Ansoft HFSS Version 7
Training
Section 5: Boundary Module
smart software for high-frequency design
5-1
Synopsis
General Overview
■
■
■
Boundary Types, Definitions, and Parameters
Source Types, Definitions, and Parameters
Interface Layout
Assigning Boundaries
■
■
■
■
Boundary Setup Exercise Part 1: Define Boundaries in
Example Model
■
■
Face Selection
Precedence
Assumptions (the ‘outer’ Boundary)
Details of Port Definition and Creation
■ Size and Position
■ Mode Count
■ Degenerate Modes
■ Calibration, Impedance, and Polarization
■ Gap Source Ports
Boundary Setup Exercise Part 2: Add ports to Example Model
smart software for high-frequency design
5-2
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/
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. an antenna array)
smart software for high-frequency design
5-3
HFSS Boundary Descriptions: Perfect E and
Perfect H/Natural
E perpendicular
Parameters: None
■
Perfect E is a perfect electrical conductor*
■
■
Perfect E Boundary*
E parallel
■
Perfect H is a perfect magnetic conductor
■
Perfect H Boundary
■
E continuous
■
‘Natural’ Boundary
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
smart software for high-frequency design
*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!!
■
5-4
HFSS Boundary Descriptions: Finite
Conductivity
Parameters: Conductivity and
Permeability
■
E perpendicular , attenuatin g
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*
smart software for high-frequency design
5-5
HFSS Boundary Descriptions: Impedance
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
N=
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
■
■
Length (in direction of current flow) ÷
Width = number of ‘squares’
Impedance per square = Desired
Lumped Impedance ÷ number of
squares
smart software for high-frequency design
5-6
HFSS Boundary Descriptions: Radiation
Parameters: None
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
smart software for high-frequency design
■
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.
5-7
HFSS Boundary Descriptions: Radiation,
cont.
■
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
90
theta (deg)
Reflection of Radiation Boundary in dB, vs.
Angle of Incidence relative to boundary
normal (i.e. for normal incidence, θ = 0)
■
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.
smart software for high-frequency design
5-8
HFSS Boundary Descriptions: Symmetry
Conductive edges, 4 sides
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
smart software for high-frequency design
5-9
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)
smart software for high-frequency design
5-10
HFSS Boundary Descriptions: Master/Slave
Boundaries
Perfectly Matched Layer
(top)
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
smart software for high-frequency design
5-11
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
smart software for high-frequency design
5-12
HFSS Source Descriptions: Port
EXAMPLE STANDARD PORTS
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
smart software for high-frequency design
5-13
HFSS Source Descriptions: Incident Wave
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
smart software for high-frequency design
5-14
HFSS Source Descriptions: Voltage Drop and
Current Source
Example Current
Source (along trace
or across gap)
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
smart software for high-frequency design
5-15
HFSS Source Descriptions: Magnetic Bias
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)
smart software for high-frequency design
5-16
Sources/Boundaries and Eigenmode
Solutions
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
smart software for high-frequency design
5-17
The HFSS Source/Boundary Setup Interface
Menu and Toolbar
Side Window
Coordinate Fields and
Snap Options
Pick Options
Controls selection options
in graphical window
Source/Boundary List
Shows all sources and
boundaries currently
assigned to the project
and their status; allows
selection for viewing,
editing, and deletion
Source/Boundary Control
Allows Naming, contains execution
controls (Assign, Clear, Units...)
Graphical View Window
Shows geometry, permits
point-and-click selection,
vector definition, and
assignment.
Source/Boundary Selection Buttons
Source/Boundary Drop-Down
Lists all source or boundary types,
based on radio button selected
Boundary Attributes Field
Region Layout changes to provide
entry fields for selected source or boundary
characteristics and options.
smart software for high-frequency design
5-18
Boundary Manager: Object/Face Selection
The Graphical Pick options (1)
control the result of clicking in the
graphical view window.
■ Object: mouse-click selects
exterior of entire object
■ Face: mouse-click selects
closest face of object
■ Boundary: mouse-click selects
2.
closest existing boundary
condition (if any)
■ To shift your focus to an object or
face deeper into the model, use the
right mouse menu (2) choice Next
Behind, or the hotkey “N”
■ Selected faces will highlight in a grid
NOTE: The same graphical view manipulation
pattern; selected objects will have
shortcuts for rotation, panning, and zooming found
their wireframe highlighted
in the Draw module also work here; the visibility
icon also assists object/face selection by ‘hiding’
■ Multiple faces may be selected
exterior objects.
simultaneously; a second click
deselects already-selected faces
smart software for high-frequency design
■
1.
5-19
Boundary Manager: Object/Face Selection,
cont.
■
3.
4.
■
The Edit menu (3) provides further
Select options, including Faces
Intersection
■ Faces intersection opens a list
box containing all objects in
the model
■ Selecting two touching objects
from the list will prompt the
interface to automatically find
all intersecting faces
■ Note: only exterior faces in
intersection are selected, not
faces of one object which are
inside the volume of the other
The Edit menu Select option By
Name (4) provides a list of all faces
in the model, numbered and sorted
by object, for selection.
smart software for high-frequency design
5-20
Boundary Assignment: General Procedure
■
■
2. Select face(s)
5. New Boundary will
appear in list
1. Select source or boundary and type
4. Name and Assign
3. Fill in Parameters as necessary
■
■
Select Source or
Boundary radio button,
and desired type from
the drop-down listing
Select the face or faces
on which you wish to
apply the
source/boundary
condition
■ (Above 2 steps
interchangeable)
Fill in any necessary
parameters for the
source/boundary
Name the
source/boundary, and
press the Assign button
smart software for high-frequency design
5-21
Boundary Assignment: Precedence
■
Boundary assignments are
order dependent:
■
■
In the pictured example, the ‘radiation’
boundary overlays the orange rectangle
(on the back face) which was earlier
assigned as the port. Ports, however,
always take precedence, and show at the
bottom of the boundary listing.
Boundaries assigned later
supercede those
assigned earlier over any
shared surfaces
Ports are the exception;
they always supercede
any earlier or later
assignments
■ Ports will sort to the
bottom of the
boundary list to
reflect this fact
■ Boundaries can be
re-prioritized using
the Model menu
smart software for high-frequency design
5-22
Boundary Assignment: Default Boundary
■
Any exterior face of the
modeled geometry not
given a user-defined
boundary condition is
assumed to be a Perfect E
■
■
■
Default boundary called
outer
Imagine entire model
buried in solid metal
unless you instruct
otherwise
To view boundaries and
see if you missed an
assignment, use the
Boundary Display pick from
the Model menu
Graphical window shows
both user and autoassigned boundaries
smart software for high-frequency design
■
5-23
Boundary Setup Exercise Part 1
■
■
■
NOTE: The model for this exercise is nearly
identical to that used in the Material Setup
exercise, but has been split in half along the axis
of the microstrip and coax feed to demonstrate
symmetry boundary application as well.
■
We will practice by
assigning boundaries to a
Coax to Microstrip
transformer model
This exercise is only Part 1
of the entire operation;
excitation assignment will
be covered after a detailed
description of HFSS
sources and port
assignment
In the Maxwell Project
Manager, find the project
entitled “bnd_exer” and
Open it
Once open, proceed to
Setup Boundaries/Sources
smart software for high-frequency design
5-24
Boundary Setup Exercise: Trace Metalization
NOTE: Since solid Material parameters are already
applied, there is already a boundary on the exterior of the
metal objects “pin”, “pin1”, and “pin2”. We only need to
apply the surface metalization for the actual microstrip
trace line, and define outer radiation, ground plane, and
symmetry boundaries.
5.
1. Select the Boundary radio Button.
2. From the list of available boundaries, select Perfect E.
3.
3. Set the Graphical Pick option to Face.
4. Click in the graphical window as if you are touching the
trace. The nearest face of the air box will highlight, since it
is between your view and the trace.
7.
4.
5. Right-click to bring up the pop-up menu and select Next
Behind, or use the “N” key on the keyboard to shift focus
deeper. Continue this operation until the trace is selected.
NOTE: If you appear to have selected the bottom-most
face of the model, you have gone too far. Use the rightclick menu to pick Deselect All and start over.
6.
1.
2.
6. In the Name field, type in “trace_metal”, and click the
Assign button.
7. The boundary should appear in the boundary list at left.
smart software for high-frequency design
5-25
Boundary Setup Exercise: Radiation
1. The Boundary radio button should remain selected.
2. From the list of available boundaries, select Radiation.
3. Leave the Graphical Pick option set to Face.
4. Click in the graphical window to touch the air volume
surrounding the structure on the three faces indicated. You
may wish to rotate to facilitate your selection.
3.
NOTE: Had this model been constructed with the air solid
sitting on top of the substrate solid, instead of containing
the substrate solid, we would have to pick specific faces on
three sides of the substrate object as well.
4. (Back, right side, top)
6.
5. In the Name field, type in “absorbing”, and click the
Assign button.
6. The boundary should appear in the boundary list at left.
1.
5.
2.
NOTE: We have assigned a Radiation boundary over
where the microstrip port will need to be! This will be
superceded in a step in part 2 of this exercise, following the
Source discussion.
smart software for high-frequency design
5-26
Boundary Setup Exercise: Ground Plane
1. The Boundary radio button should remain
selected.
2. From the list of available boundaries, select
Perfect E.
3. Leave the Graphical Pick option set to Face.
4. Either rotate the model view to bring the lower
face to the front, and click on it, or click as though
touching the lower face of the air volume and use
the “N” key to shift focus deeper to the lower
surface of the air volume and substrate.
3.
5. In the Name field, type in “ground_plane”, and
click the Assign button.
4.
6.
6. The boundary should appear in the boundary list
at left.
1.
2.
NOTE: Since this is being assigned a Perfect E
boundary, we could have allowed the automatic
“outer” boundary to take care of this face if we
wished.
5.
smart software for high-frequency design
5-27
Boundary Setup Exercise: Symmetry Plane
1. The Boundary radio button should remain selected.
2. From the list of available boundaries, select Symmetry.
3. Leave the Graphical Pick option set to Face.
4.
4. Click on the face of the model which bisects the
microstrip trace and coax. Once a face is selected, the
options for the Symmetry boundary appear below the
graphical view. Click again in the model to select the cut
faces of the ‘thru_hole_in_wall’ and “coax_outer” cylinders
as well. (You may wish to zoom in to assure you have the
correct faces selected.)
3.
NOTE: Again, if we had defined our air volume to sit atop
rather than to contain the substrate, we would need to
select the substrate face too.
7.
5. In the parameter space for the boundary, click the
radio button for Perfect H type symmetry (E-fields
tangential to surface).
4.
6. In the Name field, type in “mag_symmetry”, and click
the Assign button.
4.
1.
6.
5.
2.
7. The boundary should appear in the list at left.
THIS CONCLUDES PART 1 OF THE BOUNDARY
SETUP EXERCISE. DO NOT EXIT THE BOUNDARY/
SOURCE MANAGER.
smart software for high-frequency design
5-28
HFSS Ports: A Detailed Look
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...
■
■
■
■
■
■
■
■
...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.
smart software for high-frequency design
5-29
HFSS Ports: Setup Interface
Name Field
Ports are always named
“portN”. Box also includes
Assign, Clear, and Options
buttons.
Mode Entry Field
Set port mode solution requirements.
Set polarization. Shows impedance
and calibration definitions applied, if
any.
Lumped Gap Source Port Option
Activating enables Port Impedance entry fields.
Impedance and Calibration Line Fields
‘Edit Line’ dropdown allows setting,
clearing, and relating Imped. and Calib.
lines.
Impedance Multiplier Field
Use if symmetry planes intersect ports.
smart software for high-frequency design
5-30
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 ‘backing’ the port
would be too disruptive of
internal fields
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)
smart software for high-frequency design
5-31
HFSS Ports: Sizing
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.
smart software for high-frequency design
■
5-32
HFSS Ports: Sizing, cont.
The port solver only understands
conductive boundaries on its borders
Port too narrow (fields couple
to side walls)
■
■
■
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!!
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)
This causes an incorrect modal solution,
which will suffer an immediate
discontinuity as the energy is injected past
the port into the model volume
smart software for high-frequency design
5-33
HFSS Ports: Sizing Handbook I
10w, w ≥ h
or
5w (3h to 4h), w < h
Microstrip Port Sizing Guidelines
■
■
Assume width of microstrip trace is w
Assume height of substrate dielectric
is h
Port Height Guidelines
6h to
10h
■
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.
■
■
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
smart software for high-frequency design
5-34
HFSS Ports: Sizing Handbook II
Stripline Port Sizing Guidelines
■
8w, w ≥ h
or
5w (3h to 4h), w < h
■
Port Height Guidelines
■
w
h
Assume width of stripline trace is w
Assume height of substrate dielectric is h
Extend from upper to lower groundplane,
h
Port Width Guidelines
■
■
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
smart software for high-frequency design
5-35
HFSS Ports: Sizing Handbook III
Slotline Port Guidelines
■
■
Assume slot width is g
Assume dielectric height is h
Port Height:
Approx 7g minimum
■
■
Larger of 4h or 4g
■
g
h
Should be at least 4h, or 4g (larger)
Remember to include air below the
substrate as well as above!
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’
smart software for high-frequency design
5-36
HFSS Ports: Sizing Handbook IV
CPW Port Guidelines
■
■
Larger of approx. 10g or 10s
■
Assume slot width is g
Assume dielectric height is h
Assume center strip width is s
Port Height:
Larger of 4h or 4g
s
h
■
■
g
Should be at least 4h, or 4g (larger)
Remember to include air below the substrate
as well as above!
■
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.
smart software for high-frequency design
5-37
HFSS Ports: Sizing Handbook V; Gap Source
Ports
Gap Source ports behave differently from
Standard Ports
Perfect E
■
Perfect H
Perfect H
Any port edge not in contact with metal structure
or another port assumed to be a Perfect H
conductor
Gap Source Port Sizing (microstrip example):
Perfect E
■
Perfect H
■
Perfect H
“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
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
smart software for high-frequency design
5-38
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
smart software for high-frequency design
5-39
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
smart software for high-frequency design
5-40
HFSS Ports: Mode Count
■
Ports should solve for all propagating modes
■
■
However, requesting too many modes in the full
solution also negatively impacts analysis
■
■
■
■
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?
■
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.
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)
smart software for high-frequency design
5-41
HFSS Ports: Degenerate Modes
■
■
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)’
■
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.
■
■
Thus, mode-to-mode S-parameters may be
referenced incorrectly
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
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
smart software for high-frequency design
■
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.
5-42
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.
smart software for high-frequency design
5-43
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
smart software for high-frequency design
5-44
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.
smart software for high-frequency design
5-45
Source Setup Exercise: Part 2
■
■
We will now complete the
Setup Boundaries/
Sources exercise already
begun, by adding the two
necessary Ports to the
problem
Ports will use both
calibration and
impedance lines, but will
require only one mode on
each terminal
smart software for high-frequency design
5-46
Source Setup Exercise: Coaxial Port
1. Select the Source radio button.
2. The source list should set by default to Port.
3. Zoom in on your model, or otherwise orient
it so you have clear visual access to the
extended face of the coaxial line. Click on the
face to select it. The port parameter entry
fields will now appear.
6.
4. Leave the port name as “port1”, and the
number of modes as “1”.
3.
7.
2.
1.
4.
5.
5. Check the box for Use Impedance Line.
This enables the Edit Line dropdown menu
beside it. In the Edit Line dropdown, pick Set...
6. The side window will prompt you to Set
Impedance Start. Define a starting point for the
impedance line by clicking in the graphical
window to snap to the vertex on the inner
conductor, at the topmost point where it
intersects the symmetry plane. (A first click
may be necessary to activate the window
before selecting vertices.)
7. Click the Enter button in the side window to
confirm your point selection. The window now
shifts to request the vector information.
(proceed to next page)
smart software for high-frequency design
5-47
Source Setup Exercise: Coaxial Port, cont.
The interface will now show a vector from the
starting point you defined to the origin. (This is
merely a default ‘guess’ at the intended
endpoint.) The side window shows vector entry
fields.
8. In the graphical window, snap to the point
radial from the starting point (on the outer
conductor radius, at the topmost intersection
with the symmetry plane). The vector fields
should update to reflect a Z-directed vector.
8.
9. Press the Enter button on the side window
to confirm the vector end point. The side
window interface closes, and the completed
impedance line is displayed as a red vector
with the letter “I”.
9.
10. Check the box for Use Calibration Line. In
the enabled Edit Line dropdown to the right,
pick Copy Impedance. The vector will now
update to include a “C” indication.
11.
11. Press the Assign button to complete the
port creation. The boundary list will now
update to show “port1”
10.
smart software for high-frequency design
5-48
Source Setup Exercise: Microstrip Port
Rotate and resize the graphical window so that
you have visual access to the microstrip
termination end of the model.
1. Source radio button and Port type are
already selected.
2. Click on the 2D rectangle provided for the
microstrip port face. If the entire face of the air
volume is selected, use the Next Behind menu
pick or “N” hotkey to shift the selection.
2.
3. Leave the port name as “port2”, and the
number of modes as “1”.
5.
4. Use Impedance Line should remain checked
from the prior port assignment. In the Edit Line
dropdown, pick Set...
6.
5. The side window will prompt you to Set
Impedance Start. Define a starting point for the
impedance line by clicking in the graphical
window to snap to the vertex on the trace, at
the point where it intersects the symmetry
plane.
1.
3.
4.
6. Click the Enter button in the side window to
confirm your point selection. The window now
shifts to request the vector information.
(proceed to next page)
smart software for high-frequency design
5-49
Source Setup Exercise: Microstrip Port, cont.
The interface will now show a vector from the
starting point you defined to the prior port’s
ending point. The side window shows vector
entry fields.
12.
7. In the graphical window, snap to the point
where the ground plane intersects the symmetry
face.
8. Press the Enter button on the side window to
confirm the end point. The side window interface
closes, and the completed line is displayed.
9. Use Calibration Line should already be
checked. In the enabled Edit Line dropdown to
the right, pick Copy Impedance.
7.
8.
10. Before assigning the port, we need to set the
Impedance Multiplier for the model. Enter a
value of 0.5.
11. Press the Assign button to complete the port
assignment. You will receive an overlap
warning, because the port overlays the earlier
“radiation” boundary. After the overlap warning
message is dismissed “port2” will show in the
boundary list.
11.
9.
10.
12. We are now done with boundary
assignment. To verify our assignment, pick
Boundary Display from the Model menu.
smart software for high-frequency design
5-50
Source Setup Exercise: Verifying Boundaries
HFSS will now perform only the initial meshing
necessary to subdivide the problem into
tetrahedra, so that actual boundary application to
the finite element mesh can be viewed.
13.
14.
13. The list of assigned boundaries is on the left.
Note that it contains both boundaries we created,
plus the boundaries “i_pinn” and “outer”. The
“i_pinn” boundaries were assigned as a result of
assigning a finite conductivity metal -- copper -to the pin objects. The “outer” boundary is
applied to any surface of the model we did not
otherwise define. Highlight “outer” in the
boundary listing.
14. Press the Toggle Display button. The mesh
on the selected boundary is displayed, indicating
the surfaces on which this boundary is applied.
Note that it provides the Perfect E definition on
the outer conductor of the coax, on the outer
conductor of the thru hole, and on the front face
of the model which represents the metal module
wall.
15.
15. If you wish you may continue displaying
additional boundaries. When you are through,
press the Close button to return to the Setup
Boundaries window. There, pick Exit from the
File menu and save when prompted. (The
overlap warning will repeat on exit.)
smart software for high-frequency design
5-51