Boundary and Excitation Training -...
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Transcript of Boundary and Excitation Training -...
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Boundary and Excitation TrainingFebruary 2003
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Boundary Conditions
Why are They Critical?For most practical problems, the solution to Maxwell’s equations requires a rigorous matrix approach such as the Finite Element Method (FEM) which is used by Ansoft HFSS.
The wave equation solved by Ansoft HFSS is derived from the differential form of Maxwell’s equations.
For these expressions to be valid, it is assumed that the field vectors are:single-valued, bounded, and have acontinuous distribution (along with their derivatives)
Along boundaries of media or at sources, Field vectors are discontinuousDerivatives of the field vectors have no meaning
0=⋅∇=⋅∇
∂∂
+=×∇
∂∂
−=×∇
BD
tD
JH
tB
E
ρ
Boundary Conditions define the field behavior across discontinuous boundaries
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Boundary Conditions
Why do I Care?They Force the fields to align with the definition of the boundary condition
As a user I should be askingWhat assumptions, about the fields, do the boundary conditions make?Are these assumptions appropriate for the structure being simulated?
Model Scope/ComplexityThe infinite space of the real world needs to be made finite
Ansoft HFSS Background or Outer boundary
When applied properly, they can be used to reduce the complexitySolution TimeComputer Resources
Failure to understand boundary conditions may lead to inconsistent results
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Boundary Conditions
Application of Boundary Conditions - Case 1Emulate laboratory measurements
Verification/Validation before production
Picture courtesy of Delphi Picture courtesy of Tektronix
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Boundary Conditions
Application of Boundary Conditions - Case 2Isolate part of a structure (i.e. Exciting arbitrary transmission lines)
Not physically possible to measure in the laboratoryFull-Wave analysis not required for total system
Or total system too complexDesign work/Component level optimizationPost production problem solving
Isolated Component – Via Transition
Total System
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What are Common Ansoft HFSS Boundary Conditions?Sources
Wave Ports (External) Lumped Ports (Internal)
Surface ApproximationsSymmetry PlanesPerfect Electric or Magnetic SurfacesRadiation SurfacesBackground or Outer Surface
Material PropertiesBoundary between two dielectricsFinite Conductivity of a conductor
Boundary Conditions
Largely the users responsibility
Transparent to the user
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Example
Example StructureCoax to Stripline
LAYER 2 (SIGNAL)
LAYER 3 (BOTTOM SIDE)
LAYER 1 (TOP SIDE)
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Example
Material PropertiesAll 3D (Solid) objects have material definitions
To complete the model shown previously we must include the airthat surrounds the structure.
air
Note: Substrate/Air boundary included in structure
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Example
Remember! Material Boundary conditions are transparent to the userThey are not visible in the Project Tree
Example Material Boundary: Conductors ð Surface Approximations Perfect Conductors ð Perfect E Boundary (Boundary Name: smetal)
Forces E-Field perpendicular to surfaceLossy Conductors ð Finite Conductivity Boundary
Forces tangential E-Field to ((1+j)/(δσ))(n x Htan).Assumes one skin depth – User must manually force Ansoft HFSS to solve inside lossy conductors that are = a skin depth
smetal
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Example
Surface ApproximationsBackground or Outer Boundary
Not visible in the Project TreeAny object surface that touches it ð Perfect E BoundaryDefault boundary applied to the region surrounding the geometric model
Model is encased in a thin metal layer that no fields propagate through
Override withRadiation Boundary
outer
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Excitations
ExcitationsPorts are a unique type of boundary condition
Allow energy to flow into and out of a structure.Defined on 2D surfaceArbitrary port solver calculates the natural field patterns or modes
Assumes semi-infinitely long waveguideSame cross-section and material properties as port surface
2D field patterns serve as boundary conditions for the full 3D problem
Excitation TypesWave Port – External
Recommended only for surfaces exposed to the backgroundSupports multiple modes (Example: Coupled Lines) and deembeddingCompute Generalized S-Parameters
Frequency dependent Characteristic Impedance (Zo)Perfectly matched at every frequency
Lumped Port – InternalRecommended only for surfaces internal to geometric modelSingle mode (TEM) and no deembeddingNormalized to a constant user defined Zo
MeasurementsConstant Zo
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Excitations
Excitation Types – Boundary ConditionsWave Port
Perfect E or Finite ConductivityDefault: All outer edges are Perfect E boundary.
Port is defined within a waveguide. Easy for enclosed transmission lines: Coax or WaveguideChallenging for unbalanced or non-enclosed lines: Microstrip, CPW, Slotline, etc.
Symmetry or Impedance Recognized at the port edges
RadiationDefault interface is a Perfect E boundary
Lumped PortPerfect E or Finite Conductivity
Any port edge that interfaces with a conductor or another port edgePerfect H
All remaining port edges
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Excitations
Excitation Types – CalibrationPorts must be calibrated to ensure consistent results. Determines:
Direction and polarity of fieldsVoltage calculations.
Solution Type: Driven ModalExpressed in terms of the incident and reflected powers of the waveguide modes.
Definition not desirable for problems having several propagating quasi-TEM modesCoupled/Multi-Coupled Transmission Lines
Always used by the solverCalibration: Integration Line
Phase between PortsModal voltage integration path: Zpi, Zpv, Zvi
Solution Type: Driven TerminalLinear combination of nodal voltages and currents for the Wave Port.
Equivalent transformation performed from Modal SolutionCalibration: Terminal Line
PolarityNodal voltage integration path
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Excitations
Example Solution Types:
T1 T2
Integration LineMode 1
(Even Mode)
Integration Line
Mode 2(Odd Mode)
Modes to NodesTransformation
SPICEDifferential Pairs
ModalPort1 Port2
TerminalPort1 Port2
T1
T2
T1
T2
2 Modes 2 Modes
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Example
What Port Type Should I Use?Example is easy decision
Port touches background (External)Cross Section is Coax (Enclosed Transmission Line)
Wave PortSolution Type: Driven Terminal
SPICE Output
Identify Port Type &Cross Section
Assign Excitation &Calibrate Port
Define Default Post Processing
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Example
Is it Really that Simple?Yes, but the geometric model was setup with several considerations
1. Only the face of the coax dielectric was selected for the port facePort Boundary conditions define outer conductorMaterial Definitions define inner conductor
2. Uniform port cross-sectionOnly supports a single modeHigher-order modes caused by reflections would attenuate before port
Modes attenuate as a function of e-αz, assuming propagation in the z-direction.Required distance (uniform port length) depends on modes propagation constant.
Uniform cross-sectionRule of Thumb: 5x Critical Distance
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Example
How often is the Setup that Simple?If you are emulating laboratory measurements? [Case 1]
Most of the time!Laboratory equipment does not direct connect to arbitrary transmission lines
ExceptionsEmulating Complex Probes with a Port ð Understanding of Probe
If you are isolating part of a structure? [Case 2]For “real” designs - usually only by dumb luck!
User Must Understand and/or Implement Correctly:1. Port Boundary conditions and impact of boundary condition2. Fields within the structure3. Assumptions made by port solver4. Return path
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Example
Side Note: Problems Associated with Correlating Results [Case 2]Can be broken into two categories of problems
1. Complex StructureBGA, Backplane, Antenna Feed, Waveguide “Plumbing”, etcMost common problems result from
Measurement setup – Test fixtures, deembedding, etc. Failing to understand the fields in the structure ð Boundary ProblemReturn path problems – Model truncation
2. Simple StructuresUniform transmission lines
Equations or Circuit ElementsMost common problems result from
Improper use of default or excitation boundary conditionsFailure to understand the assumptions used by “correct” results (Equations or Circuit Elements)
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Return Path
Why are they critical?Any current injected into a system must return to the source
DCChooses path of least resistance
ACChooses path of least inductanceA signal propagates between the signal trace and its reference plane Reference plane is just as important as signal trace!
Why do I care?Many real designs have nonideal return paths
Effects only captured by full-wave simulatorsIsolating parts of a structure
Failure to maintain the correct return path willLimit correlation to measurementsMask or create design problems
Port and Boundary setup most common source of error in model setup
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Return Path Example
Port1
Port2 Port3
No DCReturn Path
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Return Path Example
Port1
Port2 Port3
DCReturn Path
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Return Path Example
DC & RFReturn Path
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Example
Isolate part of structure - Case 2 Isolate Transition
DeembedRecombine using Ansoft Designer - Circuit
Wave Port
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Example
Ansoft Designer - Circuit
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Example
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Example
What went wrong?Isolate Port from Discontinuity?
YesUniform Cross-Section?
NO – The cross section of the port (including its boundaries) is not maintainedMaintain Return Path?
NO – Boundary on port shorts the planes together at edgesIdentical to placing vias at port edge!
All Modes Accounted for?NO – Did not consider Parallel Plate mode
Even if we did, the via (port edge) cuts off mode ð Reason vias are used!
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Example
Stripline
ParallelPlate
Mode Matching
Ansoft Designer - Circuit
Wave Port
2 Terminals
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Example
No DCReturn Path
Port InfluencesResults
Lumped Port
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Example
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Closing Remarks
Understand Boundary Conditions!
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