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STB Analysis Breakpoint Selection and Nyquist Plot Support in Cadence

Hi,

In STB analysis, the phase margin curve may start at 180° or 0°which is a common problem in multi-loop system. According to feedback theory, loop gain should have a negative sign in a system with negative feedback.

Online discussions indicate that "accurate results require placing the iprobe on a common line that breaks all loops".

So, how do I choose the correct breakpoint? And does Cadence support Nyquist plots to analysis negative feedback loop stability issue?

JankoK 22 days ago

Hi Eleanor,

The first question is not a trivial one and it depends on a circuit to circuit basis. Even for a specific circuit, different designers would have their own personal opinions. The general answer is that the placement should consider all loops as you already figured. You can find some common sense placements in this article:

How to insert probes into schematic when doing stability (stb) analysis

Regarding your second question... you can use YvsY option on you Loop Gain plots. The procedure is explained here:

Creating a Nyquist Plot to Determine the Stability (Video)

One bonus article (actually a RAK, there is also a tarball that you can download and try)... if you want to go a step further you can take a look at our Pole-Zero (PZ) Analysis and plot Root Locus:

Pole Zero (PZ) Analysis with Spectre/SpectreX

Hope this helps!

/Janko
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Eleanor Grace 21 days ago in reply to JankoK

Hi Janko,

Thanks for your reply.

The following is original circuit diagram. You can see that the phase curve starts from 0 degrees.

Then, I follow the steps in the video to generate the real and imaginary parts of loop gain.

Finally I got the Nyquist plot as follows, which seems to have some problems. Are there any steps that need to be modified?

Thanks.
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JankoK 18 days ago in reply to Eleanor Grace

Hi Eleanor,

It is tricky to understand what is happening here without a proper circuit analysis, some loop might have a positive feedback or it might be due to how loops are broken...

I think it's best to file an official case and work with your local support team. You can do that easily on our support (ask) page.

BR

/Janko
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