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Phase noise simulation in ring oscillator

Ayrin

I have two questions regarding the phase noise simulation results (PSS + Pnoise) for ring oscillators:

(1) How much the precision of the initial oscillation frequency is important in estimating phase noise ?

(2) The results that I get for the phase noise seems a bit high. I feel there is a shift up in all the frequencies. In very low frequencies (close to carrier) the phase noise is largely positive. Comparing with the analysis presented in [Abidi, JSSC 2006] the phase noise sounds very high. Does someone has any idea that what could be wrong?

 

The ring oscillator designed to oscillate at 2.4 GHz using CML gates with a tail bias current of about 200 uA. 

 

Thanks in advance for your help.

 

Armin

 

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  • Hi Ayrin,

    You have probably resolved your issues by now - but if not - there are a few relatively straightforward items I noticed that will significantly impact the accuracy of your pnoise results  (especially since you noted you are examine the phase noise close to the carrier).

    1. I am not sure what the time length (TSTOP) of your simulation is. However, it does not appear you allowed for a finite TSTAB in your PSS analysis. The lack of a finite value for TSTAB suggests the PSS analysis will consider the transient start-up behavior in its estimate of the steady-state waveform. This will impact the PSS solution - and be especially important for frequencies close to the carrier as the time constants of the transient are likely long relative to the period of oscillation. I would recommend adding a value of TSTAB that encompasses the total time required for the start-up transients to become negligible. You might estimate this by running a transient simulation and plotting the frequency in percent or ppm from the final value at the end of the simulation and choosing a time when the frequency is within 0.1%, for example, of its steady-state value.

    2.  I noticed you did not include a value of integration parameter "maxstep" in your PSS analysis. With "errpreset" set to "conservative", the default value for the minimum integration time will be used - which is computed from the TSTOP of the simulation. If this value does not represent, say 25 to 100 points per oscillation period, the accuracy of the resulting zero-crossing estimates may be significant. As you might imagine, this will also impact your phase noise estimate.

    3. To verify if any of these are potential issues, you might plot the resulting PSS waveforms in the differential and single-ended mode analyses you performed. Compare these waveforms to the respective steady-state transient analysis waveforms from a conventional transient simulation (with a low value of maxstep - 25-100 timepoints/period). If there appears to be a signficant discrepancy, items [1] and [2] might be worth considering.

    Good luck! Phase noise analyses require some care and feeding...

    Shawn
Reply
  • Hi Ayrin,

    You have probably resolved your issues by now - but if not - there are a few relatively straightforward items I noticed that will significantly impact the accuracy of your pnoise results  (especially since you noted you are examine the phase noise close to the carrier).

    1. I am not sure what the time length (TSTOP) of your simulation is. However, it does not appear you allowed for a finite TSTAB in your PSS analysis. The lack of a finite value for TSTAB suggests the PSS analysis will consider the transient start-up behavior in its estimate of the steady-state waveform. This will impact the PSS solution - and be especially important for frequencies close to the carrier as the time constants of the transient are likely long relative to the period of oscillation. I would recommend adding a value of TSTAB that encompasses the total time required for the start-up transients to become negligible. You might estimate this by running a transient simulation and plotting the frequency in percent or ppm from the final value at the end of the simulation and choosing a time when the frequency is within 0.1%, for example, of its steady-state value.

    2.  I noticed you did not include a value of integration parameter "maxstep" in your PSS analysis. With "errpreset" set to "conservative", the default value for the minimum integration time will be used - which is computed from the TSTOP of the simulation. If this value does not represent, say 25 to 100 points per oscillation period, the accuracy of the resulting zero-crossing estimates may be significant. As you might imagine, this will also impact your phase noise estimate.

    3. To verify if any of these are potential issues, you might plot the resulting PSS waveforms in the differential and single-ended mode analyses you performed. Compare these waveforms to the respective steady-state transient analysis waveforms from a conventional transient simulation (with a low value of maxstep - 25-100 timepoints/period). If there appears to be a signficant discrepancy, items [1] and [2] might be worth considering.

    Good luck! Phase noise analyses require some care and feeding...

    Shawn
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