lab1 microstrip line fem fdtd
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Lab 1: Microstrip Line
1-1
Lab 1: Microstrip Line
In this lab, you will build a simple microstrip line to quickly familiarize yourself with the
EMPro User Interface and how to setup FEM and FDTD simulations. If you are doing
only the FEM or FDTD class, just skip the sections you are not interested in.
1. Getting Started
Once EMPro opens, use File > Save Project As to immediately define a project name
Lab1_ustrip_WG.ep and a location for your projects. You may want to first create a
folder such as C:\home\EMPro_Projects as a unique place to store EMPro projects.
2. Set Project Preferences
a. Select Edit > Application Preferences to bring up the Application Preferences
editor. Take a look at what is there. The Undo history size is not limited by
default. It should typically not be required to set a limit as only actions, and not
model states, are stored in memory.
The Clear Results on Project Load can be used to empty the Results window of
data from the previously loaded project. If unchecked, loading new projects adds
the results to those from previously loaded projects.
The Transparency Algorithm can be quite useful when trying to visualize models
with overlapping 3D shapes.
Lab 1: Microstrip Line
1-2
b. Click on the Interface tab.
c. Check that the option Edit Properties is selected.. This will automatically open
the properties editor when you define a new material.
d. Click on the Modeling tab.
e. Here you can set the default colors used to display faces, edges, and/or vertices for
parts or materials when they are first created. You can also set the color of the
components (like voltage sources).
f. Press OK to apply the changes and close the editor.
Lab 1: Microstrip Line
1-3
3. Set Modeling Units
a. In the pull-down menus, select Edit > Project Properties to bring up the Project
Properties Editor.
b. Click on the Display Units tab.
c. Check that the default Length units are millimeters.
d. Press Done.
Simulation Method
At the bottom left of the EMPro window are two buttons used to set the type of
simulations that will be performed. A geometry can be simulated using either FDTD or
FEM or both methods. Clicking the buttons enables so-called “user-interface skins” – that
is, some features are enabled or disabled based on this setting. For example, if FEM is
selected, notice that “External Excitations” becomes inactive (greyed out) because FEM
does not support such sources.
In this lab, we will use FEM first using features unique to this
method, so enable only FEM for now.
Lab 1: Microstrip Line
1-4
4. Draw the Substrate
a. The Geometry Tools button in the Geometry window should be active by default.
Select Create > Extrude. This will bring you to the Edit Cross Section tab.
b. Click on the Specify Orientation tab and verify that the origin is located at
(0, 0, 0). The default drawing plane is the XY-plane.
c. Click on the Edit Cross Section tab. Ensure that all
snapping tools are selected to facilitate the correct
placement of points.
d. Double-click on the Z-axis to switch to a flat
XY-plane view or select the Top (-Z) orientation
using the View Tools button.
e. Select the Construction Grid button.
f. Set the minor grid Line Spacing to 1
mm.
g. Set Mouse Spacing to increments of
0.1 mm.
h. Select OK.
Lab 1: Microstrip Line
1-5
i. Choose the Rectangle tool from the
shape icons.
j. Draw a rectangle from (0,0) to (15,20) in the
sketching plane. You can click the Tab key to
enter coordinates (but the window must be
active, that is, you have to have clicked inside
the drawing plane before starting the draw
action).
k. Click the Select/Manipulate tool, then
move the pointer over the bottom left
vertex, and with a right-click choose Edit Position from the pop-up menu.
l. Validate that U=0 mm and V=0 mm. Press OK when done.
m. Use the Select/Manipulate tool to highlight
the upper right vertex, and with the right mouse
button select Edit Position from the pop-up
menu.
n. Check that the values are U=15 mm and
V=20 mm and click OK.
o. Click on the Extrude tab.
p. Enter the value “2 mm” as the Extrude Distance in the W‟-direction.
Rotate the view and observe the arrow that indicates along which axis the sweep
will be performed. Experiment by changing U‟ and V‟ to see the effect on the
extrude axis. When done reset back to extrude only along W‟ direction. You can
also graphically drag the extrude vector.
q. Type Substrate in the Name field.
r. Press Done.
s. Note the new object in the Parts branch of the
Project Tree. You can open the Substrate branch
and double-click the Extrude entry to modify the
part at any time in the future (even after the part
has been combined with other parts using
Boolean operations).
Lab 1: Microstrip Line
1-6
5. Draw the Microstrip Line
a. Select Create > Sheet Body. (A sheet body is simply a 2 Dimensional shape.)
b. Click on the Specify Orientation tab. Select the TOP (-Z) orientation from
the View Tools. Note that the last view you selected is remembered and the icon
changes to indicate which
plane you last selected (e.g, no
need to select from the list, just
click the icon).
c. Edit the Z-value to be 2 mm.
(This will move the drawing
plane to the top of the
substrate).
d. Click on the Edit Profile tab.
e. Select the Rectangle tool. Click on the point U’=0
mm, V’=9 mm on the left edge of the substrate (see the
coordinate readout at bottom right of the window or use
the Tab key).
f. Click a second point at U’=15 mm, V’=11 mm on the right edge of the substrate.
g. Select the Select/Manipulate tool , then move the pointer over the bottom left
vertex, and with a right-click, select Edit Position from the pop up menu.
h. Correct the values to be (0, 9) if necessary.
i. Repeat the check on the top, right vertex.
j. Name the sheet body Line.
k. Click Done and then Save the project.
Lab 1: Microstrip Line
1-7
Note: Since no materials have been assigned, both objects appear with the default colors.
If you want to see outlines of the objects (along with their bounding box dimensions), just
select them in the Project Tree or in the Geometry window.
6. Define Materials
The assignment of materials can be done either after all the objects have been defined or
as each object is completed. The second approach has the benefit that the objects can be
more easily distinguished from each other.
A large set of materials can easily be inserted from the supplied library, but here you will
create your own defintions. These can also be stored as library parts and copied into any
other project.
a. Right-click on the Definitions > Materials branch of the Project Tree and select
New Material Definition.
Lab 1: Microstrip Line
1-8
b. Type Cu in the Name field.
c. Select Isotropic in the Electric
drop-down list.
d. Under the Electric tab, use Type
Nondispersive, Entry Method Normal,
and type in 5.8e7 S/m for Conductivity.
e. In the APPEARANCE tab, pick
yellow for the Face Color and a light
shade of yellow for the Specular
Color. The Specular Color
determines how “bright” the object
will appear when the light source
reflects directly to the viewer. If you
plan to view object edges and/or
vertices, it is useful to pick other
shades of yellow for those items as
well.
f. Click Done to add the new material to
the Project Tree.
g. Add another material and define an Isotropic, Nondispersive material called
Dielectric using the Loss Tangent Entry Method with epsr=9.9 and tanD=0.005 at
1 GHz.
h. In the Appearance tab, select shades of green for the Face Color and Specular
Color.
Lab 1: Microstrip Line
1-9
7. Drag-and-Drop Material Definitions
a. Drag the Dielectric material icon on top of the
Substrate object in the Parts branch .
b. Drag the Cu material icon on top of the Line object
under the Parts branch.
c. Experiment with the settings in the Appearance tab
of the Cu and Dielectric material defintions. Turn
Face/Edge/Vertex display on/off, change colors,
Vertex Size (0.4 is a good value), and notice the effect of Specular Color on the
object brightness when you rotate the faces towards the light source.
8. Define the Outer Boundary Conditions
The outer boundary conditions on each of the six sides of the model space must be set to
indicate how the EM simulators will interpret the space beyond them.
a. Double-click on the Simulation Domain >
Boundary Conditions branch of the Project
Tree to open the Boundary Conditions Editor.
b. Set the Lower Z boundary to PEC. This sets
the bottom of the substrate as a perfect
conductor to form the ground plane.
c. Note the Absorption Type is greyed
out because these only apply to
FDTD whose UI skin you turned off
earlier.
d. Click Done.
Lab 1: Microstrip Line
1-10
9. Set Bounding Box Dimensions
a. Double-click the FEM Padding branch.
b. The FEM Padding Editor opens up. Set the lower and
upper X- and Y-padding to 0 mm. The lower
Z-padding must also be 0 mm to make sure the
bottom PEC surface touches the bottom of the
substrate. Set the upper Z-padding to 10 mm. Since
the FEM absorbing boundary acts like a resisitve
surface if placed too close to a structure, we want to
move it some multiple of the substrate thickness and line width away.
c. The FEM bounding box indicators are 3 white line segments terminated in red
dots. You can control bounding box visibility with the corresponding icon on the
right toolbar.
Lab 1: Microstrip Line
1-11
10. Define Waveguide Ports for FEM Simulation
a. Right-click on the Circuit Components/Ports
branch and choose New Waveguide Port.
b. Click the Pick tool and then hover the mouse
over the substrate face at X=0 mm and click
to define the port. A transparent surface
appears to indicate the port cross section that
will be used by the FEM port solver.
c. On the Properties tab, change the name to
Waveguide Port1 and select the
50 ohm Voltage Source for the
Waveguide Port Definition. This will
result in 50 Ohm S-Parameters
(renormalized from the generalized
S-Parameters using the Power/Current
characteristic impedance computed by the
port solver). If you were to choose the
1W Modal Power Feed instead, you would
get the generalized S-Parameters instead.
Lab 1: Microstrip Line
1-12
d. Click the Impedance Lines tab. Define the line along which the voltage for the
Zpv and Zvi characteristic impedances will be computed (the electric field for
Mode 1 will be integrated along this line). Click the Endpoint 1 Pick tool and then
click on the midpoint of the bottom substrate edge.
e. Click the Endpoint 2 Pick tool and then click on the midpoint of the strip edge.
You could also type the coordinates directly.
f. Click OK.
g. Repeat the procedure for the 2nd
port, changing the name to Waveguide Port2.
You have to rotate the model to be able to select the opposite face.
h. Save the project.
Lab 1: Microstrip Line
1-13
11. Run an FEM Simulation
a. Click on the Simulations tab on the right workspace menu.
Choose New FEM Simulation.
b. Add an adaptive frequency sweep from 1 GHz to 10 GHz with 20 Sample Points
Limit using the Add To List button.
c. Click on the Setup Mesh/Discretisation tab. Define a Delta error of 0.02,
3 minimum adaptive passes and 10 maximum adaptive passes.
Lab 1: Microstrip Line
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d. Under the Matrix Solver tab, select the Direct matrix solver.
e. Click Create & Queue Simulation. In the Output tab, you can see entries for the
adaptive pass number, simulation frequency, mesh count, CPU and elapsed times
for the mesher, matrix size in number of unknowns, memory consumption in GB,
CPU and elapsed time for the matrix solver, and the DeltaS values compared to the
desired Delta Error:
(0.010[->0.020]). If Delta Error is met, the adaptive sweep proceeds with the
previous to last mesh (9200 unknowns – may vary from release to release due to
algorithm changes).
Lab 1: Microstrip Line
1-15
12. View Results
a. Click on the Results tab to open the corresponding window.
b. Experiment with the column selections available to help identify the data by
right-clicking any of the column titles. The top part of the window has four fixed
columns that can be used to filter the data making it easier to find the data of
interest. The bottom part shows the actual data content based on the filter
selections you make above. The column settings are saved across sessions, but can
be reset using the application preference Clear Saved Layout Now.
Lab 1: Microstrip Line
1-16
c. Click on S-Parameters under the Result Type column. Then click the Domain
column (make sure it is turned on) to put the Frequency data (AFS sweep) before
the discrete frequency points.
d. Right-click on the entry for S21 and choose View (default).
e. Notice that an entry is created under the
Graphs branch of the Project Tree. Here
you can rename, delete and open the graph.
f. Save the project.
Lab 1: Microstrip Line
1-17
13. Create Project Templates
Templates can be configured with models, materials and sources already assembled to
form the basis of new designs. Templates also contain the Construction Grid settings and
any custom units you may have defined.
a. Use the pull-down menu Edit > Application Preferences and under the General
tab, notice the location on your computer to which templates are saved.
b. Select File> Manage Project Templates.
c. Click Create New and enter a name for your template in the dialog box such as
Lab1_template and then click Create.
d. Close the dialog box.
e. Templates can be used with the command File > New Project From Template.
Lab 1: Microstrip Line
1-18
14. Define Discrete Sources
a. Make a copy of the project using File > Save Project As and call it
Lab1_ustrip_Int.ep.
b. Under the Results window, click on the Project Name Lab1_ustrip_WG and then
click Unlist Project.
c. Notice the warning icons that appear next to the graph entries. Moving the mouse
over them, you can see that the plots are now invalid because the project whose
data was being plotted is no longer in memory. Delete those two graphs (select
both and right-click on top to see the menus).
d. Select the Waveguide Ports and delete
them both.
e. Right-click on Circuit Components/Ports, and select New Circuit Component
with > No Definition.
Lab 1: Microstrip Line
1-19
f. No new feed definition is needed because when
you created the waveguide ports, two
definitions were automatically created under the
Circuit Component Definitions branch.
g. Select the Pick tool for the location of Endpoint 1. Position the mouse over the
bottom substrate edge at X=0 mm. Click on the midpoint of the edge.
h. Select the Pick tool for Endpoint 2 and select the midpoint of the top microstrip
edge. An arrow indicating the component location appears.
i. Check the coordinates and correct them manually if necessary.
j. Click on the Properties tab and name the new component Port1.
k. Select the 50 ohm Voltage Source for the Component Definition pull-down
menu.
l. Click Done.
Lab 1: Microstrip Line
1-20
m. Repeat the procedure to define Port2 at X=15 mm.
n. Double-click the FEM Padding branch and edit the
X-padding to be 5 mm in both directions. The voltage
sources are not allowed to touch an outer boundary.
o. Save the new project.
15. Simulate using FEM
a. Click the Simulations tab and click on New FEM Simulation. All the data
should still be set from the previous simulation.
b. Click Create & Queue Simulation. Check the Output tab until the simulation
finishes.
16. Compare the Results
a. Click the Results tab.
b. Use the List Project button to
reload the simulation results for
Lab1_ustrip_WG.
c. Select the two S21 entries from the
two simulations (use filters and
sort as necessary and use the CTRL key to select distinct entries). Right-click on
one of them and choose View (default).
Lab 1: Microstrip Line
1-21
d. The results are quite different at higher frequencies due to the parasitic effect of the
voltage sources. Repeat the previous step, but this time choose
Create Line Graph and then select Phase.
e. Click on the Graph Properties
icon. On the left, click the
Axes Properties tab. Edit the
Min/Max phases to go from
-180 to +180 degrees.
Lab 1: Microstrip Line
1-22
17. Define the FDTD Grid
a. Turn on the FDTD user interface skin to enable
FDTD specific features.
b. Hover the mouse cursor over the warning message inside the Circuit Component
Definitions branch – the 1 W Modal Power Feed is not valid in FDTD. Delete it
by right-clicking on it.
c. Notice the bounding box indicator for the FDTD simulator
with blue dots. FDTD bounding box sizes are typically
defined in terms of number of cells, while the FEM
bounding box is usually a half wavelength for free space
applications.
d. Double-click on Boundary Conditions in the Project Tree.
Now the Absorption Type is editable. The default
absorption model is Perfectly Matched
Layers - PML. This tends to be more
accurate than Liao, but it places
additional grid cells around the model
space defined by the number of Layers.
The default of 7 layers gives very good
absorption, but if memory is a problem,
you can reduce this to 5 and often get
similarily good results or use the Liao
boundary conditions.
e. Double-click the FDTD Grid entry in the Project Tree.
Lab 1: Microstrip Line
1-23
f. The Size tab should be active by default. The Specify Padding button should be
selected.
g. Define the Target Base Cell Sizes as 1 mm for the X-, Y- and Z-directions. Use
Merge Ratios of 0.2 (cells smaller than 0.2*Base Cell Size=0.2 mm will be
merged).
h. Define Free Space Padding as 5 in both X- and Y-directions (side walls will be
1 mm*5=5 mm away). Set the Lower Z padding to 0 and the Upper Z padding to
10 cells (top wall will be 1 mm*10=10 mm away).
i. Press Done to close the Edit Grid dialog.
j. Right-click on the Substrate object in the
Geometry window or on its name in the
Project Tree, and select FDTD Gridding
Properties.
k. Select Use Automatic Grid Regions and
change the Z-directed Target to 0.5 mm.
It is generally recommended that
substrates are meshed with at least 3 cells
along their height for better accuracy.
The Target base cell size is the maximum value
a cell in that direction can have. A cell may be smaller than this size.
The Merge field sets the range over which the base cell size may be varied to obtain cells.
In the above case the use of the selection „ratio‟ means that in generating the grid the size
will not go below 1/5th
(20%) of the base cell size. This can also be set to an absolute size
by unchecking the Ratio flag.
The Free Space Padding determines the number of base cells added around the geometry‟s
bounding box.
Lab 1: Microstrip Line
1-24
18. Creating the FDTD Mesh
The mesh is created as soon as we activate the mesh view. The grid is a set of brick-
shaped elements that is superimposed on the geometry. The cell edges do not necessarily
terminate at object boundaries. The material properties of the geometric objects are
mapped to the grid edges to define the simulation mesh for the FDTD solver. This
mapping introduces one of the principal errors in the FDTD analysis if one does not set
grid definitions well.
Check the Feed Placement in the Mesh
a. Double-click on the FDTD Mesh branch in the Project Tree
or click the mesh toggle icon on the right.
b. Select the “Right (-X)” view from the View Tools icon.
c. Press the Component Visibility icon along the right of the
Geometry window to turn off the Port Components (also works
for waveguide surfaces). This will let you see where the feed is applied in the
simulation mesh. If you have a component selected in the Project Tree, then it will
be visible regardless. You need to select something else to make it invisible.
d. The Mesh Cutplanes button whould be selected. Click the YZ Plane at
X-slice = 20 in order to view the slice of the mesh where the circuit component is
located. (The color of the components can be changed in the Modeling tab of Edit
> Application Preferences.) The cell edges above and below the voltage feed have
been automatically converted to a PEC wire along the length of the feed to make
sure that the voltage is actually applied between the ground plane and the
microstrip. They show up in a color that is hard to differentiate from the green
substrate. If you change the color of the dielectric to red, you can see the wire cell
edges.)
Lab 1: Microstrip Line
1-25
e. Rotate the view to the other side and check that the feed is applied there as well by
changing the X-slice to 5.
f. Select 3D Mesh on the bottom left of the Geometry window and then select
Faces With Edge and rotate the geometry. Notice that the yellow microstrip
disappears. That‟s because by default, the geometry of the objects is superimposed
on the mesh, so it may appear that all the materials have been correctly assigned to
the mesh, when in fact, they are not. In this case, the copper of the microstrip has
been overwritten by dielectric. The reason is that the mesh priority is determined
by the position in the Parts list or by a numeric value that is nominally the same for
all new objects.
g. Drag the Line object above the Substrate object in the
Project Tree under Parts. The effect on the mesh
should be immediately visible. If you notice “holes”
in the mesh under the microstrip, don‟t worry about
these. In FDTD, the faces have no meaning. Only the
edges of the cells are used in the simulation. To see
what FDTD actually sees in a
simulation, turn on All Edges
and then turn off component
visibility.
h. When finished, dismiss the mesh
controls by clicking on the mesh
icon or by double-clicking the
FDTD Mesh branch of the Project
Tree.
i. Save the project.
Lab 1: Microstrip Line
1-26
19. Editing Feed Components and Waveforms
a. To view the feed properties, double-
click the 50 ohm Voltage Source
entry under the Circuit Component
Definitions branch of the Project
Tree. The 1 Volt excitation with a
50 Ohm source impedance is the
typical source used for FDTD
simulations.
b. Click Done.
a. Double-click on the Broadband
Pulse definition under the
Waveforms branch.
b. Select Excite up to a Maximum Frequency and set the maximum frequency of
interest to 10 GHz and the signal level at that frequency to be -10 dBa.
The S-parameters will only be meaningful up to about 10 GHz because the largest mesh
cell of 1 mm cannot be larger than about 1/10th
of a wavelength. In the substrate at
10 GHz (epsr=9.9), we have a wavelength of about 10 mm. By limiting energy in the
pulse waveform to a lower band, the simulation will be faster.
Lab 1: Microstrip Line
1-27
20. Setting Up the FDTD Simulation
a. Open the Simulations window by clicking on the Simulations
tab along the right edge of the EMPro window.
b. Select the New FDTD Simulation button in the upper-left of the
window.
c. Click on the
Setup S-Parameters tab.
Enable S-Parameters is on by
default. Leave Port1 as the
only active port. This will
give us S11 and S21.
Enabling the other port would
require a 2nd
simulation.
d. Click on the Specify Termination Criteria tab. The default values are sufficient
for a large class of problems. The simulation will stop if the convergence of -
30 dB has been reached or after a maximum of 50,000 time steps has been
simulated.
Lab 1: Microstrip Line
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e. Select Create & Queue Simulation to run this simulation.
f. Click on the Output tab to see the simulation progress.
g. View the Summary tab. The maximum frequeny referred to here is based on
largest cell size in free space. It does not take into account dielectrics which can
reduce the maximum usable frequency substantially.
21. Viewing the Results
FDTD simulation results can be viewed dynamically even before a simulation completes.
S-Parameter graphs update automatically as the simulation progresses.
a. Click on the Results tab along the right edge of the EMPro
window.
NOTE: Perform the next two steps only if you skipped the FEM part of
the lab above.
b. Notice the 4 columns at the top. These can be customized and are used to filter the
data available for plotting. Right-click on the column headers and select different
filters to familiarize yourself with the choices.
c. The columns at the bottom are the actual data which you can sort, select and plot.
Here too, right-click on the column headers to see the choices.
Lab 1: Microstrip Line
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e. Make one of the filter columns Simulation Name and select FDTD.... Make
another filter column Result Type and choose S-Parameters.
f. Double-click on [S2,1] for the default
graph. (If you right-click instead, you
see other options.)
g. Within the plot window, select the
Graph Properties icon.
h. Notice the 3 tabs on the upper, left side
of the plot. Choose the 2nd
tab - Axes
Properties. Define the X-axis range
from 0 to 10 GHz and Y-axis range from
0 to -3 dB.
Lab 1: Microstrip Line
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i. Take a look at the marker tools and choose Point
Marker to read off some values on the trace.
Notice that if you click on a point, the marker
readout and a reference dot will be placed on the
graph.
j. Try some of the other markers as well.
22. Saving Parts in a Library
One useful feature of EMPro is the ability to easily generate libraries of any of the objects
created in a project.
a. From the right side of the Main window, select the Libraries
tab.
b. To create a new Library press the + button in the Libraries
window. A File Manger window will open allowing you to
create a suitable directory to store your Libraries in. Use
Browse Folders, and create a new one with the right mouse
button called Libraries in the EMPro_Projects folder. Name
the new library MaterialLibrary.
Lab 1: Microstrip Line
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c. Click on the Filter Materials on the right side.
d. From the Project Tree in the Main window, select both Cu and Dielectric and drag
them on top of the Materials branch at the bottom of the Libraries window.
e. Close the Libraries window.
The stored materials will be used in a later lab exercise.
As can be seen from the Libraries window, almost any item located in the Project Tree can
be saved to a Library. This means standard parts, such as PCB connectors, can be reused
in a consistent manner. But even Sensor definitions (Far Field sensors, etc.) can be stored
in a library saving time and eliminating mistakes during simulation setup.
End of Lab Exercises
Drag and Drop
the items to be
used in other
projects to the
library
Lab 1: Microstrip Line
1-32
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