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Design >> RF Design >> nmos varactor degrade phase noise
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Message started by casual on Apr 21^st, 2010, 4:06am

Title: nmos varactor degrade phase noise
Post by casual on Apr 21^st, 2010, 4:06am

I try to design a typical cmos LC VCO. I found out the phase noise degrades a lot after adding nmos varactor as compared to ideal cap.

The phase noise could be improve if add a bypass capacitor (for ac ground) however it will add additional load to the LC tank results in narrow tuning range. Is it common to do so in real practice??

I try to improve it by adding more fingers of nmos varactors. It helps a bit only.

Is there any way to improve the varactor?

Title: Re: nmos varactor degrade phase noise
Post by rfcooltools.com on Apr 22^nd, 2010, 11:23am

The common way is to switch in MIM caps to make bands within a VCO where the varactor will only be used cover the a smaller freq range temperature and process drift to connect the bands. to make a switch pick the appropriate size NMOS device to minimize off capacitance and minimize on resistance. Typically VCO's on chip use 4 bits or more of binary weighted MIM capacitors and switches to improve the frequency coverage while off loading varactor tuning.

of coarse this means that tuning requires a state machine or look up table.

Title: Re: nmos varactor degrade phase noise
Post by love_analog on Apr 22^nd, 2010, 4:19pm

Remember a non-linear varactor will convert the 1/f noise of tail current source to Phase modulation. If you had a perfectly linear cap then you will not see this increase in noise.

Title: Re: nmos varactor degrade phase noise
Post by casual on May 4^th, 2010, 7:15pm

any other ways to improve phase noise besides using more varactor fingers?

Title: Re: nmos varactor degrade phase noise
Post by love_analog on May 5^th, 2010, 12:35pm

put a ac coupling cap between varactor and output. Will make overall cap more linear.

Title: Re: nmos varactor degrade phase noise
Post by casual on May 5^th, 2010, 5:58pm

anyway, i will put programmable cap for coarse tuning. It also works as ac coupling cap :)

thank you

Title: Re: nmos varactor degrade phase noise
Post by love_analog on May 14^th, 2010, 4:12pm

typically the coarse tuning cap will not be in series with the varactor. more like parallel.

would be curious to see your schematic.