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Design >> RF Design >> Simulating a VCO
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Message started by shortckt on Feb 20^th, 2006, 12:07pm

Title: Simulating a VCO
Post by shortckt on Feb 20^th, 2006, 12:07pm

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.

Title: Re: Simulating a VCO
Post by shekar380 on Mar 17^th, 2006, 10:42pm

hello

instead of using a single MOS CAP u can connect two mos caps in series so that the gates of two transsitors act as two ends of the capacitor and the S/D terminal of the two transistors shorted together can act as thetuning control.

ground one end of the capacitor and connect a sinusoidal source with required DC level(say 1.0V) to the other end.Let the frequency ofthe source be that value for which u r designing ur VCO.

do a parametric sweep of the control voltage and measure rms(i)/ rms(dv/dt).This gives a graph showing the capacitance variation.The limits of the CAP variation gives the tuninf frequency limits accordingly.

regards
shekar

Title: Re: Simulating a VCO
Post by judean1 on Mar 21^st, 2006, 3:00am

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