Simulating a Spiral Inductor in Sonnet & Sonnet Lite
In this application note, the steps required to manually create and simulate a rectangular spiral inductor on PCB substrate are outlined. The size of the example circuit is one which can be analyzed using all Sonnet Products, including Sonnet Lite. Methods of plotting the results are also included. This application note uses version 13.52 of Sonnet.
For step by
step instructions on creating a spiral inductor, please see the "Spiral
Inductor" tutorial in the Getting Started manual.
The Sonnet project editor is used to enter circuits for analysis; an overview of the project editor interface is pictured below.
The circuit menu is where all of the structure information is defined. Use this and the Quick Start Guide as checklists to ensure that your model setup is complete. The various commands in the circuit menu are explored below.
Selecting Circuit => Box from the menu opens the Box Settings dialog box, shown below. Select some meaningful numbers for the box size and cell size. The cell size sets the grid to which the metal pattern is snapped.
Selecting Circuit => Units from the menu opens the Units dialog box, shown below. Set the desired length and frequency units. These units are used throughout this project.
You use the Circuits => Dielectric Layers command to open the Dielectric Layers dialog box to define your stackup. Specify the substrate and air layers. Sonnet Lite is limited to three dielectric layers and two metal levels, as pictured below.
Selecting Circuit => Metal Types opens the Metal Types dialog box which allows you to define materials for your circuit. There are two kinds of metal types: planar metal for metal polygons and via metal for via polygons. Define a planar metal type and a via metal type and set them as the default for new metal and via polygons as shown below.
TIP: At this point, if this setup (box size, substrate, metal types, etc.) is one you will use for multiple circuits, you can save your circuit as a template file for future use.
Once the setup is complete, you add the rectangular spiral to your circuit by selecting Tools => Add Metalization => Rectangular Spiral. This opens the Rectangular Spiral attributes dialog box which allows you to specify your rectangular spiral metal dimensions as shown below.
The resulting spiral is pictured below.
Next, you add the feed lines for the spiral on level 1.
Select the command Tools => Add Metalization => Draw Rectangle to add the rectangles for the feed lines.
TIP: Holding down the shift key while selecting the command allows you to add multiple rectangles without selecting the command again. To exit, push the Escape key.
You need to go to level 0 to add the air bridge and the vias to connect the airbridge to the spiral on the level below. You can use the level drop list on the Tool Bar, as pictured below, to change metal levels. You could also use the View => Up One Level command.
Next, you add the vias for the airbridge by using the Tools => Add Via => Draw Rectangle command. The default for vias is to point downward (toward the substrate) from the level on which they are added. If you have not changed the default setting, when you add the vias on level 0 they extend from level 1 (the spiral) to level 0 (where the airbridge will be placed).
Use the Tools => Add Metalization => Draw Rectangle to add the airbridge on level metal 0 to connect the feed line to the spiral. The airbridge is pictured below.
Return to level 1 and use the Tools => Add Ports command or the Add Ports button in the tool box to add ports to the feed lines.
If you wish to use reference planes, select the desired port, then use the Modify => Port Properties command to open the Port Properties dialog box which allows you to add reference planes to your circuit. Reference planes are not required, but can be useful in increasing the accuracy of your results. For a detailed discussion of de-embedding, please see the 'De-embedding" chapter in the Sonnet User's Guide available in PDF format through the Manuals button on the Sonnet task bar.
A reference plane de-embeds up to the specified position. The circuit with the reference planes added is shown below.
To set up the analysis of your circuit, use the Analysis => Setup command which opens the analysis setup dialog box shown below. This dialog box allows you to specify the sweep type, frequency points, and analysis run options.
Estimate Memory
The project editor allows you to estimate the amount of memory required to analyze your circuit before continuing. To do so, select the Analysis => Estimate Memory command. An output window, pictured below, appears on your display with information about memory use for your analysis. The memory requirement for the model is predicted. Sonnet Lite is limited to 32 Mbytes. Note that the memory is twice what it would be if we used lossless materials.
Clicking on the View Subsections button in the Estimated Memory window, opens the subsection viewer which allows you to see the subsectioning used by the analysis engine, em.
To analyze your circuit, click on the Analyze button on the project editor tool bar. The analysis monitor, shown below, provides you feedback on the analysis progress. Once the analysis is complete, you can click on the icons in the analysis monitor tool bar to view the data. Various views of the data are shown in the sections below. Note that the Far Field Viewer is not available with Sonnet Lite or LitePlus.
S-parameter data is plotted by default in the response viewer, but the data can be viewed in many formats. See the Curve => Add New Curve Group command.
There is a built-in equation capability in the Response Viewer (Equation => Add Equation Curve). The definition of inductance2 is the series inductance (in nH) between any pair of ports assuming a PI-model. Any parasitics to ground are not included in the inductance calculation.
The Quality Factor ( Q) of your project.
You can also generate a simple PI model equivalent circuit from within the Response Viewer (Output => PI-Model File). The series inductor and parasitic elements are computed. Note that with the default model options, the Lmax value is 100 nh. Since the inductance in this case was around 132 nh, it was necessary to change the model options and change the Lmax value in order to include the inductance in the output.
Sonnet Lite is a valuable tool for modeling spiral inductors which allows you to generate the geometry directly within the software. After analysis, the data can be plotted in many formats, including inductance and Q factor, versus frequency.
Reference: KB-5804
