Problem with 8bit VCO for control frequency

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vco problem...

Dear all :
I'm doing 8bit VCO(complementary) for control frequency.
when I sim tank and cap array (only NMOS varactor)
is 580f and 850f for control bit min and max and step is 1f.
According these value we can get freq=4.4G~5.4G ,step=5Mhz.


when I run transient, the fmax=5.39G but fmix=4.8G ,
I don't know what happen .
someone have this experience or have some suggestion ?

Thanks ..
 

vco problem...

Did you include the parasitic capacitance of the switch?
 

vco problem...

I didn't include it. cadence should include it..

but when I series IND in control bit output , output freq is 4.5G~5.4G
why ??
 

Re: vco problem...

Very simple answer:

The big (real) transient and small signal oscillation frequency differ. That is because many caps are nonlinear.

The effective cap could be measured if you apply a DC bias voltage a sine voltage source and measure the cophase component of the current amplitude of the cap. That matches very close to the simulated transient frequency. If the nonlinearity is to big amplitude noise is converted into phase noise!
 

    S

    super

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vco problem...

I agree rfsystem . good
 

vco problem...

"The big (real) transient and small signal oscillation frequency differ" . you mean have two frequency in VCO ??

or this a AM-PM conversion?
 

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