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Home > ANSYS SIwave教學> SIwave and HFSS Correlation with Roughness
本文以SIwave v6與HFSS v14來模擬10 inches長的differential pair,藉此例說明HFSS模擬很長的傳輸線結構(均勻截面)該怎麼做比較好,也順便說明roughness model對S-parameter的影響。
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HFSS:10000mils microstrip without roughness and without wave port de-embedded (wrong)
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HFSS:10mils microstrip without roughness and with 9990mils wave port de-embedded
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HFSS:10mils microstrip with roughness, real thickness, and 9990mils wave port de-embedded
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6.1 為何step4.2的結果,S11在高頻出現些微誤差?
SIwave v7 result:結果正確
SIwave2014 result (wrong):S11在12~20GHz區間凹陷不合理 ,此問題是R15解梯形截面才會遇到
work around的辦法是:取求解的最高頻寬遠大於所需的有效頻寬,把出問題的頻段推出我們在意的頻寬範圍
SIwave2016 result: 結果正確,問題在R17已修正
Use low loss Huray model for roughness
Using SIwave v7.0 to solve trace with "Trapezoid" cross section will get smaller S11 than SIwave v6.0 and HFSS.
3.1 仿Polar的SM layer結構,且Delta S=0.1
3.2 仿Polar的SM layer結構,且Delta S=0.01
3.3 仿SIwave的SM layer結構,且Delta S=0.1
對於10000mils(10 inch)這麼長的結構,並不適合直接在HFSS跑,因為要得到夠精確收斂的值,需要極大的RAM size與時間;反之,如果Delta S設的不夠小,則跑出的S-parameter會與step1的模擬結果差很大。
建議的作法,是在HFSS內只建一小段長度的傳輸線,其餘用wave port de-embedded推出即可。
4.1 Use Finite Conductivity BC (wrong)
Finite conductivity BC無法表現梯形截面的傳輸線
4.2 Use Real Thickness and Set Delta S=0.0001
5.1 Use Finite Conductor (wrong)
5.2 Use Real Thickness, Huray roughness on top surface, and Set Delta S=0.0001
只要delta S設的夠小,與Step1的SIwave模擬結果:S21在4GHz差0.22dB,在12GHz處差0.7dB。是HFSS此例中斜坡面沒設定roughness所造成的
5.3 Use Real Thickness, Huray roughness on top and slanted surface, and Set Delta S=0.0001
只要delta S設的夠小,與Step1的SIwave模擬結果幾乎一樣:S21在4GHz差0.06dB,在12GHz處差0.2dB。
6.1 為何step4.2的結果,S11在高頻出現些微誤差?
Ans:誤差主要兩個來源
一是太長的wave port de-embedded原本就會在高頻引入些許誤差。Refer to here
二是此例的S11值極小,若要得到足夠的數值精度,HFSS的solution frequency要再設高一些,refine mesh次數再多一些