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Design >> RF Design >> Ring oscillator phase noise problem
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Message started by aaron_do on Jul 9^th, 2013, 10:25pm

Title: Ring oscillator phase noise problem
Post by aaron_do on Jul 9^th, 2013, 10:25pm

Hi all,

I designed a ring oscillator using 40nm CMOS technology, and I noticed that the noise plateaus around 100kHz and below. Above 100 kHz it has a 1/f² shape. Has anybody encountered this phenomenon before?

I tried taking the basic transconductor cell and simulating it using noise analysis, and the flicker noise is still there.

thanks,
Aaron

Title: Re: Ring oscillator phase noise problem
Post by Ken Kundert on Jul 10^th, 2013, 2:06am

See http://www.designers-guide.org/Theory/cyclo-preso.pdf. Particularly the discussion of linewidth on page 30.

-Ken

Title: Re: Ring oscillator phase noise problem
Post by aaron_do on Jul 10^th, 2013, 2:14am

Thanks for the reply....

I just found the problem. I had "lorentzian" selected in the pnoise options. It resulted in the phase noise flattening out at lower frequencies.

Aaron

Title: Re: Ring oscillator phase noise problem
Post by aaron_do on Jul 10^th, 2013, 2:45am

Hi Ken,

seems that you might mean I should look at page 31 right? It says that the phase noise as expressed as a voltage spectrum should flatten out near the carrier. Does this have anything to do with selecting "lorentzian" in the pnoise options?

I wonder what is the relevance of this? Since for my application I am concerned with jitter, I guess I should be more concerned with phase noise as expressed as phase rather than voltage.

thanks,
Aaron

Title: Re: Ring oscillator phase noise problem
Post by Ken Kundert on Jul 10^th, 2013, 4:52am

Small signal methods do not naturally predict the flattening at low frequencies. However, the latest versions of the various RF simulators fit a Lorentzian shape to the data, which then shows the flattening.

I recommend that you read the entire paper and try to understand cyclostationary noise as a whole. It is hard for me to give you guidance on this subject with just a few sentences. You really need to understand the subtleties to assure you are doing everything correctly.

-Ken

Title: Re: Ring oscillator phase noise problem
Post by aaron_do on Jul 10^th, 2013, 9:15pm

Ok. Thanks for the help.

Aaron