Lab2 Microstrip Lpf Fem Fdtd

Lab 2: Microstrip Low Pass Filter

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This lab explores further features of the EMPro User Interface. You will build a

microstrip lowpass filter.

1. Create New Project from Template

In the first lab, a Project Template was saved containing units, parts, and construction grid

settings. The template could contain anything that can also be stored in a library. But the

template also saves any parameters that you may have defined, and these cannot be saved

to a library.

a. From the File Menu select File> New Project from Template.

b. Select the

Lab1_template and

click OK. (Note the

option to always use a

template for any new

projects.)

c. Select the two objects

Substrate and Line

and delete them with

the right-mouse button

menu.

2. Importing Material Definitions from a Library

a. Select the Libraries tab on the right side of the EMPro window.

b. Select the previously defined library from the Libraries list.

c. Select Materials in the

Filters list.

d. Drag and drop the two

materials from the Library

window to the Project Tree

under the materials branch.


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e. Edit Dielectric to εr=3.0.

f. Save the project as Lab2_LPF.ep.

3. Using EMPro Default Library Objects

a. Click the Create Box macro from the

EMPro Library toolbar at the bottom left of the

main window.

b. Enter a Depth (Y-axis) of 10 mm, Height

(Z-axis) of 0.64 mm and Width (X-axis) of

12 mm. (Note: The dimensions cannot be

negative values. Also, the dimensions will be

centered about the origin.)

c. Note the new entry in the Project Tree under

Parts called EMPro Library Part. This entry is

a special type of an Assembly (group of

objects). Right-click the name and rename it to

Substrate. If you open the branch, you see the

actual object called box and an entry below

called EMPro Library Primitive which

references the Library Box dialog.

d. Drag and drop the material Dielectric on top of Substrate. The material will be

placed underneath the box branch. The reason is that for a general assembly of

parts, dragging a material on it will apply the material definition to each and every

object in that assembly.

e. Double-click the Extrude entry and select the Extrude tab.

f. Enter -0.64 mm for the Extrude Distance. This will redraw the box with the

extrusion extended downward. Alternatively you can keep the +0.64 mm and enter

-1 for the W‟-Direction. Click Done.


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g. Another method to edit the box is to double-click EMPro Library Primitive,

which will reopen the Library Box dialog. Try this and note how the Height value

remains at 0.64 mm even though we have edited the direction of extrusion. This is

because the Library Primitive is the source part we based our variation on. So we

have a way of returning to the original „as inserted‟ primitive. Click Cancel to

avoid resetting the edit you just performed. You can easily detach the box object

from the assembly by dragging it directly under the Parts branch and then deleting

the Assembly.

4. Drawing the Filter

The Microstrip pattern will consist of 5 items; an input and

output line, two capacitive elements and an inductive element

between them. This forms a PI filter arrangement in LowPass

configuration. The sizes of the elements are indicated in the

table.

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In 2 mm 0.46 mm Cu

C1 2.7 mm 3.8 mm Cu

L1 2.6 mm 0.46 mm Cu

C2 2.7 mm 3.8 mm Cu

Out 2 mm 0.46 mm Cu

a. Select Create New > Sheet Body.

b. Select the TOP (-Z) orientation from VIEW TOOLS.

c. Click on the Specify Orientation tab. Using the Direction Picking tool choose

Origin from the drop-down Menu.


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d. Position the cursor on the center of the left hand

edge of the substrate - a blue dot shows up when

you are over the edge to indicate the exact midpoint.

When exactly over the blue dot, click to confirm the

new origin.

e. The Origin coordinates should now show

x = -6 mm, y=0 and z=0.

f. Click on the Edit Profile tab.

g. Name the sheet body In.

h. Select the Rectangle tool.

i. Move the cursor over the left

edge around U=0 and V= -0.5.

Look at the bottom right of

the window to see the

coordinates.

j. Press the Tab key to bring up the Specify Position

dialog and type in the values U=0 mm, V=0.23 mm.

Click OK.

k. Move the mouse cursor to the top and right and press

Tab again. Set the Width=2 mm and

Height=0.46 mm.

l. Click OK and then Done to exit the Sheet Body

command.

Note: If you make any mistakes these can

be undone using the Geometry Undo/Redo

buttons next to the Name field (not the

top-level buttons on the left, top of the EMPro window.)

m. The second element is a capacitive stub of

dimensions Width=2.7 mm, Height=3.8 mm. The

process for drawing this stub is the same as the previous steps.

The position of the Reference Origin for C1 is the center of the

output edge of In (X= -4 mm). The first rectangle vertex will be

at (0, -1.9 mm) in this coordinate system.

You have to move the mouse very precisely to capture the edge

midpoint. Sometimes, it is easier to turn off the visibility of a

larger, enclosing object (the substrate in this case) before starting a new modeling

operation. If you do not snap to an anchor point, but instead type an origin, there is

no locked connection between the objects for later parameterizations or edits.

n. Go to the Specify Orientation tab.


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o. Make simple edits to the position of the U‟,V‟,Z‟-Axes by

clicking on the U’,V’ or Z’-axes arrows and dragging them.

This way to position the local coordinate system is not very

accurate and is shown here to make sure you don‟t accidentally

drag them when trying to rotate or pan the 3D view. Notice

how the Origin coordinates update as you drag the axes. Note

that the geometry moves with the coordinate system. The

geometry is fixed to the U‟,V‟-coordinate system.

p. Click on Advanced Mode. Here the Anchor point for the

Reference Coordinate System is shown. The Local Coordinate

System Origin shown in the Basic Mode is the sum of the

Reference Coordinate System anchor point + Translations +

Rotations. Notice the label Center of Edge. This is what you

obtain when you snap to an edge or face using the Origin Direction Picking tool.

Notice the Reference Coordinate System anchor point indicator. If you type a new

Anchor point location, this indicator moves, and the Local Coordinate System also

shifts because the translations defined are applied to the new anchor point location.

The label changes to Parameterized Position which would allow you to use a

parameter to define it.

q. Use the Geometry Undo button to go back to the original rectangle position.

r. Click Done to save the object.


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s. Open the branch for In and double-click on the Cover

entry. In the Specify Orientation tab, move the axes

around as you just did. Click Done. C1 stays attached to

the anchor point as expected. Click the Main Window

Undo button on the top left.

t. The same process could be used to add the

second capacitive element, but as an

alternative, this will be added by using the

Copy command. Right-click C1 in the

Project Tree and select Edit > Copy. Select

the Parts heading in the Project Tree, right-

click and select Edit > Paste.

u. Rename C1 (Copy) to C2. Right-click C2

select Specify Orientation. Select the button

Advanced Mode. Enter 5.3 mm (2.7 mm +

2.6 mm) for U’. Press Enter on the keyboard.

Notice that the anchor is the same as it was for

C1, which means that every time you edit the

input line position, both C1 and C2 would correctly

move along without any need to parameterize

coordinates explicitly. This is what is meant by

constraint based modeling. Click Done.


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v. Now add the remaining two lines calling them L1 (2.6 mm width and 0.46 mm

height) and Out (2 mm width and 0.46 mm height). Either use copy and edit the

dimensions and axis origin or create the lines from scratch.

w. Select the 5 sheet body parts in the Project Tree using the SHIFT key on the 1st

and the 5th

part in the list. Right-click on any of them and select Material >

Assign Material. Choose the Cu material and click OK.

x. Save the project.

Note: As you create or delete objects, or if you move objects far away from each other

and then close together again, the origin used to rotate the model with the left mouse

button may no longer give the desired “geometric center”. Instead of rotating around

itself, the objects seem to rotate around some distant origin. You can use the

Zoom to Extents icon to reset this rotation origin.


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5. Setting up the FEM Simulation

a. Use File > Save Project As to create Lab2_LPF_WG.ep.

b. Turn off the FDTD UI Skin on the bottom left of the window

to activate the FEM only UI commands.

c. Right-mouse button over the Circuit Components/Ports

entry and choose New Waveguide Port.

a. Using the Pick tool, hover the mouse

over the face at X= -6 mm and click

when highlighted to define

the port cross section.

b. On the Properties tab, change the name to Waveguide Port1 and select the

50 ohm Voltage Source for the Waveguide Port Definition.


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c. Click the Impedance Lines tab. Define the line along which the voltage will be

computed using the Pick tools. Select the edge midpoints when you see the blue

dots highlight being careful to click right on top of it.

d. Click OK.

e. Repeat the procedure for the 2nd

port, changing the

name to Waveguide Port2. You have to rotate the

substrate to be able to highlight the opposite face.

f. Double-click the FEM Padding entry in the Project Tree. Change the Lower

and Upper X- and Y- and Lower Z-values to 0 mm and the Upper Z-value to

10 mm.

g. Check the Boundary Conditions

Editor to make sure there is a PEC

bottom boundary for the ground

plane.

h. Save the project.


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6. Running the FEM Simulation

a. Click the Simulation tab and click on New FEM Simulation.

b. Add an Adaptive sweep from 0.1 to 15 GHz.

c. Click on the Setup

Mesh/Discretisation tab and

select Refine at specific

frequency and type 5 GHz.

By refining in the passband,

we help the accuracy and

convergence because in the

passband, energy is going

through the entire filter

structure. At the default,

highest frequency of

15 GHz, there is little energy

in the middle of the filter and

the mesher would refine very

slowly in that region if at all

d. Click Create & Queue Simulation. Check the Output tab until the simulation

finishes.


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e. Open the Results window. Change the filter columns to Project

Name, Result Type, Simulation Name and Domain. Filter for

S-parameters and Frequency (AFS). Right-click on S21 and

select View (default).

f. You can change the scale under Graph Properties from -20 dB to 0 dB and from

0 to 15 GHz if necessary. You can also change the Color and Weight of the

traces to make them thicker using the Plot Properties tab.


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7. Advanced Visualization for FEM

a. In the Results window, select the Lab2_LPF_WG Project Name and then click on

the Advanced Visualization button.

b. In the next dialog, select the Simulation 000001.

c. Click the Mesh button next to Dielectric and the Vis and Shd buttons for

Dielectric and Cu.

d. Click the Plot Properties tab on the bottom, left.

Activate the Surface and Volume Mesh options.

e. Go back to the Properties tab and experiment

with the colors, transparency, Materials vs.

Objects tabs, etc.


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f. Turn the Mesh flags off on the materials and also turn off the mesh displays in the

Plot Properties tab. Then click on Enable for the Z-0 plane.

g. Click on Options and change the Max scale value to 30000. Select the button

Keep Max/Min Values.

h. Click the Animate button.

i. Click the Edit button for the selected plane and shift the plane up and down using

the slider.

j. Switch to the Solution Setup tab on the bottom, left and cycle through the

frequencies. Notice how the Min/Max scale values stay locked to the values you

set.


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k. Back in the Plot Properties tab, turn off Shaded plot, and turn on Arrow plot. Set

the X- and Y-arrow density to 3 under the Options dialog. Use Options > Black

background to get a better view of the arrows.

l. When done experimenting with the various features in the Advanced Visualization

window, close it.


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8. Setting Mesh Priority for the FDTD

Mesh priority is an explicit method for ranking the relative priorities of overlapping

objects for meshing. Although it is used by both FDTD and FEM, it is most important for

the FDTD simulations due to the fact that only the grid edges carry material information

while in FEM, the volume and surfaces retain knowledge of the materials. The FEM

mesher contains additional algorithms that typically do not require a user to think about

mesh priority or the order of objects in the Parts branch.

In the first lab, you learned that just moving the substrate part below the microstrip gave

the substrate lower priority and the microstrip was no longer overwritten in the mesh, but

users do not always want to sort their objects based on this criterion.

a. Use File > Recent Projects > Lab2_LPF.ep to load the

model before FEM ports were defined.

b. Turn the FDTD UI skin on again.

c. Right-click on the Parts branch and select View

Parts List (All Parts).

d. Right-click on the object box and select

Meshing Order > Move Down. This will

lower the priority to 49.

e. Notice that you can edit any of the properties you see in that table using the pop-up

menu – visibility, whether it is included in the simulation or not, material

properties, and others.

f. Close the Parts List window.


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9. Creating the FDTD Grid

a. In the Project Tree, double-click on FDTD Grid to

bring up the Editing FDTD Grid dialog.

b. Define Base Cell Sizes of 0.3 mm in all directions,

uncheck the Ratio boxes, and set Merge valules to

0.1 mm.

c. Define Free Space Padding as 0 for Lower-Z as this

will be the ground plane and 10 in all other directions.

d. Press Done.

10. Align Grid with Object Boundaries

A default grid is just a regularly spaced set of bricks and therefore will usually not

coincide with the edges of objects. EMPro has some simple controls that will make such

alignments using automatic, localized, adapted grids. Increasing the base cell size would

make a dense mesh everywhere which is not optimal in terms of memory consumption.

a. Turn on the Mesh visibility by double-clicking FDTD Mesh in the Project Tree or

clicking the icon on the right toolbar (if you forgot what it looks like, read the

bubble-up help).

b. Turn on the XY-Plane and move the grid up the geometry using the slice selection

tools. Next to the object visibility icon, click the % icon to reduce the opacity.

The mesh does not completely coincide with the drawn filter pattern.


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c. Right-click on the box part and select FDTD Gridding Properties.

d. Select Use Automatic Grid Regions, activate only the Z-direction, and type

0.2 mm for target size and 0.1 mm as the smallest cell size (Ratio unchecked).

e. Enable Use Automatic Fixed Points. Click Done.

f. Right-click on the part In, select FDTD Gridding Properties and turn on Use

Automatic Fixed Points.

g. Click Apply. Then click the button Copy to Clipboard.


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h. Select the other 4 sheet bodies C1, C2, Out and L1. Then right-click and Edit >

Paste. Check the FDTD Gridding Properties on one of them to verify that the

copy was successful.

i. The mesh now aligns nicely with the object boundaries. Take a

look at the substrate cross section and notice (use the Measure

icon) how the substrate thickness of 0.64 mm is exactly meshed

into 4 cells.

j. Alternatively, you can turn on View Mesh Information at the bottom of the

window and click the bottom grid edge to see its exact position and other

information.


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11. Defining Discrete Sources

The filter will be excited by two centered feeds at the input and output. Based on our

geometry, these two ports will be at XY-locations (-6, 0) and (6, 0) and will start at

Z = -0.64 mm and finish at Z = 0 mm.

a. Right-click on the Circuit Components branch of the Project Tree, and select

New Circuit Component with > New Feed Definition.

b. Select the Left (+X) orientation from View Tools and zoom into center.

c. Select the Pick tool and snap

to the midpoint of the bottom

edge.

d. Repeat for the 2nd

point.

e. Click the Properties tab and type

the name Port1.

f. Press Done.

g. Repeat these steps to set up Port2, except that you

now choose New Circuit Component with > No

Definition (reuse the 50 ohm Voltage Source just

created) and change the View to Right (-X).


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12. Edit Waveform Definition

a. Edit 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 -3 dBa.

c. Click Done.

13. Defining the Outer Boundary

a. Double-click on the Simulation Domain > Boundary Conditions branch of the

Project Tree to open the Boundary Conditions Editor.

b. Set Lower Z boundary to PEC.

c. Click Done.

d. Save the Project.


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14. Running the FDTD Simulation

a. Click on the Simulations tab along the right edge of the

EMPro main window.

b. Select the New FDTD Simulation button.

c. Click the

Setup S-Parameters tab. We

will only run one simulation

with Port1 active to speed up

the simulations.

d. The Specify Termination

Criteria tab should already be

set up for a -30 dB

convergence level and 50,000

time steps.

e. Click Create & Queue Simulation.

f. Check the Output tab for progress. The simulation should run quite quickly and

converge after about 3900 time steps.


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15. Viewing the Results

a. When the simulation is complete, open the Results window

clicking on the tab on the right side of the window.

b. Filter the S-parameters under the column Result Type. Select

the FEM and FDTD S21 entries and righ-click to View (default).

c. Edit the Graph Properties, so that the scale of the plot is from -20 dB to 0 dB and

from 0 to 15 GHz. Edit the Plot Properties where you can change the trace weight

and label names (double-click and type a new one). We see that the voltage source

based simulations are quite close to the modal port based simulations, which shows

(compare to microstrip lab) that the effect of the voltage source parasitics is also a

function of the type of structure being simulated and can often be neglected. Also

note that we have done no convergence testing by running the simulations with

finer meshes.


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16. OPTIONAL – FEM Simulation with Lumped Voltage Sources

Time permitting, you can set up an FEM simulation using the lumped voltage sources and

compare all 3 simulation results to each other. Don‟t forget to define the FEM Bounding

Box leaving about 5 mm space in front of the voltage sources (otherwise the absorbing

boundary will absorb the voltage source) and save the project before running the

simulation.

End of Lab Exercises


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Lab2 Microstrip Lpf Fem Fdtd

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Transcript of Lab2 Microstrip Lpf Fem Fdtd