VCO can not start up when it is put into the PLL loop?

[eejli]

If I simulate the VCO alone it starts oscillation very easily using power down control.

But if I put this VCO into the PLL loop it can not start up. What could be the reason?I tried to adjust the convergence parameter but no help. I have a power down control.

Thanks.

 

[flatulent]

Is the control voltage proper? It may be that the variable capacitance diode is forward biased.

 

[eejli]

Thanks, Flatulent

The tank cap array is MOS varactor and it has always capacitance even negatively biased. I will try to give the tuning voltage a initial value.