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Measurements >> Phase Noise and Jitter Measurements >> wrong fundamental freq from pss
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Message started by wakizashi on Nov 20^th, 2007, 3:03am

Title: wrong fundamental freq from pss
Post by wakizashi on Nov 20^th, 2007, 3:03am

Hello,

I have a simple differential ring oscillator, 4 stages, fosc=20MHz. During initial tran analysis in pss the frequency is evaluated wrongly. It is 10MHz although from the pss-tran analysis waveform it is 20MHz. Pnoise also finds fundamental as 10MHz.
Therefore pnoise is evaluated vs 10MHz not 20MHz. My questions:
- why pss evaluates the frequency wrongly even I see correct fosc in tran (which is run before pss)?
- can I use 2nd harmonics in pnoise analysis to evaluate noise against the correct oscillation frequency?

thank you for your replies.

Title: Re: wrong fundamental freq from pss
Post by ACWWong on Nov 20^th, 2007, 3:30am

Have you set an appropriate tstab for the pss analyses ? This additional time for stablization can be arrived at by running a transient analysis.
Is 20MHz the lowest frequency in the testbench ? If you have other sources or circuits (such as divide by 2) at 10MHz then the pss solution will converge on these. In the case you have a divide-by-2 circuit then you can do pnoise relative to the 2nd harmonic on the 20MHz oscillation node.
If you still have problems you can plot the pss voltage waveforms versus time to see the pss solution arrived at to help you debug. If the pss solution is in your opinion incorrect, then any pnoise analysis based upon it will not be valid.

hope this helps.

Title: Re: wrong fundamental freq from pss
Post by wakizashi on Nov 20^th, 2007, 5:17am

hi,

yes, tstab is enough because I can plot the waveform with started oscillation in pss-tran. the frequency of this waveform is 20MHz. there are no dividers in the circuit. only these 4 delay stages of ring oscillator.
pss-fd also shows 1 harmonic on 20MHz. but in log file I see that fundamental frequency found is half of this. Im not sure why this happens.

anyhow, thank you for the reply.

Title: Re: wrong fundamental freq from pss
Post by buckaroo on Dec 1^st, 2007, 1:00am

yeah, when i simulate xtal oscillator, i encountered the problem too. i'm confused, the pss voltage spectrum, (my osc is 32k)32k is much stronger than 16k, however, pss told me fundmental freq is 16K...
sometimes, i setup reltol to a small value, it works well, but sometimes, this mothod is useless

wakizashi wrote on Nov 20^th, 2007, 5:17am:

Title: Re: wrong fundamental freq from pss
Post by pancho_hideboo on Dec 1^st, 2007, 1:49am

Quote:

It is 10MHz although from the pss-tran analysis waveform it is 20MHz.

Are there 10MHz component in DFT result ?

Quote:

- why pss evaluates the frequency wrongly even I see correct fosc in tran (which is run before pss)?

Maybe 10MHz component is generated in your oscillator. So fundamental frequency must be 10MHz.
Show me spectrums of both transient analysis and PSS analysis.

Quote:

- can I use 2nd harmonics in pnoise analysis to evaluate noise against the correct oscillation frequency?

Yes you can.
Set "Sweeptype" as "relative" and "Relative Harmonics" as "2" in Pnoise setting.