Chapter 5 Current and Pattern Display, Electromagnetic Optimization

We have discussed basic technique and geometry construction in the last two chapters. We have used the MGRID, IE3D and MODUA for construction, simulation and display in chapters 3 and 4. In this chapter, we will discuss the use of MGRID and PATTERNVIEW for the current and pattern display.

Before the IE3D 9.0, we have been using the CURVIEW application program to perform current distribution, pattern calculation and display. The PATTERNVIEW was implemented later for pattern display, comparison and processing. PATTERNVIEW is certainly better than the CURVIEW in pattern display.

Starting from IE3D 9.0, for better integration, we have implemented all the features of CURVIEW into the layout editor: the MGRID. All the current distribution and pattern handling features are enhanced on the MGRID. The CURVIEW is completely replaced, even though it may still be offered in the IE3D 9.0. The CURVIEW should be phased out after the version 9.0 and we will not document its usage here. The old users are suggested to switch to MGRID and PATTERN for current and pattern handling.

In order to display the current distribution and radiation patterns of a structure, we need to run a simulation on the structure and save the current distribution data file and pattern data file. Our first example is the spiral inductor we built in Chapter 4.

Section 1. Simulation of a Spiral Inductor and Extraction of L and Q.

Step 1Run MGRID. Open c:\ie3d\practice\cspiral1.geo. Select Simulate in Process menu.

Response:

The Simulation Setup dialog comes up.

Step 2We want to sweep a wide frequency range and see how the frequency response looks like. Enter Start Freq = 0.05 GHz, End Freq = 10 GHz, Number of Freq = 200. Hit Enter key.

Response:

200 frequency points starting from 0.05 GHz to 10 GHz with step 0.05 GHz are entered in the list.

Step 3It is not suggested to enable Current Distribution File for so many frequency points. Otherwise, the file will be big. We want to check the frequency response first. Select Adaptive Intelli-Fit (AIF). Uncheck Current Distribution File. The Radiation Pattern File will be automatically un-checked when we un-check the Current Distribution File. Disable Automatic Edge Cells (AEC). AEC is good for high accuracy results. However, it will slow down the simulation. Here, we just want to have some fast results. Select OK to continue.

Response:

The MGRID will display a dialog “Errors or Warnings Detected in Port Validation” (see Figure 5.1).

Explanation:

Ports are the most difficult topics for a general user of non-Electromagnetic simulation expert to manage. Starting from the IE3D 9.0, we try to implement some validation routine to validate the ports and give suggestions to the users before a simulation. Discussions on the port validation errors and warnings are documented in the Table 5.1.

Step 4There are “High Warning”s on both ports 1 and 2. However, if you check the “v1. vs. v2” statements in the descriptions, you will see that the v1 and v2 are not very far away. For the 1st statement in description list box in the dialog, you will see “… is too big (0.108909 vs. 0.085)”. It indicates that the substrate thickness is about 20% thicker than the warning limit. It is not too much. For this spiral example, the 20 mils out of 21 mils in the substrate is some conductive material. The insulation layer is just 1 mil thick. The “effective” distance between the trace and the ground should be smaller than 21 mils due to the conductivity. It should not be so serious. Please select CONTINUE to start the simulation.

Figure 5.1 The warnings issued in port validation by MGRID.

Table 5.1 The classification of error or warning messages on port validation on MGRID.

Error/Warning / Seriousness / Description
Error / Very Serious / The users should try to fix the problem before he can do the simulation.
High Warning / Quite Serious / Normally, it is quite serious. However, the user should check the statement (v1. vs. v2) in the description of the item. If the values of v1 and v2 are close, it should not be a big problem.
Medium Warning / Somewhat Serious / It is not as serious as the High Warning. You should also check the statement (v1. vs. v2) in the description.
Low Warning / Not Serious / It should not be very serious warning. However, it is possible it may cause accuracy problem.
Notes: / It is impossible for MGRID to detect all the possible problems for you; Some detected problems may not be as serious as MGRID may think; It does not mean that your circuit is ok if MGRID did not issue any warning.

Response:

IE3D will be invoked and it will take less than 1 minute to simulate the structure on modern PC. After the simulation, MODUA is invoked to display the s-parameters. It can be in either Cartesian coordinate or Smith Chart form.

Step 5If the display is not the s-parameters in Cartesian coordinate, you can select “Define Display Graph” in Control menu of MODUA. Select “dB and Phase of S-Parameters”. Select OK to continue. Select “dB[S(1,1)]” and “dB[S(2,1)]”. Then, select OK to display the s-parameters in Cartesian coordinate (see Figure 5.2).

Figure 5.2 The frequency response of the spiral inductor.

Explanation:

It is a typical spiral inductor’s response. The S21 is close to 1 (or dB[S21] is close to 0 dB) at DC and decreasing with frequency. The S11 is increasing with frequency. Due to the lossy substrate, the S11 cannot go up to the 1. Some users may be interested in the L and Q-values of the inductor. The simple equivalent circuit of the spiral inductor is an L and an R in series. However, a user should understand it is just a very low frequency approximation. The wide-band equivalent circuit is far more complicated than even usually used and more complicated -network. In our calculation, the Q-value is calculated as the Im(Zin)/Re(Zin) when the spiral is fed as a 1-port differential feed. It may be different from other more precise model. On the IE3D, we do allow the extraction of L and Q even though we know the values are no longer meaningful at high frequency.

Step 6Select LC-Equivalent in Process menu. Select OK for the Multiple Frequency LC-Equivalence dialog. Select “With Shunt R” for the Shunt R Option in Port Definition Style for Equivalence dialog. Select OK to continue. MODUA will start the extraction for you. It will issue a warning on negative inductance. It is due to the fact that the equivalent circuit is no longer valid at some high frequency. If we enforce the equivalence, we will get negative L-value. Select YES to continue. MODUA will finish the extraction in one minute. After it finishes, it will issue a message stating that the results are saved into a file called: cspiral1.txt. Select OK to continue. The spiral1.txt file will be opened on the NOTEPAD accessory of Windows.

Explanation:

Part of the low frequency data is listed in Table 5.2. We can import the data into Microsoft Excel or similar application programs to plot the curves. Figure 5.3 shows the L and Q-values vs. frequency plotted on Microsoft Excel. As you can see, the L-values decreases and eventually goes below 0 at about 1.5 GHz, meaning that the equivalent circuit is no longer valid at 1.5 GHz. In fact, the circuit is no longer accurate beyond about 0.25 GHz. It is indicated by the Error Factor (see Table 5.2). From our experience, when the Error Factor exceeds 0.25-0.3, the equivalent circuit will depart from the original s-parameters.

In case we want to obtain the LC-equivalent circuit compatible with SPICE format. We should perform the extraction at one single frequency only. MODUA will extract the equivalent circuit at the frequency. The user can save the extracted results into SPICE format by selecting the Save SPICE File in FILE menu of MODUA.

Table 5.2 The LC-equivalent circuit parameters at low frequency.

Freq (GHz) / Error Factor / Q-Factor / Series R (Ohms) / Series L (nH) / Shunt R (Ohm) / Shunt C (pF)
0.05 / 0.06005 / 2.2928 / 3.1269 / 22.923 / 1.17E+05 / 1.5893
0.1 / 0.12107 / 4.454 / 3.1396 / 22.631 / 46819 / 1.6183
0.15 / 0.18259 / 6.5366 / 3.0881 / 22.343 / 16674 / 1.6275
0.2 / 0.24494 / 8.5289 / 2.959 / 21.984 / 8174.6 / 1.6327
0.25 / 0.30848 / 10.384 / 2.7348 / 21.545 / 4825.7 / 1.6377
0.3 / 0.37367 / 12.018 / 2.3954 / 21.026 / 3179.3 / 1.644
0.35 / 0.44097 / 13.327 / 1.9191 / 20.43 / 2245.4 / 1.6525
0.4 / 0.51092 / 14.203 / 1.2843 / 19.764 / 1661.2 / 1.6636
0.45 / 0.58407 / 14.565 / 0.46986 / 19.033 / 1269.3 / 1.6775
0.5 / 0.661 / 14.395 / -0.54348 / 18.243 / 992.38 / 1.6943

Figure 5.3 The Q- and L-values vs. frequency.

Section 2. Average Current Display.

Step 1Save the opened file c:\ie3d\practice\cspiral1.geo as c:\ie3d\practice\cspiral1a.geo. Select Simulate in Process menu.

Explanation:

This time, we want to simulate it as selected frequency points with Current Distribution File saved.

Step 2Select Delete All in the Frequency Parameters group to remove all the frequency points. Enter Start Freq = 0.5, End Freq = 2 and Number of Freq = 4. Hit Enter key.

Response:

The 4 frequency points: 0.5, 1.0, 1.5, 2.0 are entered into the list.

Step 3Enter Start Freq = 5 and hit Enter.

Response:

The f = 5 GHz is also entered into the list.

Step 4Uncheck AIF, Check Current Distribution File. Select OK to continue. MGRID will issue the warnings again. Select Continue to start the simulation. The simulation will be done in a short time.

Response:

After the simulation, MODUA is invoked to display the s-parameters. Another MGRID is invoked for meshed structure and current distribution display.

Explanation:

Before the version 9.0, CURVIEW is used for the post-processing display of the meshed structure and current distribution display. For better integration of the package, the features of CURVIEW have been completely integrated into the MGRID.

The display on the meshed structure is very similar to the display of original structure. The meshed structure is shown in the main window of MGRID as top view, and in the 3D view window. In fact, for the post-processing, the 3D view window is the main display window. There is another window showing the layers of the meshed structure. This window is also used to show the color bars for current display.

The users may find that menus on the MGRID main window are changed. The users can no longer use all the editing features of MGRID except the users can still select polygons and vertices.

Step 5Please press down the left mouse button on the 3D view and move. The 3D view of the spiral will change angles. Please hit any of the following 6 keys: ←, ↑, →, ↓, Home, End. You will see the view angles are also changed. To zoom the view, press down the “Ctrl” key and window the portion you want to view. To pan the window, press down the right mouse button on the 3D view and move. Use the above commands to adjust the 3D view to appropriate angles and size.

Step 6Select Display Current Distribution in Process menu of the MGRID main window.

Response:

The Current Distribution Display Parameters dialog is shown in Figure 5.4.

Explanation:

You can select the types of display on the combo box on the top left. The default is Average Current Distribution. At the time of this writing, there are 6 types of displays for you to select. They are documented in Appendix L. The listbox on the left lists all the 5 frequency points. A user must choose one frequency once at a time.

On the right hand side of the listbox, a user can choose the parameters for the control of the display. For example, a user can choose to display electric current or magnetic current. The user can choose whether he wants to display the boundaries of each cell. The user can choose to use dB or linear for the scaling of the color.

The lower left corner lists the layers of the structure. A user can choose which layers will be displayed. The lower right corner lists the ports and excitations. The user can choose the source type: (1) Voltage Source, (2) Current Source, and (3) Wave Source. The user can also define the magnitude and phase of the source, and the terminating impedance of each port. Please read Appendix R for the meaning of the different sources.

Figure 5.4 The Current Distribution Display Parameters dialog.

Step 7Select OK to accept the default settings for the Current Distribution Display.

Response:

The 3D view is updated with colorful display. A new window showing the port excitation appears (see Figure 5.5).

Explanation:

The spiral becomes red color. The color window shows a color bar for the scaling of the colors. Different colors at different locations of the spiral indicate the current magnitudes are different. In fact, at the low frequency of 0.5 GHz, the color is always red on the spiral, indicating that the current density does not change much on the whole spiral.

The Excitation window shows the sources magnitude and phase, the incident wave, reflected wave, voltage, current and terminating impedance at each port. The incident and reflected waves are always referenced to the 50-ohm system.

What is shown in Figure 5.5 is the average current distribution. For more information on average current distribution, please read the Appendix L.

Figure 5.5 Nearly uniform current distribution on the spiral at low frequency (0.5 GHz).

Figure 5.6 Significantly non-uniform current on the spiral at high frequency (5 GHz).

Step 8Select Display Current Distribution in Process menu of the MGRID main window again.

Response:

The Current Distribution Display Parameters dialog comes up again.

Explanation:

This time, the Auto Color Scaling is automatically un-checked. The Max E-Current is 329.81. This is the maximum electric current density detected at 0.5 GHz for the last display. It was the reference for 0 dB in the last display. It may not be the maximum electric current for other frequencies. However, for comparison, we will not change it for the display of other frequency points. If you want to get the maximum electric current density at a specific frequency point, you should check Auto Color Scaling for each frequency’s display.

Step 9Select “Freq = 5 GHz” in the frequency listbox. Select OK to continue.

Response:

The 3D view is updated. We will see more different colors on the spiral (Figure 5.6).

Explanation:

At higher frequency, the current will become more distributive. The more colors on the spiral indicate that the current is stronger at some locations than at other locations. If we want to distinguish the colors more, we can select the Display Current Distribution menu item again, and change the “Mag Scale” and “dB Step” values. We will not do it here. Interested users can try it out.

Section 3. Vector Current Display.

What are shown in Figures 5.5 and 5.6 are the average current distribution. It indicates where the current are strong or weak as a time average value. However, we cannot get the direction of the current at specific locations. As it is discussed in Appendix L, in the frequency domain, current distribution is a complex vector function. In the other word, the direction of the vector is normally time varying. Therefore, it is meaningless to display the time average vector current. Instead, we will be interested in the vector current at specific locations at different times. From the directions at specific times, we will know the polarization properties of antenna structures. We will demonstrate how we can display the vector current at a specific time.

Step 1Select Display Current Distribution in Process menu of the MGRID main window again.

Response:

The Current Distribution Display Parameters dialog comes up.

Step 2Select Vector Current Display in the combo box at the upper left corner (see Figure 5.7). Make sure the frequency is 5 GHz.

Explanation:

You may find that there are a few parameters activated: (1) Vector Shape, (2) Vector Size, and (3) Cycle Count. On the vector current display on the CURVIEW, the size of the vectors indicates the magnitude of the current density at a specific location at a specific time. It is realized that the magnitude of current density may differ in a few orders at different locations. The difference of orders may cause huge difference in the vector size. On the MGRID, we implement a better scheme with same size of vectors. The magnitude of the current density is represented by the colors of the vectors in terms of dB.

The vectors on MGRID are cone shaped vectors. The parameter of Vector Shape defines the ratio of the diameter of the cone bottom circle over the length of the cone. Its default value is 0.5. The parameter of Vector Size defines the size of the vectors. Its default size is calculated automatically based upon the size of the meshing. The Cycle Count defines the display time. What we are simulating is the frequency domain response of the current distribution. The current will be changing as the sine function of time and frequency. In some sense, when we display the vector current distribution in time, we only need to display them at some fractions of a cycle. The Cycle Count that takes the value from 0 to 1 defines the time in a cycle.

Figure 5.7 The dialog after Vector Current Distribution is selected.

Step 3Make sure the parameters are like what are shown in Figure 5.7. Select OK to continue. The Vector Current Distribution will be shown.

Step 4You can see the vectors are pointing at the directions of the trace, as expected. The color of the trace is in uniform brown color. The colors of the vectors are changing, indicating the strength of the current at different location at the time of 0.25 cycle. Select Display Current Distribution in Process menu again. Change the Vector Size to 2 mm.

Response:

The size of the vectors is increased and shown in Figure 5.8.

Step 5Select Current Distribution Display in Process menu. Change the Cycle Count from 0.25 to 0.5. Select OK to continue.

Response:

The colors of the vectors are changed indicating the change of vector current distribution at different time.