Stabindex and S1/S2 in Sprobe Cadence

RE: Stabindex and S1/S2 in Sprobe Cadence

Frank Wiedmann — Tue, 21 Jan 2025 14:25:39 GMT

The Spectre documentation does not seem to have a lot of information about the Stability Index. The AWR documentation has some more information under https://support.cadence.com/apex/techpubDocViewerPage?path=awrSimulationAnalysis/awrSimulationAnalysis24.1_ISR1/A.5._Stability_Analysis_Methods.html#A.5.StabilityAnalysisMethods-A.5.4.MicrowaveOfficeApproachtoInternalStabilityAnalysis . If we assume that Spectre is using the same definition, a Stability Index < 1 at all frequencies (from 0 to infinity) would indeed mean that the circuit is stable according to the Nyquist Criterion.

RE: Stabindex and S1/S2 in Sprobe Cadence

YufeiLiu — Mon, 20 Jan 2025 17:48:24 GMT

Thank you for your reply. I have read that article. Since for normal SP simulation with Ports, similar S11/S22 > 0dB or negative Z11/Z22(real) represent unstable situations. Just curious if it's possible to ignore S1/S2 >1 or negative real impedance in Z1/Z2 in Sprobes when Stabindex <1.

RE: Stabindex and S1/S2 in Sprobe Cadence

Frank Wiedmann — Mon, 20 Jan 2025 12:15:19 GMT

I suggest that you take a look at https://support.cadence.com/apex/ArticleAttachmentPortal?id=a1O3w000009lgsvEAA . It says that "StabIndex values below 1 are considered stable."