Hi,

Can anyone please tell me that how to setup MOS Varactor in Spectre (Schematic) while designing a LC VCO. Should I put a variable (Vcontrol) at gate and use it as one plate and short drain and source to use it as other plate of capacitor.  If yes, then what are the steps to sweep the control volatge to see the frequency change?

I am new to VCO design. Please help!

Thanks.

Hi,
First of all in designing the mos varactor, you should fix what type of device you are going to use and the region of operation of the device ( depending upon where the voltage at the control pin), as mos can act as a varactor in all the three region, viz. accumulation, weak inversion and strong inversion.
Try using a PMOS based varactor, where the device acts in inversion region and you can integrate it with the charge pump LPF combination in future. Further simulating and charactersing PMOS based varactor in inversion region can be done witht he BSIM3v3.x model files with certain other apporoximation.
In this varactor design, the gate acts a one plate and the source-drain short acts a the another one, donot forget to connect the substrate to VDD, to always keep the transistor in inversion region. Apply the varialble voltage to the gate and connect the other terminal of the mos cap to the oscillator arm.
Hope this helps.
regards,
V