Jess Chen
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Sven,
I agree that the VCO resets but I would point out two observations:
1. Since the argument of the VCO can be thought of as a trig function, the "reset" events are transparent to the VCO output. It is only at the phase detector that such events become important.
2. In a phase domain model of the VCO, where the output is a sawtooth waveform, the output is not a true steady state quantity. I usually pull the VCO integrator, and the reference integrator, into the PFD model. This has couple of advantages. First, small signal analysis is performed about a true DC quantity (i.e. frequency). And since the DC analysis is valid, the models of the various components can be written to map out lock ranges with just the DC analysis. Second, by placing the integrator (a resettable integrator to be more precise) inside the PFD, the PFD accurately models the hysteresis observed in the relationship between average output and input phase error. Furthermore, the reset value of the integrator can be adjusted to accurately simulate the frequency slewing properties of the PFD. Where was I going with this? In a phase domain model, I think the resettable integrator belongs in the PFD, not the VCO. Furthermore, for small signal analysis, the integrator indeed introduces 90 degrees of phase lag. For large signal analysis, the behavior is highly nonlinear, probably to the point that the usual nonlinear methods, such as describing functions, bear little fruit.
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