Quote:By modeling pll in phase domain, we can analyze its stability.
But this method is error-prone and also difficult to model the parasitic effect.
I remember Andrew mentioned pstb somewhere, is it the one we are looking for?
I don't have access to pstb yet but if it's what I think it is (phase/gain margin after a pss anaylysis) then yes, that's what I was referring to.
Quote:Transient analysis is helpful but through transient analysis we can not know
how much margin there is and maybe for step response it's stable but for some
other stimulus it's unstable.
Usuallly, if the system is marginally stable the step response rings a long time. You would have to have some sort of unobervable unstable state for the step response not to show it and if you did have one, the loop gain could miss it as well. As long as the step has a sharp enough rise, it should excite any observable unstable state. The only time I've seen the stimulus make a difference is when the instability depended on the size of the disturbance. In that case, the AC analysis will be less reliable than the transient analysis.
Quote:By the way, can we insert a AC voltage source between LPF and VCO to break the
loop to do the stability analysis? because at this node, the signal is voltage
rather than phase.
I agree: the node between the LPF and VCO is a convenient place to insert the AC source for a phase domain model or a pstb approach with a voltage domain model.
-Jess