Getting Started with Momentum

This chapter is intended to help you get started with Momentum. It illustrates basic tasks and exercises using Momentum. The subsequent chapters contain more in-depth reference information and examples.

Layout Basics

There are two basic ways to create a layout:

This section describes how to create a layout automatically from a finished schematic, and how to use the basic features in a Layout window to create a layout directly.

Note
When you are through working in layout, release the Layout license so that it is available to another user. To do this, select File > Release Layout License from the Layout window.

Creating a Layout from a Complete Schematic

If all data is contained in the schematic, it is very simple to create a layout.

  1. In the Schematic window, build the schematic shown here, then choose the menu command Layout > Generate/Update Layout (the schematic is the source representation). The Generate/Update Layout dialog box appears.

    By default, the layout will begin with P1, at 0,0, with an angle of 0 degrees. There is no existing layout, so the Equivalence (the layout component that corresponds to the starting component in the schematic) is shown as not created.

    At this point, all of the elements in the schematic are highlighted, indicating that they all need to be generated.
  2. Click OK .
    The Status of Layout Generation dialog box appears. It displays the number of designs processed, the number of items regenerated (created) in the layout, the number of items that are oriented differently in the layout than in the schematic, and the number of schematic components that were not placed in the layout.

    The program automatically opens a Layout window and places the generated layout in it. The orientation of the layout is different from that of the schematic, because the layout is drawn from left to right across the page, beginning at the Starting Component .

Creating a Layout Directly

Launch Advanced Design System and create a project. To display a Layout window, choose Window > New Layout from the ADS main window, or choose Window > Layout from an open Schematic window.

The following tasks are performed the same way in a Layout window as they are in a Schematic window:

Connecting Components with a Trace

As in a Schematic window, you can connect components without having them actually touch. In a Layout window, this is done by placing a trace between the component (in the same way a wire is used in the Schematic window). Either choose Component > Trace , or click the Trace button in the toolbar.

Unlike wires in a Schematic window, a trace in a Layout window may be inserted alone (click twice to end insertion).

Shapes

In Layout you can insert shapes, using either Insert commands or toolbar icons:

With each shape, you can either click and drag to place it, or define points by coordinate entry (choose the shape, then choose Insert > Coordinate Entry ).

Note
The final segment of polygons and polylines can be entered by pressing the space-bar rather than double-clicking the mouse.

Experiment by drawing different shapes to get the idea of how each is created.

Layers

In a Layout window, items are placed on a layer. The name of the current insertion layer is displayed in the toolbar and in the status bar.

Changing the Insertion Layer

There are many ways to change the insertion layer:

Copy to a Different Layer

Experiment with copying shapes from one layer to another; use the command Edit > Advanced Copy/Paste > Copy to Layer . Note that the copied shape is placed at exactly the same coordinates as the original. Move one to see them both.

Default Layer Settings

Choose the command Options > Layers to display the Layer Editor. This is where you can edit the parameters of any defined layer, add layers, or delete existing layers.

Clicking Apply updates layer definitions but does not dismiss the dialog box.

Experiment with layer parameters. Note that you can toggle the visibility of all items on a layer. Protected means you can not select items on that layer.

Other Layout Defaults

The Options > Preferences command displays the Preferences for Layout dialog box. This dialog box has 11 tabs; clicking a tab brings the corresponding panel to the front.

Click the Grid/Snap tab.

This panel is where you set the snap grid and display grid parameters.

The display grid appears on the screen as a series of vertical and horizontal lines or dots that you can use for aligning and spacing items in the drawing area.

Adjust Grid Visibility and Color
  1. In the Display area, choose Major, Minor, or both.
  2. Choose the Type of display (Dots or Lines). You may have to zoom in to see the grid display.
  3. Click the colored rectangle next to the word Color, and choose the desired color for the grid. Click OK to dismiss the color palette.
  4. Click Apply . Experiment with different settings
    Note
    The drawing area color is the Background color under the Display tab.

Adjust Snap and Grid Spacing

The ability to display a major grid as an increment of the minor grid enables you to gauge distances and align objects better in a layout.

  1. In the Spacing area, change the Minor Grid display factors for both X and Y. The larger the number, the wider the grid spacing.
  2. Click Apply . Experiment with different settings. If a display factor makes the grid too dense to display, it is invisible unless you zoom in.
  3. Now experiment with the Major Grid.

Adjust Pin/Vertex Snap Distance

Pin/vertex snap distance represents how close the cursor must be to a pin of a component or a vertex of a shape before the cursor will snap to it.

A large value makes it easier to place an object on a snap point when you are unsure of the exact location of the snap point. A small value makes it easier to select a given snap point that has several other snap points very near it.

Place several components and several shapes in the drawing area and experiment with different settings of Pin/Vertex Snap.

Screen pix specifies sizes in terms of pixels on the screen. For example, if you choose 15, the diameter of the snap region is 15 pixels.

User Units specifies sizes in terms of the current units of the window. For example, if you are using inches and choose 0.1 user units, the diameter of the snap region is 0.1 inch.

Experiment with Snap Modes

Snap modes control where the program places objects on the page when you insert or move them; you can change snap modes when inserting or moving a component, or drawing a shape. When snap is enabled, items are pulled to the snap grid.

Experiment with different snap modes turned on or off to see how they affect the placement of items in a Layout window.


Snap Mode Priority




You can restrict or enhance the manner in which the cursor snaps by choosing
any combination of snap modes. This table lists the snap modes, and their priorities.
 Pin 1
Vertex

2
Midpoint
Intersect
Arc/Circle Center
Edge 3
Grid 4

Angle Snapping automatically occurs when only Pin snapping is enabled and you place a part so that the pin at the cursor connects to an existing part. The placed part rotates so that it properly aligns with the connected part.

For example, if you have a microstrip curve at 30° and place a microstrip line so that it connects to it, the microstrip line will snap to 30° so that it properly abuts the curve.

Enable Snap toggles snap mode on and off. You can also toggle snap mode from the Options menu itself, and there are snap mode buttons on the toolbar.

Except for pin snap, the pointer defines the selected point on the inserted object.

When you set all snap modes OFF, you can insert objects exactly where you release them on the page. This is sometimes called raw snap mode. Like other snap modes, the raw snap mode also applies when you move or stretch objects.

Pin When a pin on an object you insert, move, or stretch is within the snap distance of a pin on an existing object, the program inserts the object with its pin connected to the pin of the existing object. Pin snapping takes priority over all other snapping modes.

Vertex When the selected location on an object you insert, move, or stretch is within the snap distance of a vertex on an existing object, the program inserts that object with its selected location on the vertex of the existing object.

In vertex snap mode, a vertex is a control point or boundary corner on a primitive, or an intersection of construction lines.

Midpoint When the selected location on an object you insert, move, or stretch is within the snap distance of the midpoint of an existing object, the program inserts that object with its selected location on the midpoint of the existing object.

Intersect When the selected location on an object you insert, move, or stretch is within the snap distance of the intersection of the edges of two existing objects, the program inserts that object with its selected location on the intersection of the existing objects.

Arc/Circle Center When the selected location on an object you insert, move, or stretch is within the snap distance of the center of an existing arc or circle, the program inserts that object with its selected location on the midpoint of the existing arc or circle.

Edge When the selected location on an object you insert, move, or stretch is within the snap distance of the edge of an existing object, the program inserts that object with its selected location on the edge of the existing object. Once a point snaps to an edge, it is captured by that edge, and will slide along the edge unless you move the pointer out of the snap distance.

Because edge snapping has priority 3, if the cursor comes within snap distance of anything with priority 1 or 2 while sliding along an edge, it will snap the selected location to the priority 1 or 2 item.

Grid When the selected location on an object you insert, move, or stretch is within the snap distance of a grid point, the program inserts that object with its selected location on the grid point.

All other snap modes have priority over grid snap mode.

Hint
Whenever possible, keep grid snapping on. Once an object is off the grid, it is difficult to get it back on.

Use 45 or 90° angles to ensure that objects are aligned evenly, and to reduce the probability of small layout gaps due to round-off errors.

Drawing Tips

This chapter provides suggestions and examples when drawing layouts to be simulated using Momentum. These examples take into consideration things that are necessary during the drawing phase to ensure that requirements for Momentum are met.

Using Grid Snap Modes

Working in Layout enables you to control all geometry precisely. The variety of available snap modes can help you draw and position shapes. As an example, if you are connecting shapes, you should have the vertex snapping mode turned on. You will also want to review snap mode settings prior to adding ports to a layout.

Snap grid spacing also helps control the positioning shapes. In general, snap grid spacing should be about 1/2 of the minor grid point value. For example, if the minor grid points are 1.0, then the snap grid spacing should be 0.5. This makes it easy to know how your geometry will snap into place.

For more information on snap modes and snap grid spacing, refer to Chapter 2 of Layout .

Choosing Layout Layers

The standard set of layout layers begins with the layer named default . Do not use this layer for drawing your circuit. The first valid layer for Momentum is cond . For more information on layout layers, refer to the Schematic Capture and Layout manual.

Keeping Shapes Simple

In general, when drawing shapes, you should use a minimal number of vertices per shape, since this will make the mesh easier to compute. When drawing curved objects, consider using a relatively large value for Arc/Circle Radius (under Options > Preferences > Entry/Edit ). This will minimize the number of vertices, and facets used to represent the shape.

Merging Shapes

Merging shapes is often useful for eliminating small geometry overlaps and can also, in some cases, result in simpler mesh patterns. For example, if you draw a layout using multiple polygons and you suspect there is some overlap, merging the polygons will prevent the system from returning an overlap error. To merge the shapes, select each shape that you want to merge, then choose Edit > Merge .

Viewing Port and Object Properties

Any item selected in a layout, including ports, shapes, or other components, has associated properties. One way to view the properties is to select the object of interest, then choose Edit > Properties. This method also enables you to change object properties. If you want to change properties that have been attributed to an object for Momentum, you should change these properties only through the Momentum Menu selections.

To view the properties of the entire layout, choose Options > Info.

Adding a Port to a Circuit

You can add a port to a circuit either from the Schematic window or a Layout window. The procedures below include considerations for adding ports to a circuit that will be simulated using Momentum.

Adding a Port to a Schematic

  1. Select a port using by choosing the menu item Component > Port , or the port icon from the tool bar:
  2. Position the mouse where you want the port and click. Verify the connection has been made.
  3. Select the port and choose Edit > Component > Edit Component Parameters .
  4. The parameter layer= indicates the layer that the port is applied to. When you convert your schematic to a layout, all components will be assigned to layout layers. You can assign the port to a specific layer at this time. To change the layer, select layer= from the Select Parameter list, and choose a new layer from the Parameter Entry Mode listbox. Make sure that the layer you choose will also be mapped as a strip or slot metallization layer. For more information about metallization layers, refer to Substrates.
  5. Click OK to accept the new layer specification.

When you generate a layout from the schematic, you should verify that the port is positioned correctly. For more information, refer to the procedure for adding a port to a layout and to Considerations.

Adding a Port to a Layout

To add a port:

  1. Determine where you want to position the port. A port can be applied to:
    • The pin of a component
    • The edge of a component or object (such as a rectangle or polygon), usually at the midpoint
    • The surface of an object
      Note
      Do not place a port on the corner of an object. When you add a port to an edge, make sure the appropriate snap modes are enabled. It is a good practice to enable the modes Midpoint Snap and Edge Snap (under the Options menu) to ensure accurate simulation results.
  2. If you want to add a port to the surface of a polygon, you may need to disable the following or other snap modes (under the Options menu):
    • Pin snap
    • Edge snap
    • Midpoint snap
  3. Identify the name of the layer on which the component or shape is entered. If you don't know the name of the layout layer, choose Options > Layers . Select each layer, noting the color in the dialog box. When the color matches that of the one in the Layout window, note the name of the layout layer. Dismiss the dialog box.
  4. Select a port using by choosing the menu item Component > Port , or the port icon from the tool bar:
  5. The Port parameters dialog box is displayed. The parameter layer indicates the layer that the port will be applied to. The parameter defaults to the currently active drawing layer.
  6. This should be the same as the layer you noted in step 3. To change the layer, select layer= from the Select Parameter list, and choose a new layer from the Parameter Entry Mode listbox. Make sure that the layer you choose is also a strip or slot metallization layer. Do not apply ports to shapes that are on layers that are mapped to via metallization layers. For more information about metallization layers, refer to Substrates.
    Note
    If you cannot find the layer name, choose Options > Layers , then select the layer name of interest from the Layers list. Note the layer number in the Number field. In the Port dialog box, select the layer= parameter and, under Parameter Entry Mode, select Integer Value . Enter the layer number, then click Apply .
  7. Click Apply to accept the new layer specification.
  8. Position the mouse where you want to place the port and click. The port is added to the circuit.
    Note
    If you are applying a port to an edge, the port must be positioned so that the arrow is outside of the object, pointing inwards, and at a straight angle. Generally, this happens automatically when you add a port to an edge. You may need to zoom in to verify this.
  9. The Instance Name in the Port dialog box is incremented if you want to add another port. Verify the layer= parameter, then use the mouse to add the next port.
  10. When you are finished adding ports, click OK to dismiss the dialog box.

Considerations

Keep the following points in mind when adding ports to circuits to be simulated using Momentum:

Examples

Designing a Microstrip Line

This section is made up of an exercise that takes you through the process of creating a schematic, converting to a planar (Layout) format, preparing the layout for simulation, simulating, and generating analysis plots.

This exercise uses many default settings and a simple circuit (a microstrip line with step in width), and illustrates how quickly a design and analysis can be accomplished.

In this exercise, you will:

Terms such as substrates and meshes may be unfamiliar, so they are explained in the course of the exercise.

This and later exercises assume that you have an introductory working knowledge of Advanced Design System, such as understanding the concept of projects, and being familiar with Schematic and Layout windows and placing components.

Drawing the Circuit

The basic steps to making the microstrip line with step in width circuit include:

The schematic and layout representations are shown here. The sections that follow describe how to create both.

Creating a New Project

You should start this exercise in a new project.

  1. From the Main window, choose Tools > Preferences . Ensure that Create Initial Schematic Window is enabled. Click OK .
  2. From the Main window, choose File > New Project . The New Project dialog box appears.
  3. In the Name field, type step1 .
  4. In the Project Technology Files section, choose ADS Standard: length unit - mil
  5. Click OK.

A Schematic window appears, which is where you will enter the design.

Adding Microstrip Components to the Schematic

The steps in this section describe how to select a component. In the next section, it will be placed in the Schematic window.
Refer to the this figure to select the microstrip line component (MLIN) from the Microstrip Transmission Lines palette:

Placing the Components

The steps in this section describe how to place two microstrip lines in the Schematic window.

  1. Move the crosshairs to the Schematic window and click once to place the component. A schematic representation of the component is placed in the Schematic window.
  2. Move the cursor so that the crosshairs are directly over the right pin of the first component, and then click once to place a second component.

Cancelling Commands

If you continue to click without ending the current command, you will add another component with each click.

  1. Click the arrow button.

    The crosshairs disappear.
  2. You can also end a command by pressing the Esc key.
    You will use the end command frequently in this and other exercises, so be sure you are familiar with it.

Editing Component Parameters

Below the schematic representation of each component are some of the editable parameters of the component. This section describes how to change the width of one of the strips. The result is a microstrip step in width transmission line.

  1. Click twice on the second component that you placed. The Libra Microstrip Line dialog box opens.
  2. In the Select Parameter field, select the W (width) parameter. When the field to the right shows the value of the width, change the value in this field to 35 mil. Click Apply.
  3. Click OK to dismiss the dialog box.
    Note
    If a parameter for a component is displayed in the Schematic window, you can also edit that parameter by clicking on the value and entering a new value.
  4. Verify that the width of component on the left is set to 25 mil, and if needed, change the value of this parameter.

Adding Ports to the Circuit

To complete the circuit, you must add ports, one at the beginning of the microstrip step in width and one at the end. For Momentum, ports identify where energy enters and exits a circuit. This section describes how to add ports.

  1. In the menu bar, click the Port button.

    Move the cursor over the Schematic window and note the orientation of the ghost icon of the port. The ports should be positioned as shown in the schematic below at the end of these steps.
  2. You may need to rotate the port to the necessary orientation. If so, click the Rotate button and move the cursor back into the Schematic window. Note the rotation of the port outline, and repeat until it is correct.
  3. Move the cursor over the open pin on the left side of the left component, then click.
  4. The command to insert a port remains active. To insert a second port, change the orientation appropriately, move the cursor over the open pin on the right side of the right component, then click.
  5. End the current command. Your schematic should now look like this figure. It is a microstrip line step in width. The width of the first part of the line is 25 mil, and it increases to a width of 35 mil. The overall length is 200 mil.
    Note
    All of the components are connected. Diamond-shaped pins indicate that pins are not connected, and you will need to select and move components to make complete connections.


    Saving the Design

It is good practice to save your work periodically. This section describes how to save the schematic.

  1. Choose File > Save Design . When the Save dialog box appears, enter the name of the project, in this case, type step1 .
  2. Click OK .

Generating the Layout

A powerful feature of Advanced Design System is the ability to convert a schematic to a layout automatically. Since Momentum requires a circuit be in Layout format, this gives you the option of drawing your circuits either as schematics or as layouts. Note that if you do choose to draw in a Schematic window, footprints of the components you use must also be available in Layout. Components that are available in Layout include transmission lines and lumped components with artwork.

This section describes how to covert the microstrip line step in width schematic that you just finished to a layout.

  1. In the Schematic window, choose Layout > Generate/Update Layout . The Generate/Update Layout dialog box appears. It is not necessary to edit fields.
  2. Click OK .
  3. A Status of Layout Generation message appears indicating that the conversion is complete
  4. Click OK .
  5. A Layout window appears, showing a layout representation of the schematic. This window may be hidden by the Schematic window, so you may need to move some windows to locate the Layout window.
  6. From the Layout window, choose File > Save Design . Name the layout step1 . You now have a layout and a schematic as part of your project.

Creating a Simple Substrate

A substrate is required as part of your planar circuit. The substrate describes the media where the circuit exists. An example of a substrate is the substrate of a multilayer circuit board, which consists of:

The steps in this section describe how to define a substrate.

For the microstrip line step in width example, a substrate with the following layers will be used:

  1. From the Layout window, chose Momentum > Substrate > Create/Modify . The Create/Modify dialog box opens, showing the Substrate Layers options and parameters.
  2. In the Substrate Layers field, select FreeSpace . Move to the Substrate Layer Name field and change it to read Air . Leave all other parameters at their default values and click Apply.
  3. Keep the default layers Alumina and ///GND/// , but highlight and Cut any other layers that may be showing. Click Apply .
    You currently see three of the four substrate layers that you need. The fourth (metal) layer can be found by clicking the Metallization Layers tab.
    The metal layer is automatically positioned between the Substrate layers of Alumina and air. The microstrip line is assumed to be on this layer.
  4. Click OK to dismiss the dialog box.
  5. To save the substrate with the project, choose Momentum > Substrate > Save As. Type step1 in the Selection field and click OK . The substrate step1.slm is saved in the project networks folder.

Some other information about saving substrates:

Precomputing the Substrate

In order to perform a simulation, Green's functions that characterize the behavior of the substrate must be computed. Substrates that are supplied with Momentum already include the computations. If you are:

In these cases, the attempt to perform the computations will be ignored since the calculations are already saved in a database. Additional calculations only be performed if you extend the computation frequency range.

The steps in this section describe how to precompute the substrate. Note that for this example, computations will not be performed. The substrate that was defined in the previous example is, in fact, identical to one that is supplied with Momentum.

  1. Choose Momentum > Substrate > Precompute... . This brings up the Precompute Substrate Functions dialog box. Set the Minimum Frequency to 1 GHz and Maximum Frequency to 4 GHz. This is the range that the simulation will cover. You should check that computations exist for the entire range that you intend to simulate, but if you do not, an error will be displayed if you need to run any computations.
  2. Click OK .
    • If a dialog box opens to ask if you want to compute the substrate, click Yes . Calculations will be performed.
    • If a dialog box informs you that substrate calculations exist, click OK . The calculations will not be recomputed.

A status window also opens. You can leave this status window on the screen as you proceed with this exercise.

Note
The preceding applies to Momentum microwave mode only. In Momemtum RF mode the substrate is precomputed for all frequencies and it is not necessary to set a mimimum and maximum frequency.

Setting up Mesh Parameters

A mesh is required in order to perform a simulation. The simplest method for generating a mesh is to skip the Mesh menu entirely and let Momentum automatically generate the mesh for your circuit, but you can choose to edit parameters that control how the mesh is generated.

A mesh is a grid-like pattern of triangles and rectangles that is applied the surfaces of a circuit. Using the mesh, the current within each triangular or rectangular area is calculated, and any coupling effects in the circuit can also be calculated during the simulation. From these calculations, S-parameters are then calculated for the circuit.

Complete details about mesh definitions and generation are in Mesh.

The steps in this section describe how to set the frequency for mesh computations. All other options will remain at the default values.

  1. In the Layout window, select Momentum > Mesh > Setup... . A Mesh Setup Controls dialog box appears.
  2. Set the value in the Mesh Frequency field to 4 GHz and click OK .

Precomputing the Mesh

If you want to view the mesh before simulation, you can precompute the mesh. The mesh will not be precomputed for this exercise.

This completes the process for generating a mesh.

Performing the Simulation

The Momentum simulation process combines the Green's functions that were calculated for the substrate, plus the mesh information, and solves for currents in the circuit. Using the current calculations, S-parameters are then calculated for the circuit.
There are a variety of options to select to set up a simulation. Details are in Simulation. This section describes the minimal steps for running a simulation.

  1. From the Layout window, choose Momentum > Simulation > S-parameters . The Simulation Control dialog box appears.
  2. Set the Sweep Type to Linear and confirm that the following parameters are set:
    • Start = 1 GHz
    • Stop = 4 GHz
    • Frequency Step = 1 GHz
      By choosing this sweep type, a linear simulation will be performed over the 1-4 GHz frequency range, selecting the frequency points to be simulated based on the step size.
  3. Click Add to Frequency Plan List . The dialog box should resemble the figure here.
  4. Ensure that the field Open data display when simulation completes is enabled.
  5. Click Simulate . The simulation will be performed. The simulation progress and its completion will be indicated in the Simulation Status window.
  6. When the simulation is complete:
    • The layout is overlaid with the mesh that was generated and used during the simulation process.
    • The S-parameter simulation results are automatically displayed on both rectangular plots and Smith charts in a Data Display window. This is because the field Open data display when simulation is complete was enabled as a default in the simulation setup. Choose View > View All or click the View All button to view all of the plots, then zoom in on individual plots. An example of one S-parameter result is illustrated here.

      This completes the first Momentum design exercise.

Designing a Microstrip Filter

This section is made up of an exercise that takes you through the process of designing a microstrip coupled-line filter. It is similar to the previous exercise, but uses fewer defaults and explores how to:

Drawing the Circuit

This section is very similar to the drawing instructions in the previous exercise. The circuit is drawn as a schematic, then converted to a layout. If you need more information on how to perform certain steps, refer to the previous exercise.

Opening a New Project

Like the previous exercise, start the filter design in a new project.

  1. From the Main window, choose File > New Project .
  2. In the Name field, type filter.
  3. In the Project Technology Files section, choose ADS Standard: length unit - mil .
  4. Click OK in the New Project dialog box to create the new project.

Adding Components to the Schematic

The steps in this section describe how to select microstrip filter components.

  1. In the Schematic window, click the Palette List arrow. The Palette List drops down.
  2. Scroll and select T-lines Microstrip __ from the Palette List.
  3. In the component palette, locate and click Mcfil . This selects the Libra Microstrip Coupled-Line Filter Section component.
  4. Crosshairs and a ghost icon of the component appear as you move the cursor over the Schematic window. Position the cursor and click.
  5. End the command either by clicking the Cancel Command button or by choosing Insert or the arrow button.

Editing the Component

This section describes how to edit several component parameters, such as the length and width, and also how to change the parameters that are displayed below the component.

  1. To edit the component parameters, double click the component.
  2. In the dialog box that appears, select one of the parameters listed below, and edit the value. Also, enable Display parameter on schematic so that the parameter will be displayed below the component in the Schematic window. Select another parameter, and continue to set the values for these parameters:
    • W = line width = 0.25 mm
    • S = spacing between lines = 0.044 mm
    • L = line length = 1.8 mm
    • W1 = width of the line that connects to pin 1 = 0.25 mm
    • W2 = width of the line that connects to pin 2 = 0.25 mm
  3. Click OK to accept the edits and dismiss the dialog box.

Copying and Placing another Component

This section describes how to make a copy of the filter section and add it to the schematic.

  1. Click the filter component on the schematic to select it. A black outline appears around it.
  2. From the Schematic window, choose Edit > Copy .
  3. Choose Edit > Paste . Move the crosshairs so that they are directly over the left connector of the first component and click.
  4. End the current command.

Adding Ports

Ports are required on a circuit. If you forget to add ports to the schematic, you can always add them later after the schematic is converted to a layout. In this exercise, ports will be added to the layout.

With Momentum, you can define additional characteristics to a port. This also will be performed later in this exercise.

Your schematic should now look like the figure here. Be sure that all components are connected correctly.

Saving the Design

It is good practice to save your work periodically.

  1. Choose File > Save Design.
  2. Enter filter as the file name.
  3. Click OK .

Generating the Layout

This section describes how to covert the microstrip coupled-line filter schematic that you just finished to a layout.

  1. From the Schematic window, choose Layout > Generate/Update Layout . The Generate/Update Layout dialog box and a Layout window appear.
  2. From the Layout window, choose Options > Preferences . Click the Layout Units __ tab and set the Resolution to 0.001 mm . Click OK .
  3. In the Generate/Update Layout dialog box, enter -1.8 in the X field and 0.294 in the Y field. This step identifies the placement of P1 on the layout. This step is usually optional, but specifying the location will be helpful later when the box is added to enclose the filter. Click OK .
  4. A window displaying the results of the conversion is displayed. Click OK.

    A layout representation of the schematic appears in the Layout window. It should resemble the figure here.

  5. From the layout window, choose File > Save Design . The layout name is filter. You now have a layout and a schematic as part of your project.

Defining a Substrate

This circuit uses a relatively simple substrate. In this exercise, the thickness of some of the layers is modified. A more important task is that ground planes are added to the top and bottom of the substrate. These ground planes form the top and bottom of the metal box in which the filter will be enclosed. The substrate will have the following layers:

  1. From the Layout window, chose Momentum > Substrate > Create/Modify .
  2. Select the top layer in the Substrate Layers field. In the Boundary list, select Closed . The top ground plane is added to the substrate.
  3. In the Substrate Layers list, select Free_Space. Change the Thickness to 2846 um . This sets the layer of air to an appropriate thickness.
  4. In the Substrate Layers list, select Alumina. Change the Thickness to 254 um .
  5. Keep the bottom ground plane, but select and Cut any other layers that may appear in the substrate.
  6. Click Apply.

    Note that the top and bottom of the box enclosure are defined with groundplanes, the sides of the enclosure will be defined later in this exercise.

    You currently see four of the five substrate layers that you need. The metal layer for the microstrip filter can be found by clicking the Metallization Layers tab.

    In this instance, the metal layer is automatically positioned between the Alumina and air layers.

    Note the following points about this layer:

    • Metal layers are identified by a dashed line.
    • The word cond identifies the layout layer that is mapped to this position. Refer to the Layout window, and you will see that the microstrip circuit was automatically applied to the layout layer named cond during the translation process.
    • The word strip defines the layer such that the microstrips are metal and what surrounds the microstrips on that layer is air or dielectric. Other choices are slot and via. These are described in the exercise in the next chapter.
  7. Click OK to dismiss the dialog.
  8. Choose Momentum > Substrate > Save As. Use the file name filter and click Save . The substrate file filter.slm is saved as part of the project.
  9. Choose Momentum > Substrate > Precompute.... This opens the Precompute Substrate Functions dialog. Set Minimum Frequency to 1 GHz and Maximum Frequency to 40 GHz. Click OK. If a dialog box appears and asks if you want to compute the substrate, click Yes.

    There are instances when the computations will not be performed. This was discussed in the previous exercise. If they are performed, a window opens that indicates the status of the computations and when they are complete.

    You can choose Momentum > Substrate > Summary... at any time and see a summary of the substrate calculations that have been completed.

Adding Ports to the Layout

In Momentum, you can define the ports in your circuit to be one of several types. Depending on the type you choose, the ports will be characterized in different ways. This can impact the result of a simulation, because the simulation can take these characteristics into account. With different port types, you can select one that best matches the intended application of your layout.

More detailed information about the various port types can be found in Ports.

Ports are defined in a two-step process. First, ports are added to a circuit when the circuit is drawn. Then, in Momentum, you specify the port type in order to tailor the port to your circuit. Note that you can add the port components at any time as you draw the circuit, but before a port type can be specified, a substrate must be defined for the circuit.

This section describes how to add ports to a layout and how to specify a port type for them.

  1. From the Layout window, click the port icon. A Port dialog box appears. If necessary, set layer = cond . The ports must be on the same layout layer as the microstrip filter. Click OK .
  2. Move the cursor over the open port on the left side of the layout and click. Move the cursor over the open port on the right side of the layout and click. End the command. The layout should resemble the figure here.
  3. From the Layout window, choose Momentum > Port Editor. This opens the Port Properties Editor dialog box.
  4. If necessary, drag the dialog box away from the Layout window so that both ports are visible. Click the connector P1 .
  5. The Port Properties dialog box will change so that you can select a port type to be applied to P1.
  6. Select Internal from the Port Type drop down list and click Apply .
  7. Click the connector P2 . Select Internal from the Port Type drop down list. Click OK to accept the port type and dismiss the dialog box.

The internal port type is selected because it can be applied to the interior of a circuit. While normally this placement of the ports on the filter would not be considered internal, they are in this case because the filter is enclosed within the box. More information on this and other port types is in Ports.

Finishing the Box around the Filter

Boxes and waveguide enclosures are useful because they introduce vertical boundaries to the design. Adding a box enables you to look at box resonance, which can have a significant effect on S-parameters in a small band centered around the box resonance frequency. More information about boxes, waveguides, and their uses are in Boxes and Waveguides.

Recall that when you defined the substrate, you defined ground planes as the top and bottom of the metal box that will enclose the filter. This section describes how to specify the dimensions and location of the sidewalls of the box.

  1. From the Layout window, choose Momentum > Box & Waveguide > Add Box .
  2. You will enter specific x, y coordinates for the left and right sides of the box. Choose Insert > Coordinate Entry . Type -4.5 , -4.5 in the X,Y fields of the Coordinate Entry dialog box and click Apply .
  3. Type +4.5 , +4.5 in the X,Y fields of the Coordinate Entry dialog box and click Apply . The box appears as shown below (you may need to click View All to see the box). Click Cancel to close the dialog box.

Generating a Mesh

This section describes how to specify additional mesh parameters.

  1. From the Layout window, choose Momentum > Mesh > Setup....
  2. Set the following options and values:
    • Mesh Frequency = 40.0 GHz. You should set the mesh frequency to the highest frequency to be simulated.
    • Mesh Density- cells/wavelength = 20. This will be used to determine the density of the mesh. A mesh needs to be made up of small cells, but a mesh that is too dense over the entire circuit slows a simulation without adding much accuracy to results.
    • Edge Mesh is enabled, and Edge Width = 0.05 mm. The edge mesh adds a relatively dense mesh along the edges of objects. Since most current flows along the edges of objects, using a denser mesh only on the edges can improve the speed and accuracy of a simulation.
      Leave the other settings as they appear.
  3. When these selections are completed, click OK.
  4. Choose Momentum > Mesh > Precompute....
  5. Click OK to accept the Precompute Mesh dialog indicating a Mesh Frequency of 40 GHz. The mesh will be computed and displayed on the layout, like the figure here. If you have trouble seeing the mesh, choose Options > Layers and reset the color and pattern values for the cond layer.
  6. Save your work by choosing File > Save Design from both the Layout window and the Schematic window. Use the file name filter for both of them; Advanced Design System can distinguish the layout from the schematic and will not overwrite one with the other.

Performing the Simulation

This section describes how to set up and run the simulation. In this exercise, the adaptive sweep type is used in the simulation process. This the preferred sweep type, because it used a fast, highly accurate method of comparing simulated data points to a rational fitting model. Unlike the linear sweep type, where simulated data points are chosen in a linear fashion based on step size, adaptive sweep type data points are chosen based on where the most variance seen. Wherever S-parameters vary from the rational fitting model the most, more samples are taken.

  1. From the Layout window, choose Momentum > Simulation > S-parameters .
  2. Select the Adaptive Sweep Type and set these parameters to the following values:
    • Start = 1 GHz
    • Stop = 40 GHz
    • Sample Points Limit = 50
  3. Click Add to Frequency Plan List .
  4. Make sure that Open data display when simulation completes is enabled The S-parameters will automatically be plotted and displayed at the end of the simulation.
  5. Click Simulate. The simulation will be performed and its progress and completion will be indicated in the status window.
  6. Click Momentum > Simulation > Summary... and saved simulation details will be displayed. You can print the report if you wish.

Viewing Simulation Results

The S-parameter simulation results are automatically displayed on both rectangular plots and Smith charts in a Data Display window. An example of S12 plotted on a rectangular plot is shown here.

Drawing a Coplanar Waveguide in Layout

There are two basic methods for constructing coplanar waveguide in layout:

Drawing slots on a ground plane is the preferred method because a mesh with fewer edges or unknowns will result, compared to meshing the strips. If a coplanar waveguide design is drawn with strips, it can be converted to a slot pattern with a boolean operation if a bounding box is provided.

Designing a Coplanar Waveguide Bend

This exercise, like the previous exercises, takes you through all of the steps that are necessary when designing with Momentum: creating a layout, defining a substrate, adding ports to the circuit, defining a mesh, simulating, and viewing results. Unlike the previous exercises, this one explores the following design topics:

Creating the Layout

In this example, the circuit is created entirely within the Layout window, and a schematic representation is not used. Be sure to save your work periodically.
The layout consists:

  1. Set your preferences so that a Layout window will open instead of a Schematic window. From the Main window, choose Tools > Preferences and enable Create Initial Layout Window . Disable Create Initial Schematic Window .
  2. From the Main window, choose File > New Project .
  3. In the Name field, type cpw_bend .
  4. In the Project Technology Files section, choose ADS Standard: length unit - millimeter .
  5. Click OK.
  6. From the Layout window, choose Options > Preferences . Scroll to the Layout Units tab. Set the layout units to um and the resolution to 0.001. Click OK .
  7. There is a difference between the length units that you chose when you created the new project, and the units that you just set. The units chosen when you created the project set the units of the layout window in which you are working. The units chosen through Options > Preferences sets the units for objects that are drawn in the Layout window.
  8. You will rename the layout layers that are used for drawing the slots, bridges, and vias of the coplanar waveguide bend. Using more descriptive names can help identify which layer that each part of the circuit is drawn on. Choose Options > Layers . Select the Advanced tab. From the Layers list, select cond . In both the Name and Layer Binding fields, enter slot . Click Apply (located just below the Layers list). Select cond2 . In both the Name and Layer Binding fields, enter bridge . Click Apply . Select hole . In the Name field only, enter via and click Apply . Click OK to dismiss the dialog box.

Drawing the Waveguide Slots

The section describes how to draw the coplanar waveguide slots.

  1. Verify that slot is the current layer. The slots will be drawn on this layer. The name of the current layer is displayed in the tool bar and at the top of the Layout window. If slot is not displayed, select it from the list.
  2. Choose Insert > Rectangle , then choose Insert > Coordinate Entry . The toolbar arrow is activated and the Coordinate Entry dialog box opens.
  3. You will define four rectangles to make two waveguide bends. Establish the following values in the X,Y coordinate window, and click Apply after each entry:
    X, Y coordinates Define
    0, 0 First corner of first rectangle
    +45, +290 Second corner of first rectangle
    0, +335 First corner of second rectangle
    +335, +290 Second corner of second rectangle
    +110, 0 First corner of third rectangle
    +155, +180 Second corner of third rectangle
    +110, +180 First corner of fourth rectangle
    +335, +225 Second corner of fourth rectangle
  4. Click OK when finished, to dismiss the dialog box.
  5. Select View > View All to see your drawing.
  6. End the command. The drawing in the layout window should resemble the figure here.

Drawing the Bridges

The slots that make up the coplanar waveguide have been drawn on the layer slot . On a different layer, you will draw the bridges that span the two slots. This is important, because the slots and bridges will be treated differently in Momentum. This will become more clear when the substrate is applied to the circuit, later in this exercise.

  1. From the layer list on the toolbar, select the layer bridge and click OK .
    Note
    The slot layer is still visible. You will not do so here, but it can be made invisible if you go to the Layout window and choose Options > Layers , select the slot layer and deselect Vis to turn Visible off. The same action can be taken in Layers window by deselecting Vis in the slot layer.

  2. Choose Insert > Rectangle , then choose Insert > Coordinate Entry .
  3. Using the Coordinate Entry dialog box you will define two rectangles that make the bridges. Establish the following values in the X,Y coordinate window, and click Apply after each entry:
    X, Y coordinates Define
    0, +170 First corner of first bridge
    +155, +180 Second corner of first bridge
    +155, +180 First corner of second bridge
    +165, +335 Second corner of second bridge
  4. Click OK when finished, to dismiss the dialog box.
  5. End the current command. The drawing in the Layout window should now look like the following figure.

Drawing a Via

The vias must be drawn on a third layer. This is necessary for when the vias are mapped to the substrate, later in this exercise.

The position and size of a via is very important. The various snap modes in Layout can aid in drawing vias, as shown in this part of the exercise. More information on how to draw vias and other drawing tips are in Drawing Tips.

  1. From the layout layer list on the toolbar, select via.
  2. Choose Options > Preferences . Select the Grid/Snap tab. Under Active Snap Modes, enable Vertex and deselect Grid . Click OK .
    By using the vertex snap mode, the vias that you draw will snap to the vertex of a nearby object (a bridge) and not to a grid point. This ensures that the vias will be positioned precisely on the edges of the rectangles.
  3. Choose Insert > Polyline . Referring to the points identified on the drawing below, create the line segments shown by performing these steps:
    • Click once on A1 and click twice on A2.
    • Click once on B1 and click twice on B2.
    • Click once on C1, click once on C2, and click twice on C3. Then click the toolbar arrow. End the command. The drawing in the Layout window should now appear as shown below.

Opening a Substrate

This exercise uses a substrate that is supplied with Momentum. It will not be necessary to make any changes to it, but it will be reviewed to clarify how layout layers can be mapped to a substrate.

  1. From the Layout window, choose Momentum > Substrate > Open . To the question, Open a supplied substrate?, select Yes .
  2. Locate and select the substrate file cpw_bend_example.slm , then click Open . You are notified that the substrate has been opened. Click OK .
  3. Choose Momentum > Substrate > Create/Modify. Select the Metallization Layers tab.

    The metallization layers are identified by dashed lines, except in the case of vias. For the used metallization layers, you see the type of layer (Strip, Via, or Slot) and the name of the layout layer that is mapped to it (bridge, via, and slot). You can see how the metallization/layout layers are mapped in between the Free_Space, air, and GaAs layers:
    • There is a thin layer of air between the bridges and the slots.
    • The vias are mapped directly to the air substrate layer. The vias, although drawn as lines, are extruded by Momentum and effectively cut vertically through this layer of air, connecting the edges of the slots and the bridges.
    • All three types of metallization layers, strip, slot, and via, are used here. A strip defines the layer such that the bridges are metal and what surrounds the microstrips on that layer is air. A slot is the opposite of a strip: the layout layer would be treated as a ground plane, and the bends that you have drawn are treated as open areas in the ground plane. Vias represent vertical sections that cut through other layers vertically.

Adding Ports

This section describes how to add ports to the layout and how to apply the coplanar port type to them.

Coplanar is one of the several port types that are available in Momentum. It is designed specifically for coplanar waveguide circuits, where an electric field is likely to build up between two ports. Each of the two ports is excited with the same absolute electrical field, but with opposite polarity. The currents are equal but in opposite in direction. For more information, refer to Ports.

  1. The ports must be added to the waveguide slots and so they must be drawn on the same layer as the slots. From the Layout Layers list box, select slot .
  2. The ports must also be positioned in the center of each slot edge. To help position the ports, set the following snap modes:
    • Enable Options > Midpoint Snap
    • Enable Options > Edge Snap
    • Disable all other snap modes except Snap Enabled
  3. Click the Port icon. The Port dialog box appears.
  4. Verify that layer=slot is displayed in the Select Parameter field. If it is not, set layer to slot. Verify that Num=1 . If it is not, select Num and set it to 1. Click OK.
  5. Note the ghost image of the port. If it appears very large, or if it cannot be seen, cancel the command. Depending on your system defaults, you may need to change the value of the Port/Ground Size ( Options > Preferences > Placement ) and the value of the Font Definition Size ( Options > Preferences > Component Text) before your ports and labels will be an appropriate size for this layout.
  6. When you are ready to add ports, apply them in the order and direction as shown here, then end the port command.
  7. From the Layout window, chose Momentum > Port Editor . The Port Properties Editor dialog box appears. You may need to drag the box away from the layout in order to have access to the ports on the layout.
  8. In the Layout window, click P1. In the Port Editor, select Coplanar from the Port Type list. Set the Polarity to Normal. Do not change any other values. Click Apply.
  9. Click P2. Select Coplanar and set the Polarity to Reversed . Type 1 in the field Associate with port number . Click Apply.
  10. Click P3. Select Coplanar and set Polarity to Normal. Click Apply.
  11. Click P4. Select Coplanar and set Polarity to Reverse. Type 3 in the field Associate with port number. Click Apply.
  12. Click OK.

Generating the Mesh

This section describes how to use additional mesh parameters. Because this circuit is on multiple layers, it is possible to set mesh parameters for each layer. Thus, you can set a relatively dense mesh on a specific layer. In this example, you will not use an edge mesh on the bridge layer.

  1. From the Layout window, choose Momentum > Mesh > Setup . This opens the Setup Mesh Controls dialog box.
  2. Set the following Global parameters:
    • Mesh Frequency = 40 GHz
    • Number of Cells per Wavelength = 30
    • Enable Edge Mesh
      Global parameters will be applied to all layers, unless other values are specified from the Layer tab.
  3. Select the Layer tab.
  4. In the Layout Layers list, select bridge. Set the Number of Cells per Wavelength to 30 and disable Edge Mesh. These mesh parameters will be applied to the objects drawn on the bridge layer only.
  5. Click OK.
  6. Select Momentum > Mesh > Precompute.... Click OK to start the computations. The mesh will be computed and displayed on the layout, as shown here. Zoom in for a closer look at the mesh.
  7. Zoom in on the ports. You can see that additional arrows have been added to each port. The new arrow indicate the direction of the voltage over the slot.

Performing a Simulation

  1. From the Layout window, choose Momentum > Simulation > S-parameters....
  2. From the Sweep Type list, select Adaptive and set these parameters:
    • Start = 1 GHz
    • Stop = 40 GHz
    • Sample Points Limit = 50
  3. Click Add to Frequency Plan List .
  4. Make sure that Open data display when simulation completes is enabled.
  5. Click Simulate.

Viewing Simulation Results

The figure shows some of the automatically plotted simulation results.

To save your work, choose Yes to All when you are asked to save your work when you close the project.

This concludes the third exercise.

Examples of Parameterized Layout Components

In this section, we will illustrate some of the possibilities the user has to define layout parameters. We will illustrate:

Definition of a parallel plate capacitor

In this example, we will illustrate the usage of nominal/perturbed layout parameter definition use model. The size of the capacitor overlap area in both vertical and horizontal direction, as well as the widths of the feedlines will be defined as layout parameters.

The starting layout is shown in Capacitor example with l and w definition.

Capacitor example with l and w definition

First we will setup the parameter l as the size of the overlap area of the capacitor, using the Nominal/Perturbed use model. This parameter will indicate the size of the overlap area in the horizontal and vertical direction. Several vertex point perturbations will need to be defined to size of the capacitor area proportionally in all directions. The following strategy can be used:

Possible perturbation specification strategy
Vertex points Deltax (um) Deltay (um)
1 -5 5
2 5 5
3 5 0
4 5 0
5 5 -5
6 -5 -5
7 -5 0
8 -5 0

The following figure illustrates moving the vertex points indicated with "1" (the two vertex points in the layout that form the top left corner of the capacitor plates).

Figure 11 - Perturbation of selected vertex points

Similarly, the other vertex points need to be moved (you can leave the Edit/View perturbation window open).

The width w of the feedline can also be defined using the Nominal/Perturbed use model.

After setting up both the l and w parameters, the layout component can be created using Momentum > Component > Create/Update.

The following figure shows a couple of different instances of the newly created component.

Instances of the capacitor structure

Definition of an Airbridge

This example illustrates the usage of the Graphical Cell Compiler tool to define layout parameters. We will illustrate this for an airbridge example, where the length of the airbridge l and the width w of the feedlines will be defined as layout parameters.

The starting layout is indicated in Airbridge example with construction lines and consists of two feedlines on layer cond , the airbridge on layer cond2 and two vias connecting the airbridge to the feedlines on layer res. The name of this layout is chosen to be AMC2MACRO.

Airbridge example with construction lines

The two construction lines that will be used to parameterize this layout are already drawn on the layout.

Using the Graphical Cell Compiler tool, accessible in layout under Tools > Macro, the airbridge layout can be easily parameterized. To define the width parameter wid, select all objects in the layout excluding the vertical construction line and use the stretch command of Graphical Cell Compiler to define the layout parameter (Tools > Macro > Stretch). The length parameter len can be similarly defined by selecting all objects excluding the horizontal construction line and before using the stretch command. After defining the stretch operations, the artwork needs to be compiled using Tools > Macro > Compile.

More information on these commands and how to compile the can be found in the Graphical Cell Compiler manual.

Once the artwork macro is compiled, we can use it to create a new Layout Component and pick up the layout parameters defined in the artwork macro. To do this, first create a new layout (e.g., called AMC2) and include the AMC2MACRO in the new layout window. You need to add two new ports (p1 and p2) connected to the pins of the artwork macro.

Create the Layout Component using GCC macro

What is now left to do is to define Layout Component parameters and map those on the parameters of the artwork macro. To do this first setup two subnetwork parameters (e.g., w and l) in the Layout Component Parameters tab.

Create subnetwork parameters l and w

Second step is to map these newly defined parameters to the len and wid parameter of the artwork macro. To do this, double click on the layout artwork component (AMC2MACRO) and assign the artwork parameter len to l and wid to w.

Map subnetwork parameters

Once this step is completed, the Layout Component can be created using Momentum > Component > Create/Update...

The component can now be used like any other Layout component, either directly in schematic where a Momentum model will be calculated on the fly using the EM/ckt cosimulation feature or as the starting point for Advanced Model Composer tool.

Definition of a Spiral Inductor Component

In this example, we will use the MRINDELA and MLIN layout artwork components in layout to create a new Layout Component. We start off by creating a layout (called AMC3) which includes the two components as indicated in the figure below.

Spiral component

Two layout parameters will be defined for the Layout Component created from this layout: w (width of the lines) and n (number of segments of the spiral). First define these two parameters as subnetwork parameters in the Momentum > Component > Parameters dialog:

Subnetwork parameters for spiral

After this, assign these parameters to the appropriate parameters in the MRINDELA and MLIN components. (e.g., for MRINDELA)

Assign parameters in component

The next step is to create the Layout Component using Momentum > Component > Create/Update.
The following figure shows a couple of instances of the newly created component for different values of n (n=5,9,13,17) where w was chosen to be fixed (10 um).

Instances Spiral Component
 

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