边界与端口设置 电子科技大学 贾宝富 博士
2-1
HFSS Boundary List
Perfect E and Perfect H/Natural Ideal Electrically or Magnetically Conducting Boundaries ‘Natural’ denotes Perfect E ‘cancellation’ behavior Finite Conductivity Lossy Electrically Conducting Boundary, with user-provided conductivity and permeability Impedance Used for simulating ‘thin film resistor’ materials, with user-provided resistance and reactance in Ω/Square Radiation An ‘absorbing boundary condition,’ used at the periphery of a project in which radiation is expected such as an antenna structure Symmetry A boundary which enables modeling of only a sub-section of a structure in which field symmetry behavior is assured. “Perfect E” and “Perfect H” subcategories Master and Slave ‘Linked’ boundary conditions for unit-cell studies of infinitely replicating geometry (e.g. a slow wave circuit & an antenna array) 2-2
HFSS Boundary Descriptions: Perfect E and Perfect H/Natural
r E perpendicular
Ø
Parameters: None
Perfect E is a perfect electrical conductor*
Perfect E Boundary*
r E parallel
Perfect H is a perfect magnetic conductor
Perfect H Boundary
r E continuous ‘Natural’ Boundary
*NOTE: When you define a solid object as a ‘perf_conductor’ in the Material Setup, a Perfect E boundary condition is applied to its exterior surfaces!!
Forces E-field perpendicular to the surface Represent metal surfaces, ground planes, ideal cavity walls, etc. Forces H-field perpendicular to surface, Efield tangential Does not exist in the real world, but represents useful boundary constraint for modeling
Natural denotes effect of Perfect H applied on top of some other (e.g. Perfect E) boundary
‘Deletes’ the Perfect E condition, permitting but not requiring tangential electrical fields. Opens a ‘hole’ in the Perfect E plane 2-3
Perfect H for 2D Aperture (I) Ø
Monopole Over a Ground plane
Perfect H
Perfect H Surface Interior to the Problem Space Behaves Like an Infinitely Thin 2D Aperture
2-4
Perfect H for 2D Aperture (II) Ø
Small Hole Can be “Cut” in infinitely Thin Septum Between the Upper and Lower Guide Using a Perfect H Surface at the Hole Perfect H
2-5
HFSS Boundary Descriptions: Finite Conductivity Ø
r E perpendicular , attenuatin g
Parameters: Conductivity and Permeability
Finite Conductivity is a lossy electrical conductor
Finite Conductivity Boundary
*NOTE: When you define a solid object as a non-ideal metal (e.g. copper, aluminum) in the Material Setup module, and it is set to ‘Solve Surface’, a Finite Conductivity boundary is automatically applied to its exterior faces!!
E-field forced perpendicular, as with Perfect E However, surface impedance takes into account resistive and reactive surface losses
User inputs conductivity (in siemens/meter) and relative permeability (unitless) Used for non-ideal conductor analysis*
2-6
HFSS Boundary Descriptions: Impedance Ø
Parameters: Resistance and Reactance, ohms/square ( Ω/χ)
Impedance boundary is a direct, userdefined surface impedance
EXAMPLE: Resistor in Wilkenson Power Divider Resistor is 3.5 mils long (in direction of flow) and 4 mils wide. Desired lumped value is 35 ohms.
3 .5 = 0.875 4 Rlumped 35 = = 40 Ω / square Rsheet = .875 N
Use to represent thin film resistors Use to represent reactive loads Reactance will NOT vary with frequency, so does not represent a lumped ‘capacitor’ or ‘inductor’ over a frequency band.
Calculate required impedance from desired lumped value, width, and length
N=
Length (in direction of current flow) ÷ Width = number of ‘squares’ Impedance per square = Desired Lumped Impedance ÷ number of squares 2-7
HFSS Boundary Descriptions: Radiation Ø
Parameters: None
Boundary is λ/4 away from horn aperture in all directions.
Note boundary does not follow ‘break’ at tail end of horn. Doing so would result in a convex surface to interior radiation.
A Radiation boundary is an absorbing boundary condition, used to mimic continued propagation beyond the boundary plane Absorption is achieved via a secondorder impedance calculation Boundary should be constructed correctly for proper absorption Distance: For strong radiators (e.g. antennas) no closer than λ/4 to any structure. For weak radiators (e.g. a bent circuit trace) no closer than λ/10 to any structure Orientation: The radiation boundary absorbs best when incident energy flow is normal to its surface Shape: The boundary must be concave to all incident fields from within the modeled space
2-8
HFSS Boundary Descriptions: Radiation, cont.
Reflection of Radiation Boundary in dB, vs. Angle of Incidence relative to boundary normal (i.e. for normal incidence, θ = 0)
Radiation boundary absorption profile vs. incidence angle is shown at left
20
Reflection Coefficient (dB)
Reflection Coefficient (dB)
0
-20 -40
-60 -80 -100 0
10
20
30
40
50
60
70
80
theta (deg)
ETM
θ
90
Note that absorption falls off significantly as incidence exceeds 40 degrees from normal Any incident energy not absorbed is reflected back into the model, altering the resulting field solution!
Implication: For steered-beam arrays, the standard radiation boundary may be insufficient for proper analysis. Solution: Use a Perfectly Matched Layer (PML) construction instead.
Incorporation of PMLs is covered in the Advanced HFSS training course. Details available upon request.
2-9
HFSS Boundary Descriptions: Symmetry Conductive edges, 4 sides
Ø
Parameters: Type (Perfect E or Perfect H)
This rectangular waveguide contains a symmetric propagating mode, which could be modeled using half the volume vertically.... Perfect E Symmetry (top)
...or horizontally.
Symmetry boundaries permit modeling of only a fraction of the entire structure under analysis Two Symmetry Options:
Symmetry boundaries also have further implications to the Boundary Manager and Fields Post Processing
Perfect H Symmetry (left side)
Perfect E : E-fields are perpendicular to the symmetry surface Perfect H : E-fields are tangential to the symmetry surface
Existence of a Symmetry Boundary will prompt ‘Port Impedance Multiplier’ verification Existence of a symmetry boundary allows for near- and far-field calculation of the ‘entire’ structure 2-10
HFSS Boundary Descriptions: Symmetry, cont.
TE20 Mode in WR90
Geometric symmetry does not necessarily imply field symmetry for higher-order modes Symmetry boundaries can act as mode filters
Perfect E Symmetry (top)
Properly represented with Perfect E Symmetry
Mode can not occur properly with Perfect H Symmetry
As shown at left, the next higher propagating waveguide mode is not symmetric about the vertical center plane of the waveguide Therefore one symmetry case is valid, while the other is not!
Implication: Use caution when using symmetry to assure that real behavior in the device is not filtered out by your boundary conditions!!
Perfect H Symmetry (right side)
2-11
HFSS Boundary Descriptions: Master/Slave Boundaries Perfectly Matched Layer (top)
Ø
Parameters: Coordinate system, master/slave pairing, and phasing
Master Boundary
Slave Boundary
Master and Slave boundaries are used to model a unit cell of a repeating structure
V-axis
Origin
WG Port (bottom)
U-axis
Constraints:
Ground Plane
Unit Cell Model of End-Fire Waveguide Array
Also referred to as linked boundaries Master and Slave boundaries are always paired: one master to one slave The fields on the slave surface are constrained to be identical to those on the master surface, with a phase shift.
The master and slave surfaces must be of identical shapes and sizes A coordinate system must be identified on the master and slave boundary to identify point-to-point correspondence 2-12
HFSS Source List
Port
Incident Wave
Used for RCS or Propagation Studies (e.g. Frequency-Selective Surfaces) Results must be post-processed in Fields Module; no S-parameters can be provided Applies to entire volume of modeled space
Voltage Drop or Current Source
Most Commonly Used Source. Its use results in S-parameter output from HFSS. Two Subcategories: ‘Standard’ Ports and ‘Gap Source’ Ports Apply to Surface(s) of solids or to sheet objects
‘Ideal’ voltage or current excitations Apply to Surface(s) of solids or to sheet objects
Magnetic Bias
Internal H Field Bias for nonreciprocal (ferrite) material problems Applies to entire solid object representing ferrite material 2-13
HFSS Source Descriptions: Port (I)
2-14
HFSS Source Descriptions: Port (II) EXAMPLE STANDARD PORTS
Ø
Parameters: Mode Count, Calibration, Impedance, Polarization, Imp. Multiplier
A port is an aperture through which guided electromagnetic field energy is injected into a 3D HFSS model. There are two types:
EXAMPLE GAP-SOURCE PORTS
Standard Ports: The aperture is solved using a 2D eigensolution which locates all requested propagating modes Characteristic impedance is calculated from the 2D solution Impedance and Calibration Lines provide further control Gap Source Ports: Approximated field excitation is placed on the gap source port surface Characteristic impedance is provided by the user during setup 2-15
Impedance, Calibration and Polarization Lines Ø
Ø
Impedance line, calibration line and polarization line are optional in port setup. They are located in the port and have a starting point and an end point.
I,C, and/or P Line Port = cross section of waveguide
2-16
Impedance Line Ø
Ø Ø Ø
Without impedance line, HFSS computes port impedance from power and current: Zpi With impedance line, a voltage can be defined: ∫ E•dl . Two more port impedances result: Zpv and Zvi . These are not the same for non-TEM transmission lines.
2-17
Calibration Line Ø Ø
Calibration lines remove 180o phase ambiguity. This helps to obtain correct phase in S21 and S12 .
waveguide, side view
2-18
Polarization Line
Ø
Imposes polarization in case of ambiguity, e.g. in square or circular guides with degenerate modes.
Port = cross section of square waveguide
2-19
HFSS Source Descriptions: Incident Wave (I)
2-20
HFSS Source Descriptions: Incident Wave (II) Ø
Parameters: Poynting Vector, Efield Magnitude and Vector
Used for radar cross section (RCS) scattering problems. Defined by Poynting Vector (direction of propagation) and Efield magnitude and orientation
In the above example, a plane incident wave is directed at a solid made from dielectrics, to view the resultant scattering fields.
Poynting and E-field vectors must be orthogonal. Multiple plane waves can be created for the same project.
If no ‘ports’ are present in the model, S-parameter output is not provided
Analysis data obtained by postprocessing on the Fields using the Field Calculator, or by generating RCS Patterns 2-21
HFSS Source Descriptions: Voltage Drop and Current Source (I)
Voltage Drop
Current Drop
2-22
HFSS Source Descriptions: Voltage Drop and Current Source (II) Example Current Source (along trace or across gap)
Ø
Parameters: Direction and Magnitude
Example Voltage Drop (between trace and ground)
A voltage drop would be used to excite a voltage between two metal structures (e.g. a trace and a ground) A current source would be used to excite a current along a trace, or across a gap (e.g. across a slot antenna) Both are ‘ideal’ source excitations, without impedance definitions
No S-Parameter Output
User applies condition to a 2D or 3D object created in the geometry
Vector identifying the direction of the voltage drop or the direction of the current flow is also required
2-23
Sources/Boundaries and Eigenmode Solutions
Ø
Ø
An Eigenmode solution is a direct solution of the resonant modes of a closed structure As a result, some of the sources and boundaries discussed so far are not available for an Eigenmode project. These are:
All Excitation Sources:
Ports Voltage Drop and Current Sources Magnetic Bias Incident Waves
The only unavailable boundary type is:
Radiation Boundary A Perfectly Matched Layer construction is possible as a replacement
2-24
HFSS Source Descriptions: Magnetic Bias Ø
Parameters: Magnitude and Direction or Externally Provided
The magnetic bias source is used only to provide internal biasing Hfield values for models containing nonreciprocal (ferrite) materials.
Bias may be uniform field (enter parameters directly in HFSS)... Parameters are direction and magnitude of the field ...or bias may be non-uniform (imported from external Magnetostatic solution package) Ansoft’s 3D EM Field Simulator provides this analysis and output
Apply source to selected 3D solid object (e.g. ferrite puck) 2-25
HFSS Ports: A Detailed Look Ø
Ø
The Port Solution provides the excitation for the 3D FEM Analysis. Therefore, knowing how to properly define and create a port is paramount to obtaining an accurate analysis. Incorrect Port Assignments can cause errors due to...
Ø
...Excitation of the wrong mode structure ...Bisection by conductive boundary ...Unconsidered additional propagating modes ...Improper Port Impedance ...Improper Propagation Constants ...Differing phase references at multiple ports ...Insufficient spacing for attenuation of modes in cutoff ...Inability to converge scattering behavior because too many modes are requested
Since Port Assignment is so important, the following slides will go into further detail regarding their creation. 2-26
HFSS Port Selection: Standard or Gap Source?
When would you choose to use a Gap Source Port over a Standard Port?
When the model has tightlyspaced lines
When a port reference location is difficult to determine using a Standard port
When you’d like to use a voltage gap, but want Sparameter output
Gap Source Ports (blue)
2-27
HFSS Ports: Sizing Ø
A port is an aperture through which a guided-wave mode of some kind propagates
A Coaxial Port Assignment
For transmission line structures entirely enclosed in metal, port size is merely the waveguide interior carrying the guided fields
A Microstrip Port Assignment (includes air above substrate)
Rectangular, Circular, Elliptical, Ridged, Double-Ridged Waveguide Coaxial cable, coaxial waveguide, squareax, Enclosed microstrip or suspended stripline
For unbalanced or non-enclosed lines, however, field propagation in the air around the structure must also be included
Parallel Wires or Strips Stripline, Microstrip, Suspended Stripline Slotline, Coplanar Waveguide, etc. 2-28
HFSS Ports: Sizing, cont. Ø
Port too narrow (fields couple to side walls)
The port solver only understands conductive boundaries on its borders
Ø
Result: Moving the port edges too close to the circuitry for open waveguide structures (microstrip, stripline, CPW, etc.) will allow coupling from the trace circuitry to the port walls!
Port too Short (fields couple to top wall)
Electric conductors may be finite or perfect (including Perfect E symmetry) Perfect H symmetry also understood Radiation boundaries around the periphery of the port do not alter the port edge termination!!
This causes an incorrect modal solution, which will suffer an immediate discontinuity as the energy is injected past the port into the model volume 2-29
HFSS Ports: Sizing Handbook I 10w, w ≥ h or 5w (3h to 4h), w < h
Ø
Microstrip Port Sizing Guidelines
Ø 6h to 10h
Assume width of microstrip trace is w Assume height of substrate dielectric is h
Port Height Guidelines
Between 6h and 10h
w h
Note: Port sizing guidelines are not inviolable rules true in all cases. For example, if meeting the height and width requirements outlined result in a rectangular aperture bigger than λ/2 on one dimension, the substrate and trace may be ignored in favor of a waveguide mode. When in doubt, build a simple ports-only model and test.
Ø
Tend towards upper limit as dielectric constant drops and more fields exist in air rather than substrate Bottom edge of port coplanar with the upper face of ground plane (If real structure is enclosed lower than this guideline, model the real structure!)
Port Width Guidelines
10w, for microstrip profiles with w ≥ h 5w, or on the order of 3h to 4h, for microstrip profiles with w < h
2-30
HFSS Ports: Sizing Handbook II Ø
Stripline Port Sizing Guidelines
8w, w ≥ h or 5w (3h to 4h), w < h
Ø
Port Height Guidelines
w h
Ø
Extend from upper to lower groundplane, h
Port Width Guidelines
Ø
Assume width of stripline trace is w Assume height of substrate dielectric is h
8w, for microstrip profiles with w ≥ h 5w, or on the order of 3h to 4h, for microstrip profiles with w < h
Boundary Note: Can also make side walls of port Perfect H boundaries
2-31
HFSS Ports: Sizing Handbook III Ø
Slotline Port Guidelines
Ø Approx 7g minimum
Port Height:
Larger of 4h or 4g
Should be at least 4h, or 4g (larger) Remember to include air below the substrate as well as above!
g h
Assume slot width is g Assume dielectric height is h
Ø
If ground plane is present, port should terminate at ground plane
Port Width:
Should contain at least 3g to either side of slot, or 7g total minimum Port boundary must intersect both side ground planes, or they will ‘float’ and become signal conductors relative to outline ‘ground’ 2-32
HFSS Ports: Sizing Handbook IV Ø
CPW Port Guidelines
Larger of approx. 10g or 10s
Ø
s h
Port Height:
Larger of 4h or 4g
Assume slot width is g Assume dielectric height is h Assume center strip width is s
g
Should be at least 4h, or 4g (larger) Remember to include air below the substrate as well as above!
Ø
If ground plane is present, port should terminate at ground plane
Port Width:
Should contain 3-5g or 3-5s of the side grounds, whichever is larger
Total about 10g or 10s
Port outline must intersect side grounds, or they will ‘float’ and become additional signal conductors along with the center strip. 2-33
CPW Wave Ports: Starting Recommendations Wave Port Size The standard recommendation for most CPW wave ports is a rectangular aperture Port width should be no less than 3 x the overall CPW width, or 3 x (2g + w) Port height should be no less than 4 x the dielectric height, or 4h
Wave Port Location The wave port should be centered horizontally on the CPW trace If the port is on GCPW, the port bottom edge should lie on the substrate bottom ground plane If the port is on ungrounded CPW, the port height should be roughly centered on the CPW metal layer
Wave Port Restrictions As with all wave ports, there must be only one surface normal exposed to the field volume Port should be on exterior model face, or capped by a perfect conductor block if internal The wave port outline must contact the side grounds (all CPWs) and bottom ground (GCPW) The wave port size should not exceed lambda/2 in any dimension, to avoid permitting a rectangular waveguide modal excitation 3 (2g + w) 3 (2g + w)
4h minimum
4h minimum w w
h
g h
g
Ungrounded CPW
Grounded CPW
(Port height centered on trace)
(Port height begins at lower ground)
2-34
HFSS Ports: Sizing Handbook V; Gap Source Ports Perfect E
Ø
Gap Source ports behave differently from Standard Ports
Perfect H Perfect H
Ø Perfect E
Gap Source Port Sizing (microstrip example):
Perfect H
Perfect H
Any port edge not in contact with metal structure or another port assumed to be a Perfect H conductor “Strip-like”: [RECOMMENDED] No larger than necessary to connect the trace width to the ground “Wave-like”: No larger than 4 times the strip width and 3 times the substrate height
Perfect H
Perfect E
Ø
The Perfect H walls allow size to be smaller than a standard port would be However, in most cases the strip-like application should be as or more accurate
Further details regarding Gap Source Port sizing available as a separate presentation 2-35
HFSS Port Selection Example: Parallel Traces Spaced by 8 or more times Trace Width Inputs sufficiently isolated that no coupling behavior should occur Sufficient room for Wave port apertures around each trace Use Wave Ports as shown
Spaced by 4 – 8 times Trace Width Inputs still fairly isolated, little to no coupling behavior should occur Insufficient room for Wave port apertures around each trace without clipping fringing fields Use Lumped Ports as shown
Spaced by less than 4 times Trace Width Traces close enough to exhibit coupling Even and Odd modes possible; N modes total for N conductors and one ground reference [odd mode shown at right] Lumped Ports from trace to ground neglect coupling behavior and are no longer appropriate Use multi-mode Wave Port Terminal line assignments can permit extraction of Sparameters referenced to each ‘trace’ 2-36
HFSS Ports: Spacing from Discontinuities
Structure interior to the modeled volume may create and reflect non-propagating modes
Port Extension
If the port is spaced too close to a discontinuity causing this effect, the improper solution will be obtained
These modes attenuate rapidly as they travel along the transmission line
A port is a ‘matched load’ as seen from the model, but only for the modes it has been designed to handle Therefore, unsolved modes incident upon it are reflected back into the model, altering the field solution
Remedy: Space your port far enough from discontinuities to prevent non-propagating mode incidence
Spacing should be on order of port size, not wavelength dependent 2-37
HFSS Ports: Single-Direction Propagation Port on Exterior Face of Model
Standard ports must be defined so that only one face can radiate energy into the model
Position Standard Ports on the exterior of the geometry (one face on background) or provide a port cap.
Port Inside Modeled Air Volume; Back side covered with Solid Cap
Gap Source Ports have no such restriction
Cap should be the same dimensions as the port aperture, be a 3D solid object, and be defined as a perfect conductor in the Material Setup module
2-38
HFSS Ports: Mode Count
Ports should solve for all propagating modes
However, requesting too many modes in the full solution also negatively impacts analysis
Circular waveguide, showing two orthogonal TE11 modes and TM01 mode (radial with Z-component). Neglecting the TM01 mode from your solution would cause incorrect results.
Modes in cutoff are more difficult to calculate; Sparameters for interactions between propagating and non-propagating modes may not converge well
What if I don’t know how many modes exist?
Ignoring a mode which does propagate will result in incorrect S-parameters, by neglecting modeto-mode conversion which could occur at discontinuities
Build a simple model of a transmission line only, or run your model in “Ports Only” mode, and check! You can alter the mode count before running the full solution.
Degenerate mode ordering is controlled with calibration lines (see next slide) 2-39
HFSS Ports: Degenerate Modes
In circular or square waveguide, use the calibration line to force (polarize) the mode numbering of the two degenerate TE11 modes. This is also useful because without a polarization orientation, the two modes may be rotated to an arbitrary angle inside circular WG.
Degenerate modes have identical impedance, propagation constants Port solver will arbitrarily pick one of them to be ‘mode(n)’ and the other to be ‘mode(n+1)’
To enforce numbering, use a calibration line and polarize the first mode to the line OR, introduce a dielectric change to slightly perturb the mode solution and separate the degenerate modes
For parallel lines, a virtual object between them aids mode ordering. Note virtual object need not extend entire length of line to help at port.
Thus, mode-to-mode S-parameters may be referenced incorrectly
Example: A dielectric bar only slightly higher in permittivity than the surrounding medium will concentrate the E-fields between parallel wires, forcing the differential mode to be dominant If dielectric change is very small (approx. 0.001 or less), impedance impact of perturbation is negligible 2-40
HFSS Ports: Impedance Definitions
HFSS provides port characteristic impedances calculated using the powercurrent definition (Zpi)
For many transmission line types, the powervoltage or voltage-current definition is preferred
For a Coax, the impedance line extends radially from the center to outer conductor (or vice versa). Integrating the E-field along the radius of the coaxial dielectric provides the voltage difference. In many instances, the impedance and calibration lines are the same!
Incident power is known excitation quantity Port solver integrates H-field around port boundary to calculate current flow
Slot line, CPW: Zpv preferred TEM lines: Zvi preferred
HFSS can provide these characteristic impedance values, as long as an impedance line is identified
The impedance line defines the line along which the E-field is integrated to obtain a voltage Often it can be identical to the calibration line 2-41
HFSS Ports: Phase Calibration
A second purpose of the calibration line is to control the port phase references
Which of the above field orientations is the zero degree phase reference? Calibration Line defines...
The 2D port eigensolver finds propagating modes on each port independently The zero degree phase reference is chosen at a point of maximum E-field intensity on the port face. This occurs twice, with 180 degrees separation, for each 360 degree cycle Therefore the possibility exists for the software to select inconsistent phase references from port to port, resulting in S-parameter errors All port-to-port S-parameter phases, e.g. S21, will be off by 180 degrees
Solution: The calibration line defines the preferred direction for the zero degree reference on each port.
2-42
HFSS Ports: Impedance Multiplier
Whole Rectangular WG (No Symmetry) Impedance Mult = 1.0
When symmetry is used in a model, the automatic Zpi and impedance linedependant Zpv and Zvi calculations will be incorrect, since the entire port aperture is not represented.
Half Rectangular WG (Perfect E Symmetry) Impedance Mult = 2.0
Half Rectangular WG (Perfect H Symmetry) Impedance Mult = 0.5 ...and for Quarter Rectangular WG? (Both Perfect E and H Symmetry) Impedance Mult. = 1.0
Split the model with a Perfect E symmetry case, and the impedance is halved. Split the model with a Perfect H symmetry case, and the impedance is doubled.
The port impedance multiplier is just a renormalizing factor, used to obtain the correct impedance results regardless of the symmetry case used. The impedance multiplier is applied to all ports, and is set during the assignment of any port in the model. 2-43
