Traditionally, s-parameters visualization and process are on the MODUA. You also perform circuit simulation on a circuit consisting of s-parameter black boxes and lumped elements such R, L, C and mutual L on MODUA. On IE3D 14, we have implemented more s-parameter visualization and processing capabilities into MGRID. MODUA is no longer the default visualization tool even though we still need to perform circuit simulations and mixed EM and circuit simulations on MODUA.
We will discuss how we can use MGRID for parameter visualization and processing. With s-parameters processing integrated into MGRID, we normally don‟t need MODUA for s-parameter processing. By default, we suggest users to select “No Display” for Post-Processing. In case, you want to display the s-parameters immediately after simulation, you should select Add Graphs in Simulation Setup to pre-define some graphs before simulation starts. Certainly, we can always define graphs after simulations.
Step 1 Run MGRID and open file: “.\ie3d\practice\c_bend.geo”.
It is a simulated structure while the results are automatically saved into the .geo file. Please note that any change to the structure and meshing parameters will cause removal of the s-parameters in the data. When you do some potential change, MGRID will warn you anyway. In case you change it and the s-parameter data is lost, you will not be able to access the data from MGRID.
However, it is still available in the file if you do not save the change into the file.
Step 2 Select Process->S-Parameters and Lumped Equivalent Circuit command or Window->S-Parameters Display->Define Plots (Note: We provide the commands with the same functionalities for your convenience). The “S-parameters and Frequency Dependent Lumped Element Models” or S-Parameters Processing dialog comes out (see Figures 3.22 or 3.19b).
Please pay attention to the message at the middle right of the dialog “The Current S-Parameters are 2-port s-parameters”. It is for the selected item or the only No.0 item in the list box. You are
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allowed to define the graphs in Simulation Setup dialog or in Window->S-Parameters
Display->Define Plots even though the s-parameters may not be available. In such a case, MGRID will indicate to you that the s-parameters are fake ones for defining the graphs only in this message.
Step 3 Please double click at the Short Name section of the No.0 item in the list box to change its Short Name from “” to “AEC Layers = 0” (see Figure 3.22).
Step 4 Select “Add Files” button on the right. Please select both c_bend3.sp and c_bend5.sp in the directory: .\ie3d\practice\output by holding down the Ctrl key. The two files will be added into the list box in the dialog. MGRID will automatically detect the strings “c_bend3” and “c_bend5”
for their short names, respectively. Please double click at the Short Name section of the two items to change them to “AEC Layers = 1” and “AEC Layers = 2”.
Figure 3.22 The S-Parameters Processing dialog.
Figure 3.23 The S-Parameters Processing dialog for s-parameter processing.
Step 5 Select the No.0 S-parameters item in the top list. Select the downward button on the right for the combo box in Processing S-Parameters (see Figures 3.22 and 3.23). It will bring out a list of possible processing features for the selected s-parameters or others.
Step 6 Select “Perform Passivity Checking on S-Parameters” from the list. Select “Go!” button.
MGRID will perform the passivity checking for you and it will create a dialog for the report (see Figure 3.24). It tells you whether the s-parameters data is passive at a specific frequency in the list. When the “Minimum Value” is non-negative, the s-parameters are passive at the frequency.
For this particular example, MGRID detect the circuit passive for all the 80 frequency points.
You can select Save Report to save the data into an ASCII file. In case, there are some
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passive points, you have the option to enforce passivity at the points. MGRID will try to adjust the s-parameters to enforce passivity. Certainly, it will change the s-parameters and the accuracy of the final results can‟t be guaranteed.
For a structure without active devices, it certainly should be passive. However, it is impossible to always guarantee an EM simulation result will always be passive even though non-passive results may happen occasionally. There are multiple reasons: (1) There are numerical errors involved. (2) There might be positive feed back loop between the source and your open structure and the positive feed back loop may cause non-passive s-parameters. There are more chances for such a case to happen especially for extension ports if the ports are not defined properly so that the port extensions, which are supposed to be not part of the circuit, are coupled with the structure. In fact, it is impossible to de-couple the port extensions and the structure. We just need to avoid the coupling is to be significant so that it will affect our results.
Figure 3.24 The passivity checking results.
Step 5 Please select OK to close the passivity checking result dialog.
There are many s-parameters processing features in the list and you can explore them. There will be more implemented in the future.
Step 6 Select Add Graph on the left. MGRID will prompt you for the Graph Type. Please select S-Parameters from the list. MGRID will set the default title of the graph as “S-S-Parameters Display”
and you can change it to whatever you like. Select OK and MGRID will prompt you for the display selection (see Figure 3.25). Please click at the No.0 item in the list box “Root=Current S-Parameters”. Check the items “dB[S(1,1)]”, “dB[S(2,1)]” and “Ang[S(2,1)]”. The repeat the selections for the “c_bend3.sp” and “c_bend4.sp”. Select OK to go back to the S-Parameters Processing dialog. The defined graph “plot_0” is listed in the list box. The preview window for
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“plot_0” is created as a tab page on the right. Click at the tab will show you the results. The s-parameters from the three files are compared (see Figure 3.26).
You can define more graphs if you like. Each additional graph will correspond to a tab page on the right. You certainly can check the data in the tab pages even though they are small. They are actually for your preview only and they don‟t contain the tables for the user defined markers for measurement.
Figure 3.25 The Display Selection for the Graph dialog.
Figure 3.26 The defined s-parameters plot is previewed in the dialog.
3-29 Figure 3.27 The defined plot is displayed on the MGRID main workspace.
Step 7 Select Continue button in the S-Parameters Processing dialog. The s-parameter plot is displayed in the MGRID workspace.
Old IE3D users may realize the big difference between IE3D 14 and older versions. The older MGRID is a SDI (Single-Document Interface). It can only show one window, and the user can open the 3D view. However, the 3D view is always on top of the drawing window and you will feel the 3D view window is in the way. We invoke MODUA to process the s-parameters, PATTERNVIEW to process radiation patterns, and another instance of MGRID to display the current and near field distributions. All the different windows from different application programs (MGRID, MODUA and PATTERNVIW) are overlapping each other and it causes confusion to users.
Starting from IE3D 14, we have integrated all the functionalities into MGRID. You can do all the different post-processing in the single interface MGRID or IE3D EM Design System. You have a big list of functionality in the Window menu of MGRID (see Figure 3.27). For this particular case, we have not saved the current distribution data (.cur file), the radiation pattern (.mpa and .pat files) and the near field (.nff). We can‟t process them right now. If you have enabled them, you will be able to access the data from the menu.
Again, the simulation results are saved either in the .geo or other related files. You can‟t modify the parameters of current structure that may affect the simulation results. Otherwise, the results will be removed even though you may still be able to access the results saved into the files.
Certainly, you can change those parameters related to the display only. For example, you can change the colors of the polygons layers, the colors and display margin of the substrate layers, etc. For changing the substrate display parameters, you should not go to Parameters->Basic Parameters to change them because there are many parameters in the dialog will affect the results. You can select Parameters->Substrate Display Parameters to access those display
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parameters not affecting the simulation results. You can also double click at the Substrates tab in the Information Bar to bring up the dialog.
For comparison, the major parameters for the 3-cases are listed in Table 3.3 and a summary is listed in Table 3.4. The simple example here demonstrates how easy you can obtain reliable accurate and reliable results from IE3D efficiently.
Table 3.3 The various simulation cases with different AEC Layer settings.
File Name C_BEND.GEO C_BEND3.GEO C_BEND5.GEO
AEC Layer 0 (or No) 1 2
AEC Ratio N/A 0.1 0.1
Multi-Layer Ratio N/A N/A 0.4
Cells 25 83 142
Unknowns 26 137 251
Zc at 10 GHz 52.429-j0.163 50.831-j0.193 50.320-j0.206
Total Time 1 2 4
Table 3.4 Summary on Simulation of the Simple Microstrip Bend.
Summary Explanation affect the s-parameters very significantly. As you can see from Figure 3.27, the dB[S(2,1)] and Ang[S(2,1)] are still quite reasonable using just single cell in the cross-section. The most difference is in the dB[S(1,1)] while the dB[S(1,1)] is below -27 dB. It mainly comes from the difference in predicting the Zc value (about 4% as shown in Table 3.3). How to find the Zc of a TLN is discussed in Appendix AQ. For such a simple structure, there is no doubt we should use AEC.
However, for some large structures, the number of unknown will become too big with AEC. We may have to disable AEC or use edge cells wisely. comes from the accuracy in modeling the Zc of the TLN. As you can see from Table 3.3, the difference between the Zc values predicted by AEC Layer = 1 and AEC Layer = 2 is less than 1%. It is very small.
However, the difference it brings to dB[S(1,1)] is 10 dB with the base at -40 dB.
Fine-tuning AEC Ratio value may improve the accuracy.
Using AEC will certainly improve the accuracy significantly. The AEC Width is also very critical. AEC Width should be about 5-20%
of the cell width. The default value of AEC Ratio = 0.1, or the AEC Width is about 10% of the standard cell size. IE3D is using non-uniform meshing. It is impossible to control AEC Width to be exact 10% or it may need to adjust for some structures. For example, if there is a narrow gap between two traces, we may need to adjust AEC Ratio to make the edge cell width not significantly larger than the gap width in order to get high accuracy results.
Normally, more and smaller edge cells will make the simulator predict more accurate loss effect.
More edge cells will capture the edge effects better. One of the major edge effects is the non-uniform current distribution in the cross-section. More and smaller edge cells can capture such a change better.
3-31 Section 3.10 Simple Ways to Build a Chamfered Bend and Other Path Structures
We discussed how we build the bend in Sections 4 and 5. MGRID allows you to enter the bend as a 7-vertex polygon. We do not mind entering the vertices one by one for this 7-vertex polygon. However, if a structure contains many bends and other geometry, it will be tedious to construct it as one ore more polygons with many vertices. You can import complicated structures from other CAD tools using GDSII, DXF, GERBER and ACIS. Numerous powerful commands are implemented into MGRID to help you build and clean structures. We will show you some simple ways to build the bend. Let‟s analyze it first.
This is a bend with the centerline defined by three vertices: (0.0375, 0.75), (0.0375, 0.0375) and (0.75, 0.0375). The three vertices and the width determine the basic shape of the bend except the corner.
We can use the Adv Edit->Build Path command to build the bend easily. Please note that the three vertices are not on the grid points. We cannot use mouse input to enter the 3 points. We can use keyboard input for them.
Figure 3.28 The object or polygon is highlighted when mouse is pointing at it.
Step 1 Run MGRID. Open the file “.\ie3d\practice\c_bend.geo”. Click at the No.2 layer with Z = 0.1 mm in the Layer Window to focus the input on the layer. Move the mouse to point at the bend.
The vertices of the bend are displayed with some markers indicating the bend polygon is highlighted.
We are going to delete the bend and rebuild it using another way.
Step 2 While the bend polygon is highlighted, click the right mouse button and you will see three useful commands in the pop-up menu:
Highlighted Object Properties Delete Highlighted Object Select Highlighted Object
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Allowing highlighted objects is a new feature on MGRID. The object can be a polygon, a text object or a port. For our case, we just want to delete the polygon.
Step 3 Please select Delete Highlighted Object from the pop-up menu. MGRID will warn you that the s-parameters results will be lost. Select Yes to continue. The bend polygon will be removed. In case the s-parameter display graph was still opened, the graph will be automatically closed.
There are many ways to delete the polygon in IE3D 14 while the above way is the simplest. You can select Edit->Select All->Polygons to select the polygon. The selected polygon will turn black color when it is selected. The status window will display “Select Polygon Mode…”. Then, you can select Edit->Delete command to delete the selected polygon.
Step 4 Select Input->Key in Absolute Location command. MGRID will prompt you to enter the X and Y-coordinates of the vertex. Enter X-Coordinate = 0.0375. Type the Tab key to shift the focus to the Y-Coordinate edit control. Enter the Y-Coordinate = 0.75. Then, hit Enter key (or select OK) to create the 1st vertex at (x, y) = (0.0375, 0.75).
Step 5 Type Shift+A. The Shift+A means that you press down the Shift key and hold it. Then, you type the A key. Finally, you release the hold Shift key. It is equivalent to selecting Input->Key In Absolute Location command. You will be prompted for the location of the next vertex.
Enter (X, Y) =(0.0375, 0.0375) at the two fields and hit Enter key. The 2nd vertex is entered at (x, y) = (0.0375, 0.0375).
The Shift+A is the accelerator for the menu item Input->Key In Absolute Location command.
It is listed in the right hand side of the menu item. You can access the menu item using the accelerator.
(a) The entered 3 vertices (b) The bend built using Build Path command Figure 3.29 The chamfered bend created using the Build Path in Adv Edit menu.
Step 6 Type Shift+R. You will be prompted for the offset of the next vertex to the last vertex. The next vertex should be located at (x, y) = (0.75, 0.0375). The last vertex is at (x, y) = (0.0375, 0.0375).
The offset should be (dx, dy) = (0.7125, 0).
The relative location between the 2nd and 3rd vertices is not straightforward. We use this command just because we want to demonstrate how we can use the Input->Key In Relative Location command. The Shift+R is the accelerator for it.
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Step 7 Enter X-offset = 0.7125 and the Y-offset = 0.0. Hit Enter key. The 3rd vertex is entered at (x, y)
= (0.75, 0.0375). We will see the 3 entered vertices forming a right angle in Figure 3.24a.
The above steps demonstrate how we can enter vertices using keyboard input (Input->Key In Relative Location and Key In Absolute Location commands).
Step 8 Select Adv. Edit->Build Path command. Enter the Path Width = 0.075 and select the Path Style as Smooth Corner (the default). Hit Enter. A chamfered bend is automatically created on MGRID as three polygons (see Figure 3.29b).
The bend is at the same location as the one created in Sections 4 and 5. However, the cut at the corner is smaller. This bend is automatically created. You may not have the control over some of the small detail. The small difference in the corner may not have significant effect to the performance of the structure. You may leave it as it is. However, if you want to make it exactly like the bend we constructed in Section 4, you need to move the vertex 1 in Figure 3.29a in the y-direction in by 0.025 mm, and the vertex 2 in Figure 3.29b in x-direction by 0.025 mm.
Step 9 While you are still in the default drawing mode, press down “Shift” and window the vertex 1 only in Figure 3.29b. What “window” means here is defined in Chapter 2. You move the mouse to the upper left corner of vertex 1. Press down the left mouse button. Drag it to the lower right corner of vertex 1. Then, you release the left mouse button.
A small rectangular marker is displayed at the vertex 1 location, indicating the vertex 1 is selected. Certainly, you can follow the standard way to select the vertex 1: Select Edit->Select Vertices command. Then, window the vertex 1 only in Figure 3.29b.
Step 10 Press down “Shift”. Press down the left mouse button and move. MGRID automatically gets into the Edit->Move Objects mode. This is a short cut to select Edit->Move Objects command. It only works when MGRID is in selection mode.
Move the mouse upward somewhere and click the left button. The “Move Object Offset to Original” dialog comes up and display the offset values (Figure 3.30).
MGRID is a very precise layout editor. The coordinates of each vertex of the polygons are described precisely using double precision floating point values. When we use the move and copy commands, we need to preserve the precision. Therefore, we always prompt you to confirm the coordinates on the move and copy commands. You can always enter precise values which mouse entry cannot capture.
There are in fact 3 snapping modes for the move and copy commands: (1) Snap to Vertex, (2) Snap to Edge, (3) Snap to Vertex. The default snapping mode is “Snap to Vertex”. To change the snapping mode, you can type F4 while you are moving the vertex. If you use the
Edit->Move Objects command, you don‟t need to keep the left mouse button pressed. You can select Edit->Change Snapping command (corresponds to F4) to change the snapping mode. In the Edit->Move Objects mode, you can also change the Moving Reference Vertex by clicking the right mouse button or use the Input->Set Moving Reference Vertex, or Shift Moving Reference by Vertex or Shift Moving Reference by Polygon command. The Moving Reference Vertex is the vertex following the cursor in the move and copy command.
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For our current moving, there isn‟t any other vertex near the vertex 1. It will not snap to any
For our current moving, there isn‟t any other vertex near the vertex 1. It will not snap to any