《現(xiàn)代微波電路和器件設(shè)計》課件7、HFSS的若干問題

1、現(xiàn)代微波電路和器件設(shè)計7、HFSS的若干問題蘇 濤2010年春HFSS是強有力的通用性工具，其中的每一個相關(guān)環(huán)節(jié)都是根據(jù)算法和模型需要設(shè)定的，也就是說，軟件通過一套參數(shù)和規(guī)則，將待分析問題和電磁數(shù)值計算需要的設(shè)定確定下來，必須了解這樣一套參數(shù)和規(guī)則。要想使用好必須 了解電磁理論 明確參量意義，及其在該軟件中的特殊設(shè)定Help/Training/Documents/Examples/Discussion/TestProject Problems TRY to solve Improve由初學(xué)者到高手的進階之路：積累電磁數(shù)值算法FEM待分析問題模型輸入輸出通用電磁分析軟件高效波導(dǎo)濾波器設(shè)計邊界條件

2、激勵邊界條件：控制平面、物體表面和交界面的性質(zhì)。邊界條件是非常重要的： Maxwell方程：場矢量單值、有界、場分布和其導(dǎo)數(shù)連續(xù)； 邊界條件：場分布不連續(xù)，其導(dǎo)數(shù)無意義 邊界條件決定了跨越不連續(xù)邊界時場的行為。HFSS使用中： 正確認識邊界條件下場分布的設(shè)定； 正確使用邊界條件 避免與實際結(jié)果不一致 減小模型的復(fù)雜度 模型空間是有限的，HFSS自動地應(yīng)用背景條件或者外部邊界條件來包圍分析區(qū)域。邊界條件的分類1、 激勵源波端口（外部）、集總端口（內(nèi)部）2、表面近似對稱面、PEC和PHC、輻射表面背景或外表面（outer）3、 材料特性不同介質(zhì)的交界面有限導(dǎo)體表面背景或外表面（outer） 分析空

3、間：計算場分布的區(qū)域，必然有限； 背景是分析空間中，包圍幾何模型，并填充那些沒有被物體占據(jù)空間的區(qū)域。 任何與背景接觸的表面自動定義為理想電邊界（PEC），并被命名為外部（outer）邊界條件?？梢匀藶楦淖冊摱x。 波導(dǎo)空間真空材料 vaccum， 外邊界 PEC 模擬表面損耗 重新定義表面為有限導(dǎo)體表面（Finite Conductivity）或者阻抗表面（Impedance Boundary） 為了模擬波輻射向無窮遠處設(shè)定外表面為輻射邊界（Radiation boundary） 優(yōu)先級：后定義的優(yōu)先，端口最高。Perfect & Perfect H 矩形諧振腔，47.55*22.15*50

4、mm3 矩形諧振腔，寬邊和窄邊各取一半與全結(jié)構(gòu)仿真相同的兩個模式。與全結(jié)構(gòu)仿真不同的三個模式：為什么兩次的結(jié)果不同？有什么區(qū)別？ 有效使用PEC和PHC，可以減小模型復(fù)雜度； 注意PEC和PHC的限制，及其對模型帶來的影響；PEC，電場垂直表面PHC，電場與表面相切波導(dǎo)TE10模式如何設(shè)定PEC和PHC？Symmetry Boundary Symmetry Boundary與Perfect E|H是類似的，多了Impedance Muliplier 利用對稱性質(zhì)，僅計算一半空間，阻抗計算要得到原來的值，需要設(shè)置Impedance MuliplierTE10Impedance Muliplier

5、2Impedance Muliplier0.5PECPHC特性阻抗278.91全尺寸仿真特性阻抗278.79Finite Conductivity Boundary 考慮導(dǎo)體表面非理想損耗，必要時要考慮表面光潔度和鍍層厚度等問題。 Impedance Boundaries 表示已知阻抗的表面。該表面的由電流引起的場和損耗將使用解析公式計算；HFSS并不真正仿真里面的場。 阻抗邊界的單位 Ohm/Square阻抗邊界給出的是，表面阻抗Zs；等效電路阻抗Z，由矩形兩邊界之間測量得到；其中，等效電路阻抗Z【例】Teflon微帶板，厚度1mm；導(dǎo)帶零厚度，寬2mm 10GHz，port ONLY 計算

6、端口特性阻抗(65.759, 0.026689)，(65.692,0.026662)Impedance Boundary作為微帶匹配負載，有誤差Lumped RLC Boundaries 集總RLC邊界條件表示任意電阻和電感/電容的組合。 不同的電路形式可以用不同的集總RLC邊界組合表示。例如：串聯(lián)的RLC電路，可以用集總RLC邊界，只有電阻值；連接集總RLC邊界，只有電感值；再連接集總RLC邊界，只有電容值。 Lump RLC作為微帶匹配負載，有誤差 Radiation Boundaries輻射邊界用在開放空間問題中，比如天線分析；其包裹有限空間，模擬波向無窮遠處傳播。 截斷無限大空間輻射問

7、題 模擬無限傳輸（暗室） 使用其邊界上的場計算遠場 有輻射損耗 一般在源1/4波長之外Amplitude andPhase Layered Impedance Boundaries多層阻抗邊界 Master & Slave Boundary周期邊界條件的描述在HFSS中使用一對：Master & Slave Boundary。在該對邊界中，將保持固定的相位差距，由此將周期問題轉(zhuǎn)換為邊界條件。注意：V8版本的例子Master 1Slave 1Master 2Slave 2WaveguideRadiatorUnit Cell WallsFeed PortSpace PortPort (should

8、 NOT touch periodic boundary)Periodic Boundary(slave)Periodic Boundary(master) PML Setup WizardPML：Perfect Matched Layer，即理想匹配層；前面的Radiation邊界就是一個理想匹配層；通過Wizard可以設(shè)置PML。高效波導(dǎo)濾波器設(shè)計邊界條件激勵注：引自Ansoft公司相關(guān)資料。Sources Power enters the model through (unlimited number of)portsvoltage sourcescurrent sourcesincid

9、ent wavesOne other kind of source:Hbias for ferritesPorts in HFSSClassical Ports: cross section of transmission lineHFSS finds propagating and evanescent modes and determines characteristic impedancesLumped Gap Source Ports: use when Classical Ports dont work (will be explained shortly)You specify c

10、haracteristic impedance of the lineClassical Port SurfacesClassical Ports Can Only be Defined on Surfaces Which Are Exposed to a Region Where The Field Does Not ExistBackgroundPerfectly Conducting ObjectsSimple 2-Port Waveguide:Ports: waveguide cross sectionsEach port bounds the BackgroundSelect fac

11、es or appropriate2D objects to define the portsExample: coax portPort is coaxscross section.To define it,select a face or a 2D object.Port and coax are inside a larger model cap behind port.Yagi Antenna With Interior Feed PortExample: Microstrip Port Ansoft recommends H=5 -10 h and W= 5 w; where h a

12、nd w are the substrate height and trace width, respectively.HWwhPECMicrostrip Set-up: poor portThis microstrip port may be too big waveguide modepossibleRemedy: create2D port object withappropriate size.Example: CPW port ground trace groundPort Port size rule of thumb: edge length = 3 (w+2s)w is tra

13、ce width, s is spacing. You may need to request port accuracy 1% to obtain good accuracy.Coplanar Waveguide (II)CPW Structure Without Lower GroundplaneRightWrongThis method allows threepropagating modes ratherthan just the CPW mode.Although port is smallerthan rule of thumb prescribesExample: Stripl

14、ine PortgroundgroundporttracePort width is several times the width of the trace.Port does not cross any ground plane.Illegal portsThe following two situations are illegal:1. A port that contains metal only e.g. the port is just the cross section of a signal trace.2. A port that is split in disconnec

15、ted parts e.g. port extends below ground plane. HFSS will not be able to find a field that “fits”.Structures with Difficult-to-Define PortsIn some signal integrity applications it can become very difficult to define traditional ports.Too little room! Solution: Lumped Gap Source PortsThese wire bond

16、ports were made easily by drawing a rectangle from the paddle to the wire ends.Traces close togetherLumped Gap Source Ports are recommended for this application - simply draw rectangle from edge of trace to ground. Vertical sides will be perfect_H boundaries.ApproximationsThe field excitation for a

17、single line is an approximation, since in reality there is field beyond the edges of the rectangle. A somewhat larger rectangle is more accurate, but not necessary when the model is much smaller than the wavelength.The field excitation for coupled ports is an approximation-the actual port calculatio

18、n does not account for the coupling to nearby conductors.Accuracy of S-parameters for Coupled Port StructuresWhen put into perspective, by considering how coupled structures are measured, this is not a bad approximation. Circuits currently are measured by adding either shorts or resistors to all but

19、 2 ports before measuring with network analyzer, which also does not consider the effect of coupled ports. Using this simulation method is equivalent to adding resistors to the other ports.Neighboring Lumped Gap Source PortsTouching (wrong)Not touching (correct)Neighboring Lumped Gap Source Ports (2

20、)Clearly, the Lumped Gap Source Port behaves very differently depending upon whether adjacent ports share an edge. If they share an edge (i.e., touching) then the shared edge is treated as Perfect-E.If they do not share an edge, then all edges which do not touch metal in the 3D model are Perfect-H.R

21、ules for Lumped Gap Source PortsYou must define calibration and impedance lines for Lumped Gap Source Ports. You must specify the port impedance.A Lumped Gap Source Port should not be defined on the interface between two materials, e.g. an air/substrate interface. Embed it in the substrate or define

22、 it on an outer face of your model.A Lumped Gap Source Port should not be backed by metal.Edges of a Lumped Gap Source Port that have an electric field component parallel to them should not touch good conductors or another port. If the source is on the outside on your model, watch out for touching Perfect_E boundaries.Coupled Li