Actually, the question you ask is a very fundamental one, one that is at the heart of all modeling and simulation efforts.

With a simulation one takes a collection of models (a collection of equations that describe the components) and assembles them into a system (a large system of equations) and solves them. The way I see it, there are only two things that can go wrong and result in you getting the wrong answer. Either one or more of the component models is wrong, or the way that you combined the model equations into a larger system and solved them is wrong. The act of assembling and solving the system of equations is the core of what simulators do, and they have been doing it in the same basic way for a long time. They may break down, but that only happens if you excite a bug or present the simulator with a pathological case. They are approximate, and in some cases their accuracy is not sufficient. However, in general they have proven themselves to be quite robust.

So I believe the question that you are really asking is how do you know that your model is correct. Except for trivial models, one usually never knows for sure whether their model is correct. The risk of error is reduced by careful model development and validation.

Even with this, there is still a good chance the simulations of the overall system are wrong. If the system puts the models into a region that the original model developer did not anticipate and is not adequately modeled, then the overall results will be erroneous. So it is important that the model developer understand the end application for the model, and that the models be validated at least in systems similar to one being simulated.

With the PLL jitter models we have several things going for us.

• The application area (PLL synthesizers) is fairly well understood.
• The models were designed with the end application in mind.
• The models were assembled into  at least one frequency synthesizer and gave results that match, at least conceptually, what one would expect.

However, you should be aware that I have never validated the models against silicon, so there is still a good chance the models are flawed in some way.

Sorry, I got rather long winded. Hope I answered your question.

-Ken