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Design >> RF Design >> Q factor of nmos varactor
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Message started by casual on May 24^th, 2010, 5:05am

Title: Q factor of nmos varactor
Post by casual on May 24^th, 2010, 5:05am

I try to simulate the Q factor of a nmos varactor by using S parameter.
Q=imag(Y11)/real(Y11)
one port setup.

I varied the width & length and check the Q.

I found that Q is much higher when length is large for a given area.
It is completely contradictory to the theory that nwell resistor is the main source of resistance, should keep length to min.

I suspect the foundry does not model the varactor properly.
Now the behavior looks like a nmos gate leakage instead.

Title: Re: Q factor of nmos varactor
Post by rfcooltools.com on May 25^th, 2010, 11:48am

What process node are you using?
Does the Q vs length consistent over length?
Is the Q vs length consistent over bias?
Describe what is connected to the 3or 4 nodes of the varactor?

J

Title: Re: Q factor of nmos varactor
Post by love_analog on May 31^st, 2010, 7:37am

can you just look at the real resistance of the varactor. What sort of model is the foundry providing you - just S parameter model?

Title: Re: Q factor of nmos varactor
Post by love_analog on May 31^st, 2010, 7:38am

BTW, typically foundry will only provide fixed templates of varactor so you cannot vary the length and expect to get correct results. Maybe only discrete lengths

Title: Re: Q factor of nmos varactor
Post by casual on Jun 2^nd, 2010, 9:54pm

As in previous post, Q becomes better if the length is larger.

The contradictory statement: In many journals, the length should be set to around the min length so that the Q is not degraded.

I simulated the Q with sp analysis. Foundry does not provide Q for nmosvar
Foundry does not provide discrete length.

Title: Re: Q factor of nmos varactor
Post by Mayank on Jul 7^th, 2010, 3:55am

Can you also try Q measurement without sp analysis ?

Simply using an AC analysis.
Q=imag(Z)/real(Z)
where Z = Vac/Iac.

--M

Title: Re: Q factor of nmos varactor
Post by casual on Jul 7^th, 2010, 4:50am

It looks correct if using AC analysis.
The Q is quite reasonable.
thank you very much!
just wonder why sp fail to give me the correct result..
I seldom use sp analysis since i am new to RF

[img][/img]

Title: Re: Q factor of nmos varactor
Post by casual on Jul 7^th, 2010, 4:52am

But it does not relate to what happened in the weird vco tuning range. :D

Title: Re: Q factor of nmos varactor
Post by vp1953 on Jul 7^th, 2010, 8:59am

Hi Casual,

Many foundries have an nmosvar and nmosvar_rf - the rf version is characterized for high frequencies whereas the non-rf version works only for low frequencies; and so if you run a rf simulation on the low frequency version, results may not be very meaningful.