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Design >> RF Design >> tuning range LC QVCO
https://designers-guide.org/forum/YaBB.pl?num=1277818019

Message started by casual on Jun 29^th, 2010, 6:26am

Title: tuning range LC QVCO
Post by casual on Jun 29^th, 2010, 6:26am

I tried to increase the current of LC QVCO tank to increase the output amplitude that drives buffers.
I found the tuning range looks odd. Near to the output common-mode (around 1.3-1.4V) it starts to have a dip & hump. The lower range is saturated. If I reduce back to the current, it looks ok and have wider tuning range.

Title: Re: tuning range LC QVCO
Post by raja.cedt on Jul 3^rd, 2010, 8:17pm

hi,
hi,
why tuning curve is non-monotonic? have you checked wehter spice is converged or not? just run with a sigle control voltage for both current. you should get bit lower frequency. If posible check how varactor is changing with voltage because at output common mode varactor voltage is zero, so check there....

Title: Re: tuning range LC QVCO
Post by wave on Jul 4^th, 2010, 1:29am

looks like you are not sweeping the control voltage in fine enough increments. That's why it appears non-monotonic. Add more points and it will smooth out --- and then probably tell you it's still non-monotonic due to a model or change in region of operation.

Title: Re: tuning range LC QVCO
Post by casual on Jul 5^th, 2010, 2:20am

it is not the matter of resolution step (checked)
The cap vs vctrl curve is smooth.

I doubt it is because of the injected power from quadrature vco.
It might due to the Q factor of the varactor. The Q is the lowest at common-mode.

Title: Re: tuning range LC QVCO
Post by Mayank on Jul 6^th, 2010, 6:51am

hmm.....interesting.

Does the Q of the varactor show a Dip at CM voltage ???
If possible, pls. post the graph of Qvaractor vs. Input Current.

if it were the case, then i doubt it would show the effect you originally quoted.
Quote:

If I reduce back to the current, it looks ok and have wider tuning range.

It should have shown same behaviour approaching from both sides.

Have you checked Raja's point whether simulator converged properly at those spiky points ?

Title: Re: tuning range LC QVCO
Post by raja.cedt on Jul 6^th, 2010, 8:04am

hi,
as mayank said please check Q vs control voltage, because all our analysis depends on based assumption that varactor Q is constant. And also if possible publish inductor Q

Thanks,
Rajasekhar.

Title: Re: tuning range LC QVCO
Post by vp1953 on Jul 6^th, 2010, 10:15am

How does the voltage amplitude vary with frequency (does it for instance peak around where you see the kink in the resonant frequency curve) - can you post a picture of that?

I also seem to recall that with cadence, using the low accuracy presets does funny stuff like this - try running a simulation with the conservative setting.

Title: Re: tuning range LC QVCO
Post by rfcooltools.com on Jul 6^th, 2010, 10:10pm

are you using parasitic RLC extraction? check the varactor dc node to see if a resonance grows there in the range you suspect.

http://rfcooltools.com

Title: Re: tuning range LC QVCO
Post by casual on Jul 7^th, 2010, 12:23am

it was prelayout sim

it is not due to accuracy setting.

Highly suspect on the Q of the nmosvar.
However I could not simulate Q nmosvar because it gave me bogus results!

see thread
http://www.designers-guide.org/Forum/YaBB.pl?num=1274702712/4#4

The Q normally is the lowest at common-mode and the weird behavior always happens near to the common-mode.

I have reduced the length of nmosvar, everything looks fine.
In the previous setting, the length is about 3x of min to increase tuning range.

Title: Re: tuning range LC QVCO
Post by raja.cedt on Jul 8^th, 2010, 11:41pm

casual..whats the reason finally? how did you solved that.

Title: Re: tuning range LC QVCO
Post by casual on Jul 10^th, 2010, 11:53pm

found that the Q of the cap array is pretty small after knowing how to extract Q using ac method rather than sp analysis.

improve the Q of the mim cap array . Just wonder why the Q is pretty small in 5GHz. How could it work in >10GHz application?

Title: Re: tuning range LC QVCO
Post by aLittleKnowledge on Jul 13^th, 2010, 4:28am

Variable capacitor Q can be quite dependent on bias. I would look at avoiding bias conditions where the Q becomes small. That might mean using a larger area device.

Title: Re: tuning range LC QVCO
Post by love_analog on Jul 29^th, 2010, 6:23am

I don't think Q can cause f-v curve non-monotonicity. Q can cause amplitude change whcih could cause a change in f, but thats usually not a very big effect.