Community RF Design Stabindex and S1/S2 in Sprobe Cadence

Stats

Replies 3
Subscribers 64
Views 743
Members are here 0

Stabindex and S1/S2 in Sprobe Cadence

Hi,

I added Sprobe to the LNA I designed to check the stability of interstages. But the results generated by Sprobe confuse me.

For StabIndex, it is always less than 1. However, at some frequency, S1 and S2 will be greater than 1, and Z1 and Z2 will also have negative real impedance at that moment. I am wondering whether the circuit is in the unconditional stable state or unstable in this case?

Thanks!

Parents

Frank Wiedmann 3 months ago

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."
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel
YufeiLiu 3 months ago in reply to Frank Wiedmann

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.
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel

Reply

YufeiLiu 3 months ago in reply to Frank Wiedmann

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.
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel

Children

Frank Wiedmann 3 months ago in reply to YufeiLiu

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.
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel

Community Guidelines

The Cadence Design Communities support Cadence users and technologists interacting to exchange ideas, news, technical information, and best practices to solve problems and get the most from Cadence technology. The community is open to everyone, and to provide the most value, we require participants to follow our Community Guidelines that facilitate a quality exchange of ideas and information. By accessing, contributing, using or downloading any materials from the site, you agree to be bound by the full Community Guidelines.