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Home > ANSYS HFSS 教學 > HFSS TDR Notices
本文始於2010,並於2019更新。以兩個相同疊構的microstrip傳輸線,但分別用厚度0與有實際厚度的PEC當參考平面,說明在HFSS內收斂性與掃頻頻寬 如果設定不足時,後者TDR會不夠準確。主要原因是:使用3D FEM模擬軟體,網格品質/adaptive mesh的收斂精度會直接影響模擬結果的準度。
This article started in 2010, and was revised in 2019. It is intended to introduce how do the mesh performance and convergence criteria affect HFSS TDR results.
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HFSS using Real Conductor Thickness as Reference Plane (not accurate enough)
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HFSS using Real Conductor Thickness as Reference Plane (more accurate)
3.1 Change [Max. Delta S] from 0.02 to 0.005, and [Min. Number of Passes] from 1 to 2
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HFSS using Perfect_E Boundary as Reference Plane
四層板很容易可以做到特性阻抗45~50歐姆。
在HFSS建一個疊構同上圖的100mils microstrip line, 求解設定[Max. Delta S] = 0.02, [Min. Number of Passes] = 2
若參考平面分別以零厚度PEC, [Finite Conductivity], and real 0.7 mils thickness copper建模,特性阻抗模擬結果如下
截至目前為止,一切看來還好,我們繼續看下去
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HFSS using Real Conductor Thickness as Reference Plane (not accurate enough)
若參考平面以有實際厚度的PEC,且這厚度從0.5mils sweep to 3mils,求解設定[Max. Delta S] = 0.02, [Min. Number of Passes] = 1,特性阻抗模擬結果如下
特性阻抗變化差異頗大,而且沒有規律的上下亂跳,明顯不合理
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HFSS using Real Conductor Thickness as Reference Plane (more accurate)
3.1 Change [Max. Delta S] from 0.02 to 0.005, and [Min. Number of Passes] from 1 to 2
特性阻抗變並沒有隨參考面厚度些微變化而有太大的改變,這就比較合理
3.2 Change [Max. Delta S] from 0.02 to 0.005, [Min. Number of Passes] from 1 to 2 and wave port de-embedded
我們更進一步使用wave port de-embedded,得到的結果更準確
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總結Conclusion
以HFSS做傳輸線的TDR是很"基本"的題目,但卻潛藏至少四點新手易犯錯誤 (蠻適合業界主管用來當面試新人的考題)
求解沒有使用"Director Solver" (Iterative Solver一般用於大型天線,不推薦用於SI)
掃頻頻寬不夠 (如果要在HFSS內直接看TDR,頻寬要取有效頻寬 的3~4倍,本文例子掃頻頻寬0~50GHz)
掃頻沒有包含DC點(或100M以下低頻成份),或沒有使用"interpolating sweep" -- TDR其實就是S11,與S參數的低頻成份有很大關係
沒有使用wave port de-embedded或網格質量不夠 (如本文step2,參考平面用實體厚度,走線用實體長度沒有wave port de-embedded