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Measurements >> Phase Noise and Jitter Measurements >> switching between voltage/phase domain PLL models
https://designers-guide.org/forum/YaBB.pl?num=1048731017

Message started by Eugene on Mar 26^th, 2003, 6:10pm

Title: switching between voltage/phase domain PLL models
Post by Eugene on Mar 26^th, 2003, 6:10pm

I am simulating PLL phase noise with a VerilogA voltage domain PLL model. (The VCO in a voltage domain PLL outputs an oscillatory voltage. A phase domain VCO model outputs a ramp. In this case, the phase domain VCO model outputs a DC voltage in steady state.) The VerilogA model does different things for different analysis. I use a phase domain model for DC analysis to precharge the loop filter. That way, I don't have to simulate long start-up transients. I switch between phase and voltage domain models with an "if" statements that depends on the type of analysis being peformed (DC or transient). For DC analysis, the VCO model generates an output voltage numerically equal to the VCO frequency in MHz, which is around 3700. For transient analysis, the model outputs a much smaller voltage, +-1 volt, representing the true oscillatory output voltage of the VCO. The 3700 volt output during the DC analysis seems to set some sort of dynamic numerical tolerance such that the subsequent transient analysis does not see much of the VCO phase noise. The resulting power spectral density is 20-30 dB too low. If I decrease reltol from 1e-3 to 1e-6, the simulation sees the noise and produces the correct power spectral density.

Does anyone know of a way to fix this problem from within the VerilogA VCO module so I don't have to remember to tighten reltol?

Title: Re: switching between voltage/phase domain PLL mod
Post by Jitter Man on Mar 28^th, 2003, 7:59am

Eugene,
You might try scaling the VCO output in DC by 1/1000 to make it more reasonably sized. Alternatively, you could try setting relref=pointlocal. Pointlocal tell the simulator to ignore the values at past times when performing relative tolerance checks.

[glb]Jitter Man[/glb]

Title: Re: switching between voltage/phase domain PLL mod
Post by Eugene on Mar 28^th, 2003, 9:10am

I was hoping for more of a VerilogA oriented solution so I could avoid changing the other models and also not have to remember to change the numerical settings whenever I instantiate this model. If I touch the other models I affect other circuits. As for numerical tolerances, as a general rule I resort to them only as a last result or as a diagnostic step. However, I suspect that a VerilogA solution does not exist. Your scaling suggestion is probably the best way to proceed. I will probably have to add a scaling parameter to my library of pll models to select the frequency units. Thanks for the help.