

Using the Si9000 sensitivity analysis to graph crosstalk
Application Note AP8187

This note describes how you can use the Si9000e sensitivity analysis to graph the effect on crosstalk (NEXT and FEXT) of changing both the separation, S1, and trace width, W1 and W2, of a differential pair while maintaining constant impedance. Begin in the Lossless Calculation tab. Select the EdgeCoupled Coated Microstrip 1B structure; use the default structure parameters but change the substrate height, H1, to 5.5 mils and calculate the impedance; differential impedance, Zdiff, should equal close to 100 ohms. Using sensitivity analysis Switch to the Sensitivity Analysis tab. Under the Impedance vs Changing Parameter section select the first Parameter, to trace separation, S1, set the Range Start Value to 7 mils and the Range Finish Value to 50 mils. In the Constant Impedance vs Changing Parameters set the Parameter to trace width, W1 and the Target Impedance to 100 ohms. Click Calculate in the Constant Impedance vs Changing Parameters section. The Si9000e Constant Impedance plot charts trace width v trace separation over the selected range of values of S1 while maintaining a constant value of 100 ohms differential impedance. A subset of the results as trace width and separation vary is shown below The results tab shows W1 / W2 / S1 changing; the impedance, ZDiff, calculates to 100 ohms but you can also see the NEXT / FEXT values for each W1 / W2 / S1 combination of parameter values. This data can be exported to other tools (for example, Microsoft Excel^{®}) for further analysis. The Si9000e sensitivity analysis includes graphing for differential, common, odd and even mode impedances along with near and farend crosstalk. Change the Display Series from Constant Impedance to NEXT / FEXT. The plot below shows NEXT / FEXT as S1 increases and W1 changes to maintain the target impedance of 100 ohms. As an exercise, repeat the above example using a stripline structure and compare the results for near and farend crosstalk. 