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for stability test use small signal model which is an integrator (consists of an ideal VCCS or CCCS plus one capacitor at output of the controlled source.)
if u need more accurate model for transent simulation consiering nonlinear behaveour of pll, make a vco with relaxation oscillator (an ideal schmitt triger plus a capacitor and a voltage controlled source charging the capacitor) and controll its frequency with a VCCS.
BTW, if u need a system model, why u are interested in HSPICE? u can use matlab or @-D-S for example!
ai... I must build a VCO's model in Hspice, beause there is no the model of varactor in my Tech-lib. And I will verify the 'lock' of my PLL in short time, so I just use this VCO wrote!
But I seldom use hspice before ( most spectreRF), could u give me a example about this VCO with relaxation oscillator.
i don't recommend you to design your VCO using varactor (only if you design a model for this varactor with MOS transistor...and in this case you will have a model for varactor )
in attachment you have a trigger schmitt example
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