Vcontrol of the VCO. How?

[faizalism]

Hi to all,

I have problem simulating VCO gain. Actually I simply change Vcont and see the output frequency. So my result will be like , 1.8V - 4GHz, 1.6V - 3.8GHz, 1.4V - 3.6V .... . I used ring oscillator same as Savoj paper: 10Gb/s Half-Rate Phase Detector. But the problem is, his VCO curve (gain) is something like this ....

How he can get Vcont (Vfine) positive and negative numbers?

Thanks in advanced.

 

[Enhancer]

I see that no problem having Vfine negative...

it is not a condition that the control volt is positive....an example of that, consider an FM modulator, so you will use that VCO and the control volt is a biopolar signal (sinusoidal or triangular)...