .................
LvW said:kooller said:Hi,
every one.
I'm now designing a pll with reference clock frequency only 6 times to the bandwidth of the pll loop, and in this case the s-domain model is no longer suitable, and z-domain model must be used to model the pll loop. .................
.....".must be used" ? Where does this information comes from ?
kooller said:Because PLL is a discrete system, in the case the fref=6fu, the s-domain model will deviate from the real situation, and I think z-model will be more suitable.
kooller said:Because PLL is a discrete system, in the case the fref=6fu, the s-domain model will deviate from the real situation, and I think z-model will be more suitable.
LvW said:Hi BIF44 !
Is your contribution a response to my question or to KOOLLER´s question ?
I don´t know what to do with it.
biff44 said:I was pointing out that you can take care of transport lag in the S domain by using a phase delay approximation where the tranfer function of the divider N would be =(e^-Ts)/N, where T is the time delay of the divider chain. This will readily give you the effective phase margin degradation of the time delay.
It might only be 10-20 degrees or so of extra open loop phase shift, but it all adds up!
biff44 said:DSP engineers use the Z domain because it works very nicely for analyzing a discrete time sampled system, since by definition the ADC is sampling at some finite clock rate. Handling time delay is obvious. It is less obvious in the S domain how to handle time delays, but can be done.
But we need to hear from the original poster about exactly what he is thinking.
...................
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LvW said:kooller said:Because PLL is a discrete system, in the case the fref=6fu, the s-domain model will deviate from the real situation, and I think z-model will be more suitable.
What do you mean with "discrete system" ? Time discrete ? Amplitude discrete ?
Why do you expect deviations from the "real situation" and what is the reason you "think" that a model in the time discrete z-domain is more "suitable" ?
What is reason for your assumptions ?
LvW said:Hi KOOLLER,
thanks for clarification and for providing the interesting gardner paper.
Now it´s clear what you mean - but I must confess I´ve no experience in the mentioned time discrete interpretation of the loop. Of course, it is true that a time continuous model of a PLL incorporating switching elements can be only an approximation (as each linear PLL model - even with a pure time continuous PD - is only an approximation).
I would recommend to you as a first approach to start with the linear model and find all parameters using conventional rules. And after this, if you detect some stability problems, you can try to follow Gardners approach. That´s all I can say at the moment.
Good luck.
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