Designer’s Guide Community

neoflash Community Fellow Offline Mixed-Signal Designer Posts: 397	Switched tuning LC oscillators Mar 19^th, 2008, 5:24am Normally, people use MIM or finger metal capacitor for high linearity. Anyone ever see paper use varactor or moscap as switched tuning capacitor array? Pls suggest reference, thanks.
Back to top	IP Logged

ACWWong

Community Fellow

Offline

Posts: 539
Oxford, UK

Re: Switched tuning LC oscillators
Reply #1 - Mar 19^th, 2008, 6:05am

lots of people use moscap for switching.... better Q can be acheived (depending on circumstances of course) using MOS rather than a switch in series with a MIM. Nearly all papers i've seen of late for CMOS switched LC VCO use MOS switched caps.

If you google it i'm sure you'll find loads of papers, but i do like this earlier example from RFIC2000:
"A LOW PHASE NOISE MONOLITHIC VCO IN SiGe BiCMOS"
Jean-Marc Mourant, Jim Imbornone, and Ted Tewksbury*

http://ieeexplore.ieee.org/iel5/6896/18556/00854418.pdf?isnumber=18556&prod=CNF&...

Back to top

IP Logged

neoflash Community Fellow Offline Mixed-Signal Designer Posts: 397	Re: Switched tuning LC oscillators Reply #2 - Mar 19^th, 2008, 7:11am use direct mos cap, controlled by digitally biasing of the backside might suffer big supply noise pushing. If there is no voltage regulator for the digital control signal, the conversion from power noise to phase noise should be high.
Back to top	IP Logged

neoflash Community Fellow Offline Mixed-Signal Designer Posts: 397	Re: Switched tuning LC oscillators Reply #3 - Mar 19^th, 2008, 7:14am any people ever try put varactor in series with switch, for switched tuning?
Back to top	IP Logged

jlee

Community Member

Offline

Posts: 40

Re: Switched tuning LC oscillators
Reply #4 - Mar 23^rd, 2008, 7:05pm

Another reference: "Design of wide-band CMOS VCO for multiband wireless LAN application" by N. H. W. Fong.

For varactor/moscap based switch cap, biasing voltage noise (LDO, current biasing) is a big concern. Try PXF simulation, and you will find the flicker noise upconversion gain is normally increased by 10dB.

Actually, well designed mimcap based switch cap has little effect on phase noise and its cmax/cmin (>4) is really higher than varactor based switch cap (<3). Q is not always the criterion for phase noise performance. Cheesy

Regards

Jason

Back to top

IP Logged

neoflash Community Fellow Offline Mixed-Signal Designer Posts: 397	Re: Switched tuning LC oscillators Reply #5 - Mar 25^th, 2008, 7:29am The binary biased varactor always make me surprised. Without good regulator, the supply noise can raise the noise floor to absurd level. However, I often see authors of Jssc using this design without regulator highlighted. Then, I just keeps being surprised.
Back to top	IP Logged