《现代微波电路和器件设计》课件7、HFSS的若干问题

1、现代微波电路和器件设计7、HFSS的若干问题苏 涛2010年春HFSS是强有力的通用性工具，其中的每一个相关环节都是根据算法和模型需要设定的，也就是说，软件通过一套参数和规则，将待分析问题和电磁数值计算需要的设定确定下来，必须了解这样一套参数和规则。要想使用好必须 了解电磁理论 明确参量意义，及其在该软件中的特殊设定Help/Training/Documents/Examples/Discussion/TestProject Problems TRY to solve Improve由初学者到高手的进阶之路：积累电磁数值算法FEM待分析问题模型输入输出通用电磁分析软件高效波导滤波器设计边界条件

2、激励边界条件：控制平面、物体表面和交界面的性质。边界条件是非常重要的： Maxwell方程：场矢量单值、有界、场分布和其导数连续； 边界条件：场分布不连续，其导数无意义 边界条件决定了跨越不连续边界时场的行为。HFSS使用中： 正确认识边界条件下场分布的设定； 正确使用边界条件 避免与实际结果不一致 减小模型的复杂度 模型空间是有限的，HFSS自动地应用背景条件或者外部边界条件来包围分析区域。边界条件的分类1、 激励源波端口（外部）、集总端口（内部）2、表面近似对称面、PEC和PHC、辐射表面背景或外表面（outer）3、 材料特性不同介质的交界面有限导体表面背景或外表面（outer） 分析空

3、间：计算场分布的区域，必然有限； 背景是分析空间中，包围几何模型，并填充那些没有被物体占据空间的区域。 任何与背景接触的表面自动定义为理想电边界（PEC），并被命名为外部（outer）边界条件。可以人为改变该定义。 波导空间真空材料 vaccum， 外边界 PEC 模拟表面损耗 重新定义表面为有限导体表面（Finite Conductivity）或者阻抗表面（Impedance Boundary） 为了模拟波辐射向无穷远处设定外表面为辐射边界（Radiation boundary） 优先级：后定义的优先，端口最高。Perfect & Perfect H 矩形谐振腔，47.55*22.15*50

4、mm3 矩形谐振腔，宽边和窄边各取一半与全结构仿真相同的两个模式。与全结构仿真不同的三个模式：为什么两次的结果不同？有什么区别？ 有效使用PEC和PHC，可以减小模型复杂度； 注意PEC和PHC的限制，及其对模型带来的影响；PEC，电场垂直表面PHC，电场与表面相切波导TE10模式如何设定PEC和PHC？Symmetry Boundary Symmetry Boundary与Perfect E|H是类似的，多了Impedance Muliplier 利用对称性质，仅计算一半空间，阻抗计算要得到原来的值，需要设置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 考虑导体表面非理想损耗，必要时要考虑表面光洁度和镀层厚度等问题。 Impedance Boundaries 表示已知阻抗的表面。该表面的由电流引起的场和损耗将使用解析公式计算；HFSS并不真正仿真里面的场。 阻抗边界的单位 Ohm/Square阻抗边界给出的是，表面阻抗Zs；等效电路阻抗Z，由矩形两边界之间测量得到；其中，等效电路阻抗Z【例】Teflon微带板，厚度1mm；导带零厚度，宽2mm 10GHz，port ONLY 计算

6、端口特性阻抗(65.759, 0.026689)，(65.692,0.026662)Impedance Boundary作为微带匹配负载，有误差Lumped RLC Boundaries 集总RLC边界条件表示任意电阻和电感/电容的组合。 不同的电路形式可以用不同的集总RLC边界组合表示。例如：串联的RLC电路，可以用集总RLC边界，只有电阻值；连接集总RLC边界，只有电感值；再连接集总RLC边界，只有电容值。 Lump RLC作为微带匹配负载，有误差 Radiation Boundaries辐射边界用在开放空间问题中，比如天线分析；其包裹有限空间，模拟波向无穷远处传播。 截断无限大空间辐射问

7、题 模拟无限传输（暗室） 使用其边界上的场计算远场 有辐射损耗 一般在源1/4波长之外Amplitude andPhase Layered Impedance Boundaries多层阻抗边界 Master & Slave Boundary周期边界条件的描述在HFSS中使用一对：Master & Slave Boundary。在该对边界中，将保持固定的相位差距，由此将周期问题转换为边界条件。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，即理想匹配层；前面的Radiation边界就是一个理想匹配层；通过Wizard可以设置PML。高效波导滤波器设计边界条件激励注：引自Ansoft公司相关资料。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