A strange question about VCO designed for PLL

[gaom9]

1.3g vco

Hi,
I am working on a Δ-Σ Fractional-N Frequency Synthesizers for 1.2G-1.4G application. Before this, I have finished a Frequency Synthesizers for 2.4G-2.7G application and it works well. But when I design this new Frequency Synthesizers based on the finished one, I met a strange question about the VCO design which did not meet before.
In the Frequency Synthesizers for 1.2G-1.4G, the VCO is separated into 8 bands by a 3-bits capacitance array. The PSS results of this VCO is shown as follow which contains its load capacitance effect.

000 1.16G-1.246G
001 1.182G-1.273G
010 1.205G-1.302G
011 1.23G-1.333G
100 1.256G-1.365G
101 1.284G 1.401G
110 1.314G-1.439G
111 1.346G-1.481G

and the phase noise is -129dBc/Hz@1MHz offset. I think this VCO works well and I added it to the PLL (this PLL structure is based on the one I have finished and only change the filter and VCO)and set a test frequency to 1.3GHz. First, I chose the 100 VCO band but I found the PLL can not be locked, and the control voltage of it dropped to near 0.2V, then I tried the other bands and find it could lock at 010 band(the control voltage is 1.1V, the VDD of it is 1.8V). After tring other test frequencies, I got a conclusion that the VCO worked on a much higher frequency region than the PSS result in the PLL system. I made a tran simulation to the VCO separately, and found it works as the PSS result, not the same as the PLL system. And then I add a ideal VCO written by Verilog-A(1.256G-1.365G) to it, it could lock at 1.3G.
What a strange question it is! Does anyone meet it before? Why?

Thank you!
Best regards!

 

[gaom9]

how vco works

Hi,
I have solved this question.
There are two kinds of AMOS cap in tsmc018rf model, one is for small var cap application, and the other is for large var cap application.
In Frequency Synthesizers for 2.4G-2.7G, I use the small one, and it works well. But when in the 1.2G-1.4G region, the var cap needed is much larger, and this small var cap can not work well in a system but well in separately simulation. Some I change it to the large kind, and it works well in a system.
The model does not show when to use the small kind and when to use the large kind.

Hope it helpful.

Best regards!