2 Replies Latest reply on May 28, 2012 9:33 AM by weiming.qian

    What to do with S-parameter causality/passivity problem




      I use HyperLynx v8.2 to extract the S-parameter model of a differential channel on Xilinx Virtex-7 Dev board. However, the Touchstone Viewer tells me that the model is non causal. What should I do with this non causal model? Is there a way to fix it? Or can I ignore it as non-causal behaviour only happens on return loss curve, which I do not care too much.


      The non-causal model and its mixed-mode model are attached.


      Cheers, Weiming

        • 1. Re: What to do with S-parameter causality/passivity problem

          Hi Weiming,


          The most common cause of non-causal data is insufficient resolution. If this is the case increasing the sampling rate will resolve this issue. Non-causal data means that the response precedes the input therefore has negative delay this is not possible for real systems. One possible reason that you are obtaining non-causal data is that you may be using logorithmic sampling which steps over certain frequencies. Try to use adaptive sampling and try to increase the resolution. If you want a complete explanation of non-causal data please go to AppNote 10029 on SupportNet. This AppNote explains causality in greater detail and other typical effects that may cause non-causality. If you are still having issues with your S-parameter model I suggest you open up a service request and we will get back to you as soon as possible.




          • 2. Re: What to do with S-parameter causality/passivity problem

            Hi Alex,


            Thanks for the information. It is very helpful indeed.


            I solve the problem by playing with the frequency ranges of the model. Increasing the upper limit from 20GHz to 30GHz solves the "non-causal" problem. However, converting s-parameter model into fitted-pole model shows "non-passive" problem. Looking closer at fitted-pole model in touch-stone viewer, the passivity plot goes negative at very low frequency. Then I increase the lower cut-off freqency from 0.1MHz to 100MHz, and regenerate the s-parameter model, then convert to fitted-pole model, everything seems fine now. I have to say, this is a very tricky job.


            Regards, Weiming