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Strange phase-noise plot in PSS

HoWei

Hi,

I am simulating a crystal oscillator (Pierce-Oscillator with Inverter), very similar to Tawnas example in

https://community.cadence.com/cadence_blogs_8/b/rf/posts/new-mmsim-12-1-harmonic-balance-features-auto-tstab-and-auto-harmonics

I applied the settings as described in 

https://community.cadence.com/cadence_blogs_8/b/rf/posts/guidelines-for-setting-pnoise-hbnoise-sidebands-to-get-accurate-results

and in the document from the $CDSHOME

Spectre Circuit Simulator and Accelerated Parallel Simulator RF Analysis in ADE Explorer Workshop.pdf

In the attached picture I compared 2 simulation methods:

In PSS/Pnoise simulation I get a strange  Phase Noise plot (RED curve).

In HB/HBnoise simulation I get a different result, but with much lower noisefloor (BLUE curve).

I am wondering what I am doing wrong in the PSS simulation (I am using 80 sidebands for Pnoise and 20 harmonics in PSS) -why this strange step at 100Hz offset ?

Why is the noise floor 15dB lower for HB simulation ?

Any idea ?

Parents

  • Hi Ho Wei,

    Those instructions are probably close to 10 years old.   For a crystal oscillator, you should be using harmonic balance, not pss (pss is not the correct tool for the job).

    Set errpreset=conservative for oscillators.   It's difficult to say what you are doing incorrectly without seeing the netlist analysis statements and options  for both pss and hb

    If you have access to Cadence Online support https://support.cadence.com, please see this Rapid Adoption Kit: SPECTRE18.1: Crystal Oscillator Simulation using SpectreRF 

    You may want to contact Cadence Customer Support (https://support.cadence.com) to get assistance from an available AE.

    best regards,

    Tawna

  • in reply to Tawna

    Hi Tawna,

    thanks for the links - it now found that when using a long stop time (tstab) in PSS I get reasonable results. The phase-noise plot now shows 3 different regions of noise slope, whereas the HBnoise shows only 2 regions of noise (as shown in the above pictures.  Maybe it was not in a steady state when using shorter "tstab" values in PSS ?

    See plot with longer simulation time:

    Anyways, I will have a look on the links and see what I can change.

    BR

    Holger

Reply
  • in reply to Tawna

    Hi Tawna,

    thanks for the links - it now found that when using a long stop time (tstab) in PSS I get reasonable results. The phase-noise plot now shows 3 different regions of noise slope, whereas the HBnoise shows only 2 regions of noise (as shown in the above pictures.  Maybe it was not in a steady state when using shorter "tstab" values in PSS ?

    See plot with longer simulation time:

    Anyways, I will have a look on the links and see what I can change.

    BR

    Holger

Children
  • in reply to HoWei

    Hi Holger,

    re: Maybe it was not in a steady state when using shorter "tstab" values in PSS

    It may take longer to converge if tstab is too short (that's why autotstab is a nice feature...it tells you the absolute minimum number of harmonics required to get an accurate hb/pss result.)

    PSS and HB will not converged unless they've reached steady state.  You cannot do a pnoise or hbnoise analysis until pss/hb has finished (you're at steady state).  

    Likely, it's a setup/training issue.  Please see the crystal oscillator RAK SPECTRE18.1: Crystal Oscillator Simulation using SpectreRF .      Sorry, I don't have time to explain in great detail. 

    best regards,

    Tawna

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