Momentum Basics

Momentum is a part of Advanced Design System and gives you the simulation tools you need to evaluate and design modern communications systems products. Momentum is an electromagnetic simulator that computes S-parameters for general planar circuits, including microstrip, slotline, stripline, coplanar waveguide, and other topologies. Vias and airbridges connect topologies between layers, so you can simulate multilayer RF/microwave printed circuit boards, hybrids, multichip modules, and integrated circuits. Momentum gives you a complete tool set to predict the performance of high-frequency circuit boards, antennas, and ICs.

Momentum Optimization extends Momentum capability to a true design automation tool. The Momentum Optimization process varies geometry parameters automatically to help you achieve the optimal structure that meets the circuit or device performance goals. By using (parameterized) layout components you can also perform Momentum optimizations form the schematic page.

Momentum Visualization is an option that gives users a 3-dimensional perspective of simulation results, enabling you to view and animate current flow in conductors and slots, and view both 2D and 3D representations of far-field radiation patterns.
If you are unfamiliar with Advanced Design System, refer to the Quick Start in the online documentation, and to Schematic Capture and Layout . For information on the interactions between Layout and Momentum, refer to Drawing Tips.

Momentum Major Benefits

Momentum enables you to:

• Simulate when a circuit model range is exceeded or the model does not exist
• Identify parasitic coupling between components
• Go beyond simple analysis and verification to design automation of circuit performance
• Visualize current flow and 3-dimensional displays of far-field radiation

Momentum Major Features

Key features of Momentum include:

• An electromagnetic simulator based on the Method of Moments
• Adaptive frequency sampling for fast, accurate, simulation results
• Optimization tools that alter geometric dimensions of a design to achieve performance specifications
• Comprehensive data display tools for viewing results
• Equation and expression capability for performing calculations on simulated data
• Full integration in the ADS circuit simulation environment allowing EM/Circuit co-simulation and co-optimization.

Momentum Overview

Momentum commands are available from the Layout window. The following steps describe a typical process for creating and simulating a design with Momentum:

1. Choose Momentum or Momentum RF mode. Momentum can operate in two simulation modes: microwave or RF. You can select the mode based on your design goals. Use Momentum (microwave) mode for designs requiring full-wave electromagnetic simulations that include microwave radiation effects. Use Momentum RF mode for designs that are geometrically complex, electrically small, and do not radiate. You might also choose Momentum RF mode for quick simulations on new microwave models that can ignore radiation effects, and to conserve computer resources. For more information comparing the Momentum and Momentum RF modes, see About the Simulation Modes.
2. Create a physical design. You start with the physical dimensions of a planar design, such as a patch antenna or the traces on a multilayer printed circuit board. There are three ways to enter a design into Advanced Design System:
   • Convert a schematic into a physical layout
   • Draw the design using Layout
   • Import a layout from another simulator or design system. Advanced Design System can import files in a variety of formats.
     For information on converting schematics or drawing in Layout, refer Schematic Capture and Layout . For information on importing designs, refer to the manual, Importing and Exporting Designs.
3. Define the substrate characteristics. A substrate is the media upon which the circuit resides. For example, a multilayer PC board consists of various layers of metal, insulating or dielectric material, and ground planes. Other designs may include covers, or they may be open and radiate into air. A complete substrate definition is required in order to simulate a design. The substrate definition includes the number of layers in the substrate and the composition of each layer. This is also where you position the layers of your physical design within the substrate, and specify the metal characteristics of these layers. For more information, refer to, Substrates.
4. Solve the substrate. Momentum calculates the Green's functions that characterize the substrate for a specified frequency range. These calculations are stored in a database, and used later on in the simulation process. For more information, refer to Substrates.
5. Assign port properties. Ports enable you to inject energy into a circuit, which is necessary in order to analyze the behavior of your circuit. You apply ports to a circuit when you create the circuit, and then assign port properties in Momentum. There are several different types of ports that you can use in your circuit, depending on your application. For more information, refer to Ports.
6. Add a box or a waveguide. These elements enable you to specify boundaries on substrates along the horizontal plane. Without a box or waveguide, the substrate is treated as being infinitely long in the horizontal direction. This treatment is acceptable for many designs, but there may be instances where a boundaries need to be taken into account during the simulation process. A box specifies the boundaries as four perpendicular, vertical walls that make a box around the substrate. A waveguide specifies two vertical walls that cut two sides of the substrate. For more information, refer to Boxes and Waveguides.
7. Set up and generate a circuit mesh. A mesh is a pattern of rectangles and triangles that is applied to a design in order to break down (discretized) the design into small cells. A mesh is required in order to simulate the design effectively. You can specify a variety of mesh parameters to customize the mesh to your design, or use default values and let Momentum generate an optimal mesh automatically. For more information, refer to Mesh.
8. Simulate the circuit. You set up a simulation by specifying the parameters of a frequency plan, such as the frequency range of the simulation and the sweep type. When the setup is complete, you run the simulation. The simulation process uses the Green's functions computed for the substrate, plus the mesh pattern, and the currents in the design are calculated. S-parameters are then computed based on the currents. If the Adaptive Frequency Sample sweep type is chosen, a fast, accurate simulation is generated, based on a rational fit model. For more information, refer to Simulation.
9. Create Momentum components. Momentum components can be used in the schematic design environment in combination with all the standard ADS active and passive components to build and simulate circuits including the parasitic layout effects. The Momentum engine is automatically invoked to generate an S-parameter model for the Momentum component during the circuit simulation. For more information on the Momentum Components and EM/Circuit cosimulation feature, refer to Layout Components.
10. View the results. The data from a Momentum simulation is saved as S-parameters or as fields. Use the Data Display or Visualization to view S-parameters and far-field radiation patterns. For more information, refer to Viewing Results Using the Data Display and Momentum Visualization.
11. Optimization. Momentum Optimization, in combination with Momentum, is an effective software tool for automated electromagnetic (EM) design optimization of planar structures. Momentum Optimization automatically adjusts the layout in order to improve the circuit performance according to your design goals, and supports designs developed using the Momentum microwave or Momentum RF mode. For more information on Momentum optimization refer to Momentum Optimization.
12. Radiation patterns . Once the currents on the circuit are known, the electromagnetic fields can be computed. They can be expressed in the spherical coordinate system attached to your circuit. For more information on radiation patterns, refer to Radiation Patterns and Antenna Characteristics
13. Momentum Visualization . Momentum visualization enables you to view and analyze, S-parameters, currents, far-fields, antenna parameters, and transmission line data. Data can be analyzed in a variety of 2D and 3D plot formats. Some types of data are displayed in tabular form.

About the Simulation Modes

An RF simulation mode to Momentums existing microwave mode in ADS version 1.5. The microwave mode is called Momentum; the RF mode is called Momentum RF. Momentum RF provides accurate electromagnetic simulation performance at RF frequencies. At higher frequencies, as radiation effects increase, the accuracy of the Momentum RF models declines smoothly with increased frequency. Momentum RF addresses the need for faster, more stable simulations down to DC, while conserving computer resources. Typical RF applications include RF components and circuits on chips, modules, and boards, as well as digital and analog RF interconnects and packages.

When compared to the Momentum mode, the Momentum RF mode uses new technologies enabling it to simulate physical designs at RF frequencies with several useful benefits. The RF mode is based on quasi-static electromagnetic functions enabling faster simulation of designs. Momentum RF has the same use-model as Momentum in ADS, and works with Momentum Visualization and Optimization. Choose_the_mode_that_matches_the_application_shows_how_each_mode_supports_Momentum_product_features._For_a_detailed_comparison_of_the_two_simulation_modes,_see_Comparing the Microwave and RF Simulation Modes.

Selecting the Correct Mode

In the Layout window, the Momentum menu label displays the current simulation mode. To select the mode, toggle the mode setting.

• To switch the mode from Momentum to Momentum RF , choose Momentum > Enable RF Mode.
• To switch the mode from Momentum RF to Momentum , choose Momentum RF > Disable RF Mode.

In each case, the menu label in the Layout window changes to the current mode.

Deciding which mode to use depends on your application. Each mode has its advantages. In addition to specifically RF applications, Momentum RF can simulate microwave circuits. The following graph identifies which mode is best suited for various applications. As you can see, some applications can benefit from using either mode depending on your requirements. As your requirements change, you can quickly switch modes to simulate the same physical design. As an example, you may want to begin simulating microwave applications using Momentum RF for quick, initial design and optimization iterations, then switch to Momentum to include radiation effects for final design and optimization.

Choose the mode that matches the application.

Momentum RF is usually the more efficient mode when a circuit

• is electrically small
• is geometrically complex
• does not radiate

For descriptions about electrically small and geometrically complex circuits, see Matching the Simulation Mode to Circuit Characteristics.

Note For infinite ground planes with a loss conductivity specification, the MW mode of Momentum incorporates the HF losses in ground planes, however, the RF mode of Momentum will make an abstraction of these HF losses.

Locating Example Projects

Examples of designs that are simulated using Momentum and Momentum RF can be found in the Examples directory.
To open an example project:

1. From the Main window, choose File > Example Project .
2. From the Open Example Projec t dialog box, position the mouse in the Directories field and double-click Momentum .
3. In the Directories field, double-click one of the example directories: Antenna , emcktcosim , Microwave , Optimization , or RF.
4. Select a project from the Files field, then click OK .

Several of these examples are referred to in the documentation to highlight applications of various Momentum features.