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Message started by Shahriar on Apr 19^th, 2012, 10:46am

Title: oscillator simulation
Post by Shahriar on Apr 19^th, 2012, 10:46am

Hi,

I'm trying to simulate a ring oscillator. I can do transient to see its frequency spectrum and did pss-pnoise to get the phase noise.

Could anyone, please, tell me if there is a way calculate or simulate the bandwidth of the oscillator from the pss-pnoise analysis? or predict phase noise from just the transient analysis?

Another question is - my free-run frequency is 400MHz. If I do a transient simulation for 10us, i can get a frequency spectrum with resolution of 100kHz using the dft function. Is there a way to get a finer resolution, say 5kHz? 10us transient simulation with post-layout takes a very long time, for running it for 200us to get 5kHz resolution might be impossible.

Thanks in advance.

Title: Re: oscillator simulation
Post by raja.cedt on Apr 19^th, 2012, 1:26pm

hello,
while running transients, simulator don't add noise, so no chance of getting PN plot.

Title: Re: oscillator simulation
Post by Shahriar on Apr 19^th, 2012, 1:52pm

Yes, I guessed so. But thought I should ask it anyway to see if someone has found something.

But what about getting something like a spectrum or measure of oscillator bandwidth from phase noise/jitter data? Could anyone, please, help me with how I can do that?

Title: Re: oscillator simulation
Post by loose-electron on Apr 21^st, 2012, 1:15pm

A ring oscillator really does not have a BW or Q so to speak.

A min max range of operable frequencies, yes, but thats about it.

Ali Hajimiri did a paper on this a while back, search IEEE JSSC

Title: Re: oscillator simulation
Post by genie_q on May 14^th, 2012, 8:25am

Hello,

Could you please tell me why Q or BW for ring VCO is not important? Razavi has a paper predict the Q of ring VCO.
I'm confusing this. I designed a differential 3-stage ring VCO, running at 100MHz. I got -110dB phase noise at 1M offset. How to link this number to the Q of ring VCO? Equation of Q=Wo/deltaW, is not usefull anymore.

loose-electron wrote on Apr 21^st, 2012, 1:15pm:

A ring oscillator really does not have a BW or Q so to speak.

A min max range of operable frequencies, yes, but thats about it.

Ali Hajimiri did a paper on this a while back, search IEEE JSSC

Title: Re: oscillator simulation
Post by sheldon on May 14^th, 2012, 4:36pm

Shahriar,

Been a while, as I remember

If you are using Spectre/ADE, add the freq meter from the PLL
Macro Model library to your schematic then run transient noise
analysis. You will also need to enable the Noise Aware PLL flow.
After the simulation is done, there will be an option to plot phase
noise in the Direct Plot. Having said that you will need to setup
transient noise analysis to give good results and the set the
accuracy on the frequency meter to give you good results.

Best Regards,

Sheldon

Title: Re: oscillator simulation
Post by loose-electron on May 20^th, 2012, 10:26am

genie_q wrote on May 14^th, 2012, 8:25am:

A ring oscillator really does not have a BW or Q so to speak.

A min max range of operable frequencies, yes, but thats about it.

Ali Hajimiri did a paper on this a while back, search IEEE JSSC

I wish this one was as simple as an LC tank oscillator and its Q.

Can you approximate it using linear means? Yes, but its really not that meaningful after you have done a few of these in the real world.

If you sweep an LC tank, it exhibits a Q and a frequency response profile that will fir into an S plane model.

A ring oscillator tends to be more flat in the pass band an the max freq and min freq do not exhibit symmetric profiles.

Trying to apply a linear system model to something that is inherently not linear does not work that well.