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Chapter 13. EM: Automated Circuit Extraction (ACE)

Automated Circuit Extraction (ACE) is a method of automatically modeling the electrical characteristics of a single signal line or a group of signal lines using the built-in transmission line models, discontinuities, lumped elements, and via models. The signal lines can be comprised of iNets and transmission line elements. Unlike EM extraction which uses an EM simulator, ACE uses the circuit simulator which leads to a meaningful simulation of a circuit in a relatively short time.

While ACE is an advanced method of electrical analysis of the signal lines and interconnects using the transmission line models, the quality of the simulation results is directly related to the configuration of the extraction. The extraction does not guarantee accurate results just because it uses the models to represent the circuit. Extraction effectively removes the tedious tasks required to construct the circuit schematic using a number of elements. The set up of an extraction should be considered the same as constructing the circuit using the schematic elements, (making sure that the circuit is represented by proper models).

The Net(s) or distributed models to be associated in ACE can be selected either in a schematic or in a layout. A separate model parameter is used to define this association. When this mode of simulation is selected, an extracted document is automatically generated under EM Structures in the Project Browser using the schematic layout and the associated STACKUP properties. The types of model used for the circuit need to be defined in the extraction options. The circuit simulator then runs and uses the built-in models to construct a circuit as per extraction setup to represent the layout. The simulation results then automatically represent the schematic for the layout.

The geometry to be simulated in the extraction comes directly from the Layout View of the commanding schematic. This concept is essential to the methodology of the extraction process; the contents of the Layout View are electrically modeled using the built-in models.

After a group of layout nets or circuit models are associated with an extraction, the electrical models of these shapes are effectively added to the schematic, and ports are added at every connection to the shapes outside of the extraction group. When simulation proceeds, the layout shapes of each of the associated nets are written into an extracted document, modeled using the built-in models and simulated using the linear simulator. These simulation results are then automatically incorporated back into the entire schematic as if including them as a schematic subcircuit with built-in models. Once set up, the extraction reflects any changes in the layout as a new analysis.

Elements that can be selected for ACE extraction include iNets, MLIN, CLIN_SYM_2-10, CLIN_ASYM2-16, MCFIL, MLEF, MCTRACE, and MTRACE2 elements. The advantage of including these elements in an ACE simulation is that ACE can calculate the coupling between these models where the models themselves may not account for the coupling. For example, MLIN models in a schematic have no way of accounting for coupling if they are close together. To account for coupling, you can use a coupled microstrip model, or include the two MLIN models in an ACE extraction.

13.1. Overview of Extraction Setup

This section presents the basic steps required to set up a Circuit Based extraction.

Consider the schematic shown in Figure 13.1, “Circuit Schematic for Automated Circuit Extraction ”, with its corresponding layout shown in Figure 13.2, “Original Schematic Layout”. The schematic consists of two sets of resistors connected by wire. The resistors shown in the layout are not extracted. The goal is to connect the resistors in layout with iNet routing and then model the interconnects using the ACE feature.

Figure 13.1. Circuit Schematic for Automated Circuit Extraction

Circuit Schematic for Automated Circuit Extraction

Figure 13.2. Original Schematic Layout

Original Schematic Layout

13.1.1. Extraction Process

Before the extraction process, perform the following steps manually:

  1. Set up Line Types in the LPF (Layout Processing File). See “The Layout Process File (LPF)” for information on how to configure the LPF. This is essential, as the extraction process only recognizes the line types defined in STACKUP. The same line types then must be used with iNet routing.

  2. The extraction process is controlled by the STACKUP element, which needs to be set up carefully to best represent the circuit and to ensure the accuracy of the results.

  3. Via modeling must be set up. A Via mapping text file and via model need to be defined to model the via during extraction.

13.1.2. Configuring the STACKUP Element

The first step in setting up the extraction process is to set up the STACKUP element. If this element is already configured for the Process Design Kit (PDK) you can use it from the kit.

The STACKUP element is used to set up the dielectric stackup for the circuit being extracted. It represents a generalized multi-layered substrate stackup. Figure 13.3, “Addition of the STACKUP Element to the Original Schematic.” shows the addition of this element.

Figure 13.3. Addition of the STACKUP Element to the Original Schematic.

Addition of the STACKUP Element to the Original Schematic.

After you add the STACKUP element you need to set its properties to represent the desired dielectric stackup. In this example, it duplicates the stackup of the MSUB elements. Double-click the STACKUP element to display its Element Options dialog box. You must configure the Material Defs, Dielectric Layers, Materials, and Line Type tabs. See “Configuring Stackup” for details on setting up the STACKUP.

Figure 13.4, “Nets routed with iNets” shows the layout nets routed with iNets. The upper set of resistors is routed on line M1 and the lower one is routed with line M2.

Figure 13.4. Nets routed with iNets

Nets routed with iNets

13.1.3. Adding the Extraction Control Element

The second step in setting up an automatic extraction is to add an EXTRACT control element to the schematic. The EXTRACT element extracts the net(s) and assigns them to a group which is used by a linear circuit simulator. The X and Y_Cell_Size and Extension parameters are not used and can be ignored. The EXTRACT element is located in the Elements Browser in the Simulation Control category. Figure 13.5, “Addition of the EXTRACT Simulation Control” shows the schematic with the EXTRACT simulation control added.

Figure 13.5. Addition of the EXTRACT Simulation Control

Addition of the EXTRACT Simulation Control

The EXTRACT element parameters must be configured for automatic extraction. See the EXTRACT element in the AWR Microwave Office Element Catalog Help for details. The following briefly reviews these parameters in the context of the extraction process:

  • EM_Doc - the name of the extracted document to be created to collect the nets associated with the extraction.

  • Name - the name for the extraction group. This name is also used for extraction through levels of hierarchy.

  • Simulator - the name of the simulator used to simulate this extraction document. This should be set to ACE.

  • X and Y Cell Size - the cell sizes used in the extracted document. This does not affect the circuit simulation.

  • STACKUP - the name of the STACKUP element used to determine the dielectric stackup of the extracted document. If let blank, it uses the first one found in the schematic or the Global Definitions.

  • Create_Enclosure - turns on or off the automatic reconfiguration of stackup after the extracted document is created.

  • Create_Shapes - determines whether or not to send the shapes to the extracted document every time a simulation is required.

  • Extension - not used in circuit extraction.

  • Override_Options - simulator options can be set on the EXTRACT block as well as directly on the EM document. With this option set to Yes, the options set on the EXTRACT block override the options set directly on the EM structure.

  • Hierarchy - determines how to treat this extraction block if used at lower levels of hierarchy, see “Extraction Through Hierarchy” for more information.

  • Extract_LineTypes - set this to No.

If the extracted network will be simulated with a transient simulator, the EXTRACT block has a SPICE Model Generation tab that controls how ACE extracted netlists are used in transient simulations.

13.1.4. Adding iNets and/or Line Models to the Extraction Group

Both iNets and certain distributed models are supported for ACE, including the MLIN, CLIN_SYM_2-10, CLIN_ASYM2-16, MCFIL, MLEF, MCTRACE, and MTRACE2 elements.

The next step in setting up the circuit extraction is to associate iNets and/or line models with the extraction group. In this example both of the iNets are associated with the extraction group. There are two ways to associate with the extraction group. One is to select the wire or model in the schematic and the other is to select the layout objects. Selecting a wire in the schematic selects the entire corresponding iNet in layout. In layout, selecting just a section of an iNet selects the entire iNet for extraction. This is often more convenient, as in the Layout View it is easier to identify nets that are in close physical proximity.

In this example, in the Schematic View select a wire, right-click it, and choose Edit Net Properties to display the Element Options dialog box. Click the Model Options tab. In the EM Extraction Options area, select the Enable check box and in Group name enter the extraction group name as "Circuit_Extract". Figure 13.6, “Model Options Tab Configured for Extraction” shows the Model Options tab with these settings.

Figure 13.6. Model Options Tab Configured for Extraction

Model Options Tab Configured for Extraction

If working in Layout View, select an iNet, right-click it, and choose Element Properties. Click the Model Options tab and assign an extraction group.

Note that you can select multiple objects in Schematic or Layout Views and assign them for extraction.

After associating nets for extractions, select the EXTRACT element in the schematic and notice that all the elements associated with the extraction are highlighted as shown in Figure 13.7, “Schematic Wires Association with EXTRACT Element”. Similarly, in the Layout View, the iNets associated with the extraction are highlighted as shown in Figure 13.8, “iNets Association with EXTRACT Element”.

Figure 13.7. Schematic Wires Association with EXTRACT Element

Schematic Wires Association with EXTRACT Element

Figure 13.8. iNets Association with EXTRACT Element

iNets Association with EXTRACT Element

13.1.5. Configuring ACE Simulation Options

ACE options are set up on the EXTRACT block. The accuracy of the circuit simulation depends on this configuration. Double-click the EXTRACT block. In the Element Options dialog box click the ACE tab in Figure 13.9, “ACE EXTRACT Setup Dialog Box”.

Figure 13.9. ACE EXTRACT Setup Dialog Box

ACE EXTRACT Setup Dialog Box

The models used to construct a circuit for an extracted document depend on the ACE options. For more information on these options, see “Options Dialog Box: ACE Tab”.

The circuit for this example is extracted using the settings shown in Figure 13.9, “ACE EXTRACT Setup Dialog Box”. Add a graph to plot S21 for the schematic or the extracted document and simulate. When an extraction is first performed (or the extracted document is first created) the set of frequencies from the schematic with the EXTRACT block are copied to the extracted document frequencies. The extracted document frequencies are not set to use the project default frequencies. Once the extracted document is set up with these frequencies, no further automatic update of the frequencies is done, unless the extracted document is deleted and recreated. A new set of frequencies can then be assigned to the extracted document and this remains unchanged. The circuit simulator constructs a circuit to represent the layout using the models specified in the Circuit Extract options. The circuit that gets simulated is equivalent to the circuit that you would create in a schematic using the element models. After the simulation is complete, click the Status Window button on the toolbar to display the Simulation Information window. Click the "Extracted netlist information" line in the Status Window to display the netlist for the circuit that was sent to the circuit simulator. The netlist displays as shown in Figure 13.10, “Netlist of the Circuit Representing the Extracted Document”. Notice that the models the circuit simulator uses to construct the circuit are the same as those in the Elements Browser.

Figure 13.10. Netlist of the Circuit Representing the Extracted Document

Netlist of the Circuit Representing the Extracted Document

You can view the circuit that represents the extracted document by adding an annotation to the 3D view of the extracted document. In the Project Browser, right-click the extracted document "Circuit_Extract_Doc" and choose Add Annotation. Expand Annotate > ERC under Measurement Type and choose EXT_CKT3D under Measurement and add the measurement as shown in Figure 13.11, “Add Annotation to Display the Circuit Representing the Extracted Document”. For more information on this measurement, see the AWR Microwave Office Measurement Catalog Help.

Figure 13.11. Add Annotation to Display the Circuit Representing the Extracted Document

Add Annotation to Display the Circuit Representing the Extracted Document

The circuit is annotated as shown in Figure 13.12, “Extracted Circuit Annotated in the 3D View”

Figure 13.12. Extracted Circuit Annotated in the 3D View

Extracted Circuit Annotated in the 3D View

13.1.6. Viewing an EM Structure without Simulating

Now that the entire extraction is configured, the extracted document is first created and is further configured for built-in model selection before simulation. If this step is not performed (simulated after setting up the extraction), the extracted document is automatically created and is simulated with the default circuit setup. To extract the structure before simulation, in the Project Browser under Circuit Schematics, select the schematic containing the EXTRACT simulation control, right-click and choose Add Extraction.

Figure 13.13. Creating the EM Document without Simulation

Creating the EM Document without Simulation

The extracted document specified by the EXTRACT control is created. Figure 13.14, “Extracted Document” shows the extracted document.

Figure 13.14. Extracted Document

Extracted Document

13.2. Advanced Extraction Topics

This section includes advanced extraction topic such as multiple extraction.

13.2.1. Multiple Extractions in a Single Schematic

You can apply a multiple extraction within the same schematic. You should use this technique if an interconnect or interconnects in the layout need to be modeled differently than the others. To enable multiple extractions, simply add another EXTRACT control to the schematic. Figure 13.15, “Schematic with Two Extraction Groups Highlighting the Elements in the Second Extraction Group” and Figure 13.16, “Layout View of Isolated Nets Highlighting the Elements in the Second Extraction Group” show a schematic and layout that have two isolated nets and two EXTRACT controls. Note that the Name parameter of the second EXTRACT control is changed to "Circuit_Extract_Line2" and the EM_Doc parameter is set to "Circuit_Extract_Doc_Line2". As in the previous section, the top line routed with M1 is added to the extraction group named "Circuit_Extract". The bottom line routed with M2 is assigned to the extraction group named "Circuit_Extract_Line2". Figure 13.17, “EM Document Produced by the "Circuit_Extract" Extraction Group” and Figure 13.18, “EM Document Produced by the "Circuit_Extract_Line2" Extraction Group” show the two extracted documents created by the two extractions.

Figure 13.15. Schematic with Two Extraction Groups Highlighting the Elements in the Second Extraction Group

Schematic with Two Extraction Groups Highlighting the Elements in the Second Extraction Group

Figure 13.16. Layout View of Isolated Nets Highlighting the Elements in the Second Extraction Group

Layout View of Isolated Nets Highlighting the Elements in the Second Extraction Group

Figure 13.17. EM Document Produced by the "Circuit_Extract" Extraction Group

EM Document Produced by the "Circuit_Extract" Extraction Group

Figure 13.18. EM Document Produced by the "Circuit_Extract_Line2" Extraction Group

EM Document Produced by the "Circuit_Extract_Line2" Extraction Group

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