4 Replies Latest reply on May 27, 2013 8:57 AM by joao.1.monteiro.ext

    Pre-Layout vs Post-Layout comparison

    joao.1.monteiro.ext

      Hi,

       

      I have selected a small net from my board and extracted the S-Parameter model in post-layout (boardsim). I modeled it in linesim with exactly the same stackup. The net is very simple, i modeled it with a bunch of transmission lines assigned to the same layer (top in this case) with the exact same lengths and widths. The characteristic impedances are the same in both boardsim and linesim for all modeled lines.

       

      The S-Parameter models diverge for frequencies above 1GHz and i can't understand why. Aren't the results suppose to be the same? I have re-checked everything.

        • 1. Re: Pre-Layout vs Post-Layout comparison
          yu.yanfeng

          If the driver/receiver pin is through pin type, the Boadsim will treat it as via and consider it's eeffect during S-Parameter extraction process, it means that Boardsim will calculates the pin hole's capacitance to Ground if you have set enable via modeling option on.

           

          You can check the through pin's capacitance in Boardsim, then  add a cap with the value to ground in the topology of Linesim,you will get same result now.

           

          Yanfeng

           

          • 2. Re: Pre-Layout vs Post-Layout comparison
            joao.1.monteiro.ext

            Boardsim is not considering the pad in S-Parameter extraction. I removed the pads modeled in linesim and the S-Parameters from both pre and post-layout match.

             

            I have Via model license, at the beggining i thought it was a license issue based on what you said. I had it deactivated first. Now I can select the pad and "Show via visualizer" in boardsim and it shows the pads correctly. But why it is not considering them in S-Parameter extraction?

            • 3. Re: Pre-Layout vs Post-Layout comparison
              yu.yanfeng

              Hi joao,

               

              t's ok that now you can see results match each other .  However I have to clarify that  there is an icon for toggling on/off of throug pin via modeling in Boardsim

               

              1.jpg

              Yanfeng

              • 4. Re: Pre-Layout vs Post-Layout comparison
                joao.1.monteiro.ext

                Yu, the problem is that they match because boardsim is not modeling the pads. I excluded the pads models from linesim just to make this check.

                 

                Yes, i noticed the button (it's pressed). Checking the "Add extra capacitance for SMD pads in via-in-pad" in Setup->Via Simulation Method in boardsim made some differences: results are similar to those when i add only the capacitor pads. However the inductor pad is not being taken into account in modelsim. I mean, adding it in linesim really changes the frequency response in comparison to what i get in boardsim.

                 

                I attach a screenshot of the net for clearence.

                What can i be missing?