Lab 2: 2: Microstrip Low Pass Filter
Lab 2: Microstrip Low Pass Filter 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 template could contain contain anything that can also also be stored 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 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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Lab 2: 2: Microstrip Low Pass Filter Filter 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 Part. This entry is called EMPro Library Part. a special type of an Assembly (group of objects). Right-click the name name and rename it to Substrate. If you open open the branch, branch, you see the actual object called box and an entry entr y below called EMPro 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 underneath the box branch. The reason is that for a general assembly 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 the Extrude tab. f. Enter -0.64 mm for the Extrude Distance. This will redraw the box with the extrusion extended extended downward. Alternatively you can keep the +0.64 +0.64 mm and enter -1 for the W‟ W‟-Direction. Click Done.
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Lab 2: 2: Microstrip Low Pass Filter g. Another method to edit the box is to double-click EMPro Library Primitive , which will reopen the Library Box dialog. dialog. Try this and note how 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. performed. You can easily detach 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 i nductive element between them. This forms a PI filter arrangement in LowPass configuration. The sizes of the elements are indicated in the table. IItteem m
L Leennggtthh
W Wiiddtthh
M Maatteerriiaall
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 > b. Select the
Sheet Body.
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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Lab 2: 2: Microstrip Low Pass Filter Filter 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 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 vertex will be at (0, -1.9 mm) in this coordinate system. You have to move the mouse m ouse very precisely to capture the edge midpoint. Sometimes, it is easier easier to turn off the visibility 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 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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Lab 2: 2: Microstrip Low Pass Filter o. Make simple edits to the position of the U‟,V‟,Z‟-Axes 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 when trying to rotate or pan pan the 3D view. Notice how the Origin coordinates update update as you drag the axes. Note that the geometry moves with the coordinate coordinate system. The geometry is fixed to the U‟,V‟-coordinate U‟,V‟-coordinate system. p. Click on Advanced Mode . Here the Anchor the Anchor point for the Reference Coordinate System Sy stem is shown. The Local Coordinate System Origin shown in the Basic Mode is the sum of the Reference Coordinate System anchor point + Translations + Edge. This is what you Rotations. Notice the label Center of Edge. 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 type a new new Anchor point location, this indicator moves, and the Local Coordinate System Syst em also shifts because the translations defined are applied to the new anchor point location. l ocation. Position which would allow you to use a The label changes to Parameterized to Parameterized Position 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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Lab 2: 2: Microstrip Low Pass Filter Filter
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 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, rightclick 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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Lab 2: 2: Microstrip Low Pass Filter 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 0.46 mm height). Either use copy 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 1 th and the 5 part in the list. Right-click on any of them and select Material > Assign Material. Choose the Cu material and click OK.
x. Save
st
the project.
Note: As you create or delete objects, objects, or if you move objects far away from each other 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 “geometric center”. Instead of rotating around itself, the objects seem seem to rotate around around some distant origin. origin. You can use the Zoom to Extents icon to reset this rotation origin.
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Lab 2: 2: Microstrip Low Pass Filter Filter 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 onl y 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 the Properties tab, change the name to Waveguide Port1 and select the 50 ohm Voltage Source for the Waveguide Port Definition. Definition.
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Lab 2: 2: Microstrip Low Pass Filter Lines tab. Define the line along which the voltage will be c. Click the Impedance the Impedance Lines computed using the Pick tools. Select the edge edge midpoints when you you see the blue dots highlight being careful to click right on top of it.
d. Click OK. nd
e. Repeat the procedure for the 2 port, changing the name to Waveguide Port2. You have 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 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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Lab 2: 2: Microstrip Low Pass Filter Filter 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 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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Lab 2: 2: Microstrip Low Pass Filter e. Open the Results window. Change the filter columns columns to Project to Project Name, Name, Result Type, Type, Simulation Name and Domain 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 change the Color and Weight of the traces to make them thicker using the Plot the Plot Properties tab.
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Lab 2: 2: Microstrip Low Pass Filter Filter 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 transparency, Materials vs. Objects tabs, etc.
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Lab 2: 2: Microstrip Low Pass Filter f. Turn the Mesh the Mesh flags off on the materials and also turn off the t he mesh displays in the Plot Properties tab. Then click on Enable 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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Lab 2: 2: Microstrip Low Pass Filter Filter 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.
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Visualizati on When done experimenting with the various features in the Advanced Visualization window, close it.
Lab 2: 2: Microstrip Low Pass Filter 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, 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 surfaces retain knowledge of the materials. 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 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 – menu – visibility, visibility, whether it is included in the simulation or not, material properties, and others. f. Close the Parts List window.
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Lab 2: 2: Microstrip Low Pass Filter Filter 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. objects. EMPro has some simple controls that that will make such alignments using automatic, automatic, localized, adapted adapted grids. Increasing the base base cell size would make a dense mesh everywhere which is not optimal in terms of memory consumption. a. Turn on the Mesh visibility visibilit y 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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Lab 2: 2: Microstrip Low Pass Filter 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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Lab 2: 2: Microstrip Low Pass Filter Filter 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 now aligns nicely with the object boundaries. 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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Lab 2: 2: Microstrip Low Pass Filter 11. Defining Discrete Sources
The filter will be excited by two centered feeds 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. Circuit Components branch of the Project Tree, and select a. Right-click on the New Circuit Component with > New Feed Definition.
b. Select the Left (+X) orientation from
View Tools and zoom into center.
Pick tool and snap c. Select the to the midpoint of the bottom edge.
d. Repeat for the 2
nd
point.
e. Click the Properties 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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Lab 2: 2: Microstrip Low Pass Filter Filter 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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Lab 2: 2: Microstrip Low Pass Filter 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 convergence level and 50,000 time steps.
e. Click Create & Queue Simulation . f. Check the Output tab for progress. progress. The simulation should should run quite quickly and converge after about 3900 time steps.
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Lab 2: 2: Microstrip Low Pass Filter Filter 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. Type. Select b. Filter the S-parameters under the column Result column Result Type. the FEM and FDTD S21 entries and righ-click to View (default).
Properties , so that the scale of the plot is from -20 dB to 0 dB and c. Edit the Graph Properties, from 0 to 15 GHz . Edit the Plot the Plot Properties where you can change the trace weight and label names (double-click (double-click and type a new one). We see that the voltage source source based simulations are quite close to the t he 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 being simulated and can often be neglected. neglected. Also note that we have done no convergence convergence testing by running the simulations with finer meshes.
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Lab 2: 2: Microstrip Low Pass Filter 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 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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Lab 2: 2: Microstrip Low Pass Filter Filter
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