Limitations of the CQMI method to assess causality of s-parameters

[yefj]

Hello , I am frequently using CQMI to see if my s-parameters are casual however I was told the CQMI can give false results.
What is the cases where I should not rely on CQMi?
https://resources.altium.com/p/ieee-p370-standard-high-speed-pcb-interconnects
Thanks.

 

[tech99]

Hello , I am frequently using CQMI to see if my s-parameters are casual however I was told the CQMI can give false results.
What is the cases where I should not rely on CQMi?
https://resources.altium.com/p/ieee-p370-standard-high-speed-pcb-interconnects
Thanks.

I think the underlying issue you have is that you need to undo the embedded test fixture parameters from your overall measured s-parameters. As s-parameters relate to power, not voltage, if the test fixture is not 50 Ohms then assumprtions about voltages will be incorrect.

 

[yefj]

I think the underlying issue you have is that you need to undo the embedded test fixture parameters from your overall measured s-parameters. As s-parameters relate to power, not voltage, if the test fixture is not 50 Ohms then assumprtions about voltages will be incorrect.

Exacrly What is the formal limitation of the IEEE370 method cqmi?
is there some definition where i know not to rely on this method?