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linear frequency tuning with varactor in LC VCO?

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saber890

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varactor linearize

Hi all

Posted an similar question earlier, didnt get no reply, so still hoping theres help to be found.

What possibilities are there to increase the linearity in the tuning voltage to frequency transition, when an MOS variable gate-oxide capacitor is used??

I know that adding capacitances will "flatten" the capacitance vs tuning voltage C(V) curve of the varactor, but it will also decrease the tuning range, and that i want to avoid.
Also, i know that larger VCO output amplitude swings impact the effective varactor C(V) curve, averaging it more.

I have no means do specify the doping level of the varactor poly-gate, this is a process that influence the varactor linearity, but this option is not available.

AND, i would like to know how to measure the linearity for the varactor C(V) curve. I know the derivative should be a constant. Should a straight line be used as a reference? In that case, how do i implement it in the waveform window in cadence?

What other methods are there to increase the linearity?? are there other ways that the varactors can be configured, are there other ways to combine varactors that would help??
 

linearity fm varactor

hi
i think the varator in rf process all are nonlinear, and i don't know why you need a linear varactor, maybe you need to reduce the AM-to-FM modulation to improve the phase noise in 1/f^3?
welcome to reply and discussing the question about vco.
 

linear frequency tuning

hi

The reason I want the varactor to be linear is that the frequency of the LC-tank VCO is given by 2*pi*f= 1 / sqrt(LC(V). If the varactor C(V) relation is linear, the LC VCO tunig curve will follow the inverse square law correctly. But obviously there are other effects, such as the AM-FM conversion that u mentioned that degrade the output frequency. What means are there to improve on this phase noise contributing factor?

regards Saber890
 

lc vco linear tuning

hi
you mentioned that "If the varactor C(V) relation is linear, the LC VCO tunig curve will follow the inverse square law correctly". why? i think the most important is the linearity of the tuning curve. and if you want to improve the pn at 1/f^3 ,pls using fully differential structure.
 

linear lc vco

You are bumping up agains physics here. Generally, the solution is to switch in banks of coarse tune caps so that you can have a smaller tuning range from the varactor, and a broad tuning range by swapping in and out capacitances in parallel


Dave
www.keystoneradio.com
 

lc varactor tuned

thank u folks for the response

first this reply is to ZYYANG, by squaring the inverse squareroot frequency expression f=1/2*pi*sqrt(LC), you will get a shift in frequency, in addition you will end up with a straight line. By using a linear varactor, the frequency squaring will give a better result.

Thanks for the info RFDave, I have heard a littlebit about the switching capacitance approace. I know the AM-FM conversion is reduced when using these capacitance switches, so that the Q is not affected and the tuning range is increased.
You mentioned physics, do you know what possibilities there are when it comes to changing the MOS varactor capacitance curves, as can be done by using the right amount of gate-doping(a shift in the flatband voltage will occure) in the MOS transistors. (i dont have this possibility, but someone surely does)
Is there any specific targeting of the varactor capacitance characteristics by any of the semiconductor producers? I know the AMOS(accumulation varactor) has a specific doping profile, with n+ source/drain in a n-well, but it is usually not available.

Saber890
 

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