Layout Components for Momentum

Layout Components are user-defined components that you can create from a layout page in ADS. The component that can be inserted in a schematic page just like any other component, and represents the (parametric) layout from the layout page. The symbol representing this component in schematic can either be a black box or a layout look-alike symbol.

Once the Layout Component is defined, you as a user can use it in one of the following ways:

You can directly include the Layout Component in the schematic. When doing a circuit simulation from the schematic environment, the Momentum planar EM solver will be called automatically during the circuit simulation (EM/Ckt co-simulation) to generate a Momentum model on the fly. The user-defined layout parameters and the most relevant Momentum simulation parameters (e.g., model type, mesh density) can be set from the schematic page. With the co -optimization feature you can also determine an optimized value for your Layout Component without returning to layout. The layout components have a built-in database mechanism that keeps track of previously calculated Momentum simulation results, that is, once the Momentum simulation is done for a certain layout component, no new EM simulations are required, unless there is a change in the parameters for this component.

The second way the Layout Components can be used is as the starting point of Advanced Model Composer (AMC), for more details refer to, the "Advanced Model Composer" manual. AMC is a generalization of Model Composer, for details refer to the "Model Composer" manual, enabling you to generate a parameterized electrical model for a Layout Component from within the layout window. After defining and creating the layout component, you can specify the ranges for the parameters (with continuous and discrete parameter range options) and launch the AMC model generation tool from within the layout environment. In contrast to the EM/Ckt co-simulation feature, AMC is an up-front model generation tool. This means that the parameterized electrical model is calculated before the component is used in a schematic design, based on a number of Momentum simulations which are run in the background. No EM simulations need to be performed when the component is used in schematic. Using an AMC component in schematic is as fast as using any other ADS built-in component. The AMC components can be added to a standard ADS design kit for easy sharing between projects and users.

Layout Components and Circuit Co-simulation

The Electromagnetic-Circuit co-simulation feature enables you to combine EM (Momentum) and circuit simulations from the schematic. From a layout in Advanced Design System, you can create and insert a (parametric) layout component in a schematic. The symbol representing this component in a schematic can either be a black box or a layout look-alike symbol.

You can set the layout and Momentum simulation parameters such as model type and mesh density from the schematic. When a circuit simulation is done in a schematic that includes a layout component, the Momentum simulation engine is automatically called as part of the circuit simulation process. The co-optimization feature enables you optimize your component during this simulation. For more information on co-optimization, refer to Co-optimization with Parameterized Layout Components

Layout components have a built-in database mechanism that keeps track of previously calculated Momentum simulation results. Once the Momentum simulation is completed for a certain layout component, no new EM simulations are required unless there is a change in the component parameters.

Note When a Layout Component is simulated during an EM/Circuit co-simulation session, a work layout design __ < name >work is created (where < _name > is the name of the layout component) with an instance of the layout component inserted. The work design is then sent to Momentum for EM simulation. The simulation results are stored back into the model database associated with the original layout component. This work design is only used temporarily and can be ignored. It is not automatically deleted after the simulation to enable you view the mesh used during the Momentum simulation. This mesh is displayed on top of the work design.

Setting up a Layout

There are two basic ways to create a layout. Use the one that best complements your own design methodology and suits your design needs:

• Layout synchronized from a schematic
• Directly drawn or imported layout

Once you have a layout, you will need to prepare it for a Momentum simulation using the following steps before you create a layout component:

1. Set up the substrate and layer mapping
2. Use Port Type Mapping to insert ports and set the port types
   You can also specify mesh settings and frequency plans if you haven't done so in the schematic window.

   Note Only single (calibrated) and internal (uncalibrated) ports are supported with the layout components. Other port types will be mapped to single or internal port types during component creation. More information on port mapping is given in Port Type Mapping.

Adding Layout Parameters

Layout components with layout parameters enable you to sweep, tune, or optimize geometrical (shape) variations of planar layout objects. This includes:

• Typical dimensions such as lengths, widths, gaps, spacing, or diameters
• Interdependent layout modifications such as length and width varying simultaneously
• Unconventional layout modifications such as varying port locations

Note There is a limit of 100 layout parameters per component.

There are two ways to create a parameterized layout:

• Using nominal/perturbed designs
• Using subnetwork parameters

These approaches are mutually exclusive and you cannot combine them when defining parameters for a single component. Nominal/perturbed parameters can be defined when the layout is flattened and contains only ports and primitive artwork shapes (such as polygons, rectangles, circles, etc.). When a combination of existing (built-in or custom defined) layout components and any other type of artwork primitives are used, the layout is hierarchical and only subnetwork parameters can be used. For more details, refer to Layout Parameters.

Note You need a Momentum license to do Momentum simulations using layout components without layout parameters and you need a Momentum Optimization license to run Momentum simulations using layout components that have layout parameters.

Using Nominal/Perturbed Designs

To define a parameter you need to provide a separate design to show the program how an incremental change in that parameter value affects your original layout. The original layout is referred to as the nominal design and the new layout is called a perturbed design . The program will guide you through this process. When you add a new parameter a separate design containing a copy of the nominal design is created. You should make your changes in this design by applying perturbations as specified in the Edit/View perturbation dialog. The modified copy is then saved as the perturbed design. You need to define a separate perturbed design for each parameter.

Note Parameters defined using the Momentum > Optimization command are not automatically converted into component parameters in ADS 2008 when the Component Parameters dialog box is opened. You must use the ADS 2006 release to perform the conversion automatically.

Use the Layout Component dialog box ( Momentum > Component > Parameters ) to set up the component parameters.

Enter a parameter name along with the nominal and perturbed values for the parameter. The nominal and perturbed values do not have to correspond to the changes in the layout implied by these parameters, however, in practice it makes most sense to use values that have a physical meaning.

Note For more information on the meaning and usage of the nominal and perturbed parameter values refer to Response Interpolation and Layout Parameterization.

Click Add to add the parameter or Edit/View Perturbation to display the Edit/View Perturbation dialog box where you can specify a Linear Stretch , Radial Stretch, or Rotation perturbation for the specified parameter.

Linear Stretch

Specifying a linear stretch for a parameter enables you to move a single selected vertex point or a group of selected vertex points linearly using a deltaX, deltaY specification.

Radial Stretch

Specifying a radial stretch for a parameter enables you to move a single selected vertex point or a group of selected vertex points radially by specifying the centerX, centerY coordinates of the center point and a factor. The following illustration shows the effect of the factor on the selected vertex points.

Rotation

Specifying a rotation for a parameter enables you to move a single selected vertex point or a group of selected vertex points by rotating over a specified angle around a center point specified by the coordinates centerX, centerY.

Note Parameters you define using the Momentum > Optimization > Parameter command will be picked up automatically as nominal/perturbed parameters.

Using Existing Layout Components

A parameterized layout can be set up using a combination of several built-in microstrip components. Use the Layout Component Parameter dialog box ( Momentum > Component > Parameters ) to specify subnetwork parameters for the layout component.

Once a subnetwork parameter is defined, it can be used to set the parameters values of one or more component instances used in the layout.

In this example, the subnetwork parameter L1 is used to set parameter value L of the MLIN instance.

Note Only the top-level design parameters can be used to set the lower level instance parameters. The use of any other variable is prohibited.

Creating a Layout Component

Use the Create Layout Component dialog box ( Momentum > Component > Create/Update ) to specify the appropriate settings for creating a layout component.

Create layout Component dialog box without recombined ports present.

Note The Add Reference Pin checkbox enables you to turn on or off the addition of an extra reference pin during layout component creation. This "extra" pin is used by Momentum to provide a reference voltage for all other pins in the layout and enables you to model ground effects (e.g., ground bounce, imperfect ground connections, etc.). The Add Reference Pin option is only available for the layout look-alike symbol selection. When selecting black box this option is unavailable (grayed out), however because black box uses standard symbols, a reference pin will appear in all schematic black box representations. This can cause problems if you are creating a structure with finite ground planes and wish to use them in the schematic. In this case, the layout look-alike component with Add Reference Pin disabled is a better choice. This option should also be disabled and the layout look-alike component used for components intended for use with the Schematic > Generate/Update command. The presence of this extra pin can cause an error during design synchronization.

Once the setup is complete, click OK to create the layout component. This layout component will have the same name as the layout design. The following actions are performed automatically:

• The technology data is saved. For more information on technology data, refer to Technology Files.

• A component symbol is created
• The component definition is updated
• The model database is initialized

An information dialog box will be displayed to confirm that the creation or update was completed successfully.

Note You need to update a layout component whenever a change is made to any layout attribute such as the parameters, ports, and the shape. You also need to reinsert or swap existing instances of the component in a schematic using Edit > Component > Swap Components.

Selecting a Symbol

You can choose between two schematic symbol types:

1. A layout look-alike symbol for which the symbol shape is a scaled copy of the layout shape.
   
2. A black box symbol with a generic rectangular shape.
   
   Scaling the layout look-alike symbol can be set in three different ways:

• Setting the minimal pin-pin distance in schematic units.
  One schematic unit corresponds with the size of most standard two port components such as resistors, capacitors, and inductors. Setting the minimal pin-pin distance to one schematic unit (1 inch) ensures that a component with a given symbol size will fit between two pins.
  
• Setting the maximum pin-pin distance in schematic units.
  The following figure illustrates the scaling when the maximal pin-pin distance is set to be equal to one schematic unit (1 inch).
  

• Mapping the layout unit dimensions to the schematic unit dimensions (recommended when combining different components in the same schematic page).

When you combine different layout components on a single schematic, a fixed, absolute scaling factor is the most appropriate choice to ensure that the relative sizes of the layout components are preserved.

Model Parameter Defaults

Each time a new layout component instance is inserted on a schematic page, the model parameter values specified during the layout component creation are applied as the initial settings for the instance. These parameters are a subset of the Momentum simulation control options in the Layout environment.

You can specify the following model parameters.

• Model Type set the mode of Momentum to be used to Momentum MW or Momentum RF
• Substrate specify the substrate description ( *.slm ) using the browse button
• Lowest Frequency lowest frequency to be used during the Momentum model generation for the adaptive frequency sweep algorithm
• Highest Frequency highest frequency to be used during the Momentum model generation for the adaptive frequency sweep algorithm
• Mesh Density number of cells per wavelength
• Arc Resolution
• Edge Mesh on/off toggle

Note If the circuit simulation requires frequency points outside the specified range, this is done through extrapolation.

Note When the Momentum Simulation Control option is set to schematic, the schematic mesh frequency is automatically set to the highest simulation frequency. When the simulation control option is set to layout, the mesh frequency is taken from the Mesh > Setup dialog in the Layout window.

Model Database Settings

When running a circuit simulation, Momentum is invoked to generate a model for the layout component. This model is stored in the layout component's model database. During the creation or update of a layout component, you have the following options on how to use the model database information that is generated.

• Delete previous database toggle deletes all previously calculated models for this component.
• Add last simulation result adds the results obtained from the last Momentum simulation invoked from the layout to the database.

Primitive and Hierarchical Components

Parameters Type	Component Type	Artwork Type
None	Primitive	AEL Macro
Subnetwork	Hierarchical	Synchronized
Nominal/Perturbed	Primitive	AEL Macro

Layout Component File Structure

The layout component is made up of a group of files that combine to define the component. Manually modifying or deleting one or more of these files may corrupt the component definition resulting in unexpected behavior.

Technology Files

During the layout component Create/Update process, four technology related files are created or modified. Three of these files define the substrate technology:

1. <substrate>_layout.lay layout (or mask) layers definition file
2. <substrate>_layout.prf layout preferences file
3. <substrate>.slm Momentum substrate definition file

The names for these files are derived from the Momentum substrate name that was chosen. The default name is the layout design name.

The fourth file is the schematic layers definition file < schematic.lay >. This file is loaded whenever a new schematic window is opened and it specifies the drawing layers for the schematic. In the case of a layout look-alike symbol, additional layers are appended to this file to draw the schematic symbol for the layout component. They can be recognized by their ` _lay ' postfix. Keep in mind that a schematic window that is already open does not dynamically update its layers definition. You will need to manually load the modified schematic.lay file.

Model Database Files

The Momentum model database is stored under the mom_dsn/<comp> directory where < comp > is the design name of the component. A model.ndx file keeps track of the models that are stored in the database.

Models are stored in a two level hierarchy. The top level differentiates entries (models) based on the model parameter settings. The bottom level differentiates entries (model samples) based on the layout parameter settings.

Two models are considered to be the same if the following model parameter settings are identical:

• Model type (ModelType)
• Simulation control (SimControl)
• Lowest model frequency (LowFreq)
• Highest model frequency (HighFreq)
• Mesh density (MeshDensity)
• Arc resolution (ArcResolution)
• Edge mesh (EdgeMesh)

For a given model, one or more (in the case of a parameterized component) samples can be generated and added to the database.

The models already available for a component can be viewed by choosing Momentum > Component > Model Database . This will open the layout Component Model Database dialog box.

In this example one model is available in the model database. By selecting the model, the parameter values for which the model is generated become visible in the Description box.

Using Layout Components in a Schematic

The layout components created in the layout window can be inserted in a schematic, either by selecting the component using the library browser (select Insert > Component > Component Library ) or by typing the name of the component in the component name entry field. The layout ports become schematic pins that can be connected to other components.

Note You need a Momentum license to do Momentum simulations using layout components without layout parameters and you need a Momentum Optimization license to run Momentum simulations using layout components that have layout parameters.

Specifying Layout Component Instance Parameters

Double click a layout component in the schematic to display the Layout Component dialog box. This dialog box has three tabs that enable you to set the parameters and their display for the layout component instance.

Model Parameters

Model Type Selection

The Model Type selection list offers the following choices:

• Momentum MW S-parameter model generated by Momentum in MW mode
• Momentum RF S-parameter model generated by Momentum in RF mode
• File Based S-parameter model available in a Dataset, CITI or Touchstone file or model given by an ADS netlist file
• Subnetwork (for hierarchical components only) the component will be netlisted based on the subnetwork topology in the schematic page of the component design and use the built-in models

Momentum Simulation Control Settings

The first three options enable you to change the settings of the most important Momentum Simulation Control parameters.

• Schematic - schematic settings overwrite the ones found in the corresponding layout design from where the component was created. Only a subset of all Momentum Control parameters can be accessed and an adaptive frequency sweep (AFS) will be performed by Momentum. For more information on adaptive frequency sampling refer to, About Adaptive Frequency Sampling.

• Layout - layout settings found in the layout design will be shown in the layout Component dialog box entries but they will be grayed-out. During a Momentum simulation the parameters specified in the layout design will be used. This allows Momentum to use parameter settings that cannot be set from the layout Component dialog box (e.g., the use of discrete frequency sweep).

Most Simulation Control parameter settings are accessible from both the Schematic and Layout windows.

Some parameters, such as Overlap , TML mesh , Frequency Sweep type , and Sample Points, can only be set from Layout control.

The Schematic Momentum control option uses the following default values for these parameters:

• Overlap - On
• TML mesh - Off
• Frequency Sweep type - AFS
• Sample Points - 50

Other parameters, such as, Layer , Primitive, and Primitive Seed cannot be set for Momentum simulations using Layout Components.

Model Database Settings

All generated models are automatically stored in the layout component model database. When performing an optimization, select the Reuse Model and Interpolate Model options to minimize the number of EM simulations.

• Reuse Model the database will be inspected, if the model for the specified parameters is available, it will be reused. When this option is not selected a new Momentum simulation is launched without inspecting the database first.

  Click the Select button to display the Select Model dialog box that lists the currently available models.

• Interpolate Model switch interpolation on or off. When this option is selected a linear interpolation scheme is used. The circuit simulator only invokes Momentum for an EM simulation if the requested model sample can not be obtained by interpolation between neighbor model samples. Neighbor model samples must be within a normalized distance (L1 measure) smaller than one from the requested sample. The normalization of the distance is with respect to the interpolation delta values specified in the Set Interpolation Options dialog box.

Click the Options button to display the Set Interpolation Options dialog box, which shows the interpolation type and the interpolation grid setting for each parameter for the current model parameter settings. You can change the interpolation grid values or reset them to their defaults (automatically calculated based on the highest model frequency and the mesh density) by clicking the Defaults button.

For more information about the model database and its model interpolation refer to Model Database Flow During Simulation.

Layout Parameters

The Parameters tab enables you to set the layout parameters (nominal/perturbed or subnetwork) similar to any other component in ADS.

Click the Optimization Setup button to specify an optimization setup for the selected parameter.

Display Parameters

The Display tab allows you to individually select which parameters will be visible on the schematic.

Port Type Mapping

Layout Port Type	Schematic Port Type
Strip	Single	Single
Internal	Internal
Differential	Single
Common Mode	Single
Ground Reference	Internal
slot	Single	Single
Coplanar	Single

The following examples illustrate how simulations with differential, ground reference, and common mode ports can be set up in the schematic in a way that is equivalent to the simulations in the layout.

Differential Ports

Ports 1,3 and 2,4 are differential port combinations. A Momentum simulation in layout with this port configuration gives a two-by-two S-parameter result with differential excitation. In a schematic, the differential ports are mapped to single ports, meaning that Momentum calculates a four-by-four S parameter result. This can then be converted to a differential excitation using the following illustration of a setup for S-parameter ports.

Common Mode Ports

Ports 1,3 and 2,4 are Common Mode port combinations. In the layout component, the Common Mode ports are mapped to Single ports. To obtain the same results as with the Common Mode ports in layout, use the setup illustrated in the following schematic.

Recombined and Ground Reference Ports

Prior to 2006A, when creating a Momentum Layout Component for a layout with Ground Reference Ports, the Ground Reference Ports were kept and pins were created in the layout look-alike symbol at the location of the ground reference ports.

In ADS 2006A Update 3, a check box has been added to the Momentum > Component > Create/Update dialog that to enables you to:

• Ignore combined port types and keep ALL pins (NEW in ADS 2006A Update 3).

or

• To NOT preserve recombined port types by unchecking the "Ignore combined port types and keep all pins" check box.

In other words, it is now possible to keep only one pin for Coplanar, Differential and Common Mode Ports. This feature also works when the Layout Component is used with Advanced Model Composer.

Note This selection is only available if "recombined port" types are present in the design. If no such port types are present, this selection will not be visable in the dialog.

Layout Component with Ground Reference Pins

Layout with 6 ports (2 single and 4 ground reference)

Ports 3 and 5 are the Ground Reference ports for Port 1 and Ports 4 and 6 are the Ground Reference ports for Port 2.The work flow for creating the layout look-alike symbol with ground reference pins is as follows:

Select Ignore combined port types and keep all pins in the Create Layout Component dialog. Set your preferences and click OK.

The Momentum Layout Component the ground reference pins are kept in the schematic view as shown in the following figure.

Momentum Layout Component look-alike symbol with 6 pins.

For design synchronization there is a 1 to1 mapping of the symbol pins, this is required for circuit simulation. In the figure below the layout ports are kept.

Layout Component without Ground Reference Pins

The work flow for creating the layout look-alike symbol without ground reference pins is as follows:

Deselect Ignore combined port types and keep all pins in the Create Layout Component dialog. Set your preferences and click OK.

In this case the ground reference pins are not kept in the Momentum Layout Component, as shown in the following figure.

Momentum Layout Component without ground reference pins.

For design synchronization there is a 1 to1 mapping of the symbol pins, this is required for circuit simulation. In the figure below the layout ports are lost.

Test procedure example

The following schematic illustrates the layout look-alike symbol, without a Global Reference pin and without the Ground Reference ports. This is accomplished by deselecting both the Add Reference Pin and Ignore combined port types and keep all pins check boxes in the Create Layout Component dialog.

Selecting Add Reference Pin while leaving Ignore combined port types and keep all pins unselected results in the following Layout look-alike symbol in the schematic.

In the following example, the Ground Reference ports are kept. Ports 3 and 4 are Ground Reference ports for port 1 and ports 5 and 6 are Ground Reference ports for port 2. In the layout component, the Ground Reference ports are mapped to Internal ports.

The following schematic illustrates the layout look-alike symbol, without a Global Reference pin and with Ground Reference ports. This is accomplished by deselecting Add Reference Pin , while selecting Ignore combined port types and keep all pins check box in the Create Layout Component dialog.

The same look-alike symbol with both Add Reference Pin and Ignore combined port types and keep all pins check boxes selected in the Create Layout Component dialog.

Note that all four EM-Circuit co-simulations yield the same S-parameter results, as shown in The following illustration.

Optimization and Tuning

During a tuning session, models cannot be generated on the fly. Consequently, tuning of parameters that influence a layout component is only possible when the requested layout component model samples are already part of the database or if the database can provide interpolated results.

If the database cannot return a valid model, tuning will fail and issue an error message indicating that the model database is incomplete. In other words, tuning is only possible within specific ranges that are covered by a sufficient number of samples.

Model Database Flow During Simulation

The following illustration shows how the model database handles a request for a model sample for a given layout component. Multiple requests may be generated during an analysis even when only one instance of the component is present. This is the case when layout component parameters are swept, optimized, or tuned.

A first model request will propagate to the Reuse Model? point. If the Reuse Model option is off, Momentum will be invoked to (re)calculate the model.

If the Reuse Model option is on, the model database will verify if the requested sample is already part of the database. If this is the case, the existing model will be reused.

If the model sample is not available yet, the component instance's Interpolate Model option will be checked. If this option is off, Momentum will be invoked to calculate a new model. The following warning message will be issued to the status window:

The model for the Layout Component is not found in the Database, a new model is generated for the requested parameter values.

If the Interpolate Model option is on, the model database will try to locate neighbor samples. Samples are considered to be neighbor samples if they are within the interpolation delta distance (L1 measured), which can be viewed and specified in the Set Interpolation Options dialog box. If no neighbor sample is found, Momentum will be invoked to calculate a new model. A warning message, which is the same as the one above, will be issued to the status window. In the case of a tuning session, the following error message will be sent to the status window.

Model Database for <instanceName> is not complete for tuning. Simulate the Layout Component first to complete the Model Database.

If at least one sample is found in the neighborhood, the database will try to interpolate in the database. If sufficient samples are not available, they will be added automatically based on the interpolation deltas that were specified (or their default settings). The number of additional samples is kept as low as possible to save computation time. When this happens, the following message is shown in the status window:

There are not enough samples available to allow interpolation, a new sample is generated and added to the Model Database.

Limitations when Using Layout Components

There are some limitations on the components that are available from the layout palettes:

• Do not use substrate components that might appear on layout component palettes. You must use the dialog box under Momentum > Substrates > Create/Modify to create or edit substrate definitions.
• Lumped elements cannot be included in a layout that is to be simulated by Momentum.
• Components from the Microstrip palette can be used, with limitations:
  • The substrate name (e.g., Subst = MSub1) is not used in Momentum. Substrate definitions must be loaded from the Momentum > Substrates menu.
  • Any electrical properties, if defined, are not used. For example, the parameters CPUA, RsT, and RsB in MFTC are ignored.
• For most of the components in Layout, only the geometry is transferred to Momentum. Any electrical properties must be defined from the Momentum menus.
• Components from the Printed Circuit Board or Waveguide palette cannot be used in a layout that will be simulated using Momentum.
• Stripline and suspended substrates components can be used, but the substrate name parameters for these components will be ignored.
• A maximum of 100 layout parameters can be used per component.