ADS Momentum Basics
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This lesson introduces the basic elements of Momentum. By the end of this lesson, you
will be able to:

Know the basic structure of Momentum

Determine which circuits are best solved by Momentum

Understand how the Momentum interface works

Recognize Momentum examples
This chapter does not include a lab. It is intended to be an overview of Momentum.
Chapter 1 - Overview of Momentum
What is Momentum?

2.5D Simulation tool for passive circuits

Motif style interface

Layout driven (accepts arbitrary geometry)

Method of Moments technique for the planar solver

Unlimited substrates and database

S-parameter results for use in circuit simulator

Plot far-field patterns
2.5D EM Simulator
Momentum can simulate planar conductor geometries, with planar substrates in a three
dimensional space. Planar conductors have currents contained in the horizontal “x-y”
plane without a vertical “z” direction current. Vias are conductors that support vertical “z”
direction current but not planar “x-y” direction current. Because of these planar
restrictions on current flow in what would otherwise be a three-dimensional space,
Method of Moments is considered a 2.5D simulator.
Integrated Environment
Momentum works directly from Layout of ADS. Conductor geometries need to be placed
in Layout; either from the schematic auto-layout, or from the Layout environment as
primitive geometries or components. Dialog boxes allow you to specify substrate
parameters for a particular technology. The substrate Green’s Functions are calculated
and stored in a database for easy reuse in subsequent simulations. You can also
specify the type and density of the mesh. The frequency range for simulation is
specified, and after simulation, the S-parameter results are available to the graphics
server for plotting. They are also available to the circuit simulator for use as a
component in a simulation. Momentum results can also be plotted as radiation patterns
on a polar plot.
Method of Moments
The mesh used for calculations is a pattern of rectangles and triangles that divide the
circuit geometry into cells. Momentum determines the current flow through each cell by
considering each cell as a small transmission line. Each cell has a “self” effect of
capacitance and inductance, just as with a transmission line, and a mutual capacitance
and mutual inductance to every other cell in the simulation space. Once all of the
inductor and capacitor values have been determined (in reality, R, L, C and G are
calculated), the simulator solves for the current flow through the circuit. From the current
and voltage through the circuit, the S-parameters are derived.
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Chapter 1 - Overview of Momentum
Basic Momentum Use Model
Simplified Flow Graph of Momentum

Optionally, start with a schematic to generate a layout (1).

Geometries must be in Layout (2) to run a Momentum analysis.

The Momentum menu is used to create/edit substrate, define ports and mesh.

Momentum mesh will appear superimposed on the geometry outline (3).

Simulation is driven from a dialog box.

The planar solver calculates S-parameters and sends the results to the graphics
server (4).

The S-parameters from Momentum are in a dataset, ready to be used as a
component in a schematic.
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Chapter 1 - Overview of Momentum
How are Circuits Analyzed?
Substrates
A Momentum simulation requires a substrate definition in order to solve for the
geometry’s electrical performance. This definition of the vertical media can be multilayered and can include vias. Each circuit solved by Momentum can only have one
substrate definition. The substrate calculator computes the Green’s functions that the
Momentum Planar Solver will require for simulation The Green’s function is the effect of
a current impulse at all points in space, across all substrate boundaries.
Mesh
The planar geometry is divided into a mesh. The mesh is generated by the Momentum
Mesh Generator, which divides the planar surfaces into triangular and rectangular cells.
The sum of the cells is called the mesh.
The goal for any simulation is to get the desired level of accuracy in the shortest amount
of time. The tradeoff of a simulation is between mesh density and simulation time. A
denser mesh translates to a longer simulation time. At some point there is a diminishing
return of accuracy for a denser mesh.
You can customize the mesh for any planar surface. Default meshes are automatically
generated if no custom mesh is set up. Momentum provides an Edge Mesh feature that
automatically meshes to capture the effect of current crowding on the geometry edges.
At higher frequencies, the Edge Mesh provides the highest level of accuracy with the
minimum mesh density, and thus the fastest solution time.
Planar Solver
Momentum solves the current of each cell by calculating the transmission line effect of
each cell and the mutual cell-to-cell interactions. Each cell in the mesh is computed as
an electromagnetic model and the proximity of all cells is considered.
The Planar Solver uses the Green’s functions, which were calculated during the
substrate definition process to determine the mutual cell-to-cell interactions.
(Recommendation to review the Appendix sections on Current Modeling and
Method of Moments)
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Chapter 1 - Overview of Momentum
When Should You Use Momentum?
Use Momentum, instead of the circuit simulator, for the following:

No simple analytical model exists

arbitrary geometry on a uniform planar substrate

Coupling effects – thin layers and close proximity

Narrow resonant frequency responses that might not be captured when using
analytical-based models

Radiation patterns – plot far-fields for antennas and others

CPW – Coplanar waveguide results with no slot mode
Use Momentum, instead of a full 3-D Electro-magnetic simulator, for the
following:

When HFSS takes too long or 3D is not required
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Chapter 1 - Overview of Momentum
The Momentum User Interface
ADS Layout Window with Momentum Menu
Circuits must be in Layout in order to use Momentum. Circuits designed in the
schematic can be transferred to layout. If any portion of the layout is not accepted by
Momentum, a message will appear in the Status Window. In addition, layouts must have
at least one port and a valid substrate definition to be solved by Momentum.
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Chapter 1 - Overview of Momentum
The Momentum Engine
There are three parts to the Momentum engine: the Substrate Calculator, the Mesh
Generator and the Planar Solver.
Substrate Calculator
The Substrate Calculator (1) computes the substrate information using dielectric
constant, thickness, and ground plane information. These results are through the use of
a Green’s Function that describes the current and charge coupling. The computed
substrate definition includes any vias, holes and air bridges. The computed substrate is
stored in a database (2) so that it can be reused. Substrate calculations vary in
computation time depending on the number of substrate layers that have been defined,
and due to the presence of vias.
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Chapter 1 - Overview of Momentum
Mesh Generator
The Mesh Generator (3) divides the planar surfaces into cells: triangles and rectangles.
It converts a via into line segments. The density of the mesh can be controlled. Each
cell contributes RLCG coefficients that need to be determined in the computation.
Planar Solver
Finally, the Planar Solver (4) uses the substrate information and the mesh to calculate
the S-parameter solution (5). Simulation data is sent to the Graphics Server as it is
computed and any engine processes are reported to the Status Server.
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Chapter 1 - Overview of Momentum
The Momentum Menu
The Momentum interface is through the Layout Window. The process for setting up and
simulating is to work through the menu from top to bottom.
1. Enable/Disable RF Mode– Enable Quasi-Static Momentum or Full Wave
Momentum
2. Substrate – Save and Open substrate definitions. Create/Edit and Pre-compute
Substrate Functions
3. Port Editor – Dialog Box for defining port types (Single, Internal, Differential,
Common Mode, Ground Reference, Coplanar)
4. Box & Waveguide – Define or delete edgewalls around geometry. Two parallel
walls for the Waveguide mode, or four walls for the Box mode.
5. Mesh – Mesh Dialog Box for setting mesh frequency, cells per wavelength and edge
mesh. Also pre-compute mesh, view mesh summary and clear mesh.
6. Simulation – Simulation Dialog Box for setting start and stop frequency and
adaptive frequency sampling (AFS).
7. Optimization – Menu for parameterizing and optimization of layouts to meet RF
specifications
8. Post-Processing – Radiation Patterns Dialog Box.
9. Agilent HFSS – Dialog box for creating Agilent HFSS Projects
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Chapter 1 - Overview of Momentum
Momentum and Momentum RF
Momentum
Momentum is a Full-Wave Method of Moments simulator
intended to be used for RF and Microwave component, circuit
and planar antenna analysis. In the fullwave approach, the
magnetic and electric Green’s functions follow from Maxwell’s
electromagnetic equations, which include all coupling,
radiation and dispersion effects in the substrate. These
Green’s functions and as a result, also the inductors and
capacitors in the RLC network are complex and frequency
dependent.
Momentum RF
Momentum RF is a quasi-static approach, where the static
magnetic Green’s function is used to calculate the inductor
values and the static electric Green’s function is used for
the capacitor values. These follow from the magnetostatic
and electrostatic solution of Maxwell’s equations.
Due to the use of the electrostatic and magnetostatic Green’s
functions, the resulting L’s and C’s are real, frequency
independent and do not include the high frequency wave effects
(radiation). For low frequencies or electrically small
distances, the static terms dominate and the quasi-static
approach will yield similar results as compared to the
fullwave approach. As the frequency increases and the
electrical length of the circuits becomes significant, the
quasi-static simulation results will gradually start to
deviate from the fullwave results.
Mesh reduction
Standard Momentum provides an easy way to construct an
equivalent network of mutually coupled inductors and
capacitors. The number of cells in the mesh determines the
number of self and mutual coupling elements in this network.
At microwave and millimeter wave frequencies, the size and the
number of rectangular and triangular cells is mainly
determined by the electrical wavelength of the signals guided
by the planar structure.
When simulating Analog/Digital, RF board interconnection and
packaging structures at the lower RF frequencies, the
geometrical complexity of the metallization patterns leads to
a much higher number of cells than needed by the mesh density
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Chapter 1 - Overview of Momentum
wavelength criterion. Being only able to use rectangular and
triangular cells greatly limits the flexibility of the mesh
maker to construct efficient meshes with a low number of high
quality cells. Meshes generated for geometrically complex but
electrically small layouts result in a large number of
rectangular and triangular cells. This makes standard Momentum
less attractive for simulating large RF board interconnection
and packaging structures as the computer memory and time
requirements are prohibitively high.
Momentum RF removes the restriction imposed by the use of
rectangular and triangular cells. This is realized by
adopting the concept of mesh reduction. Starting from an
initial mesh of rectangular and triangular cells, a reduced
mesh is constructed by merging two or more adjacent cells.
This results in a lower-resolution mesh with a lower number
of polygonal shaped cells. The mesh reduction step can be
repeated up to the highest level, in which each
disconnected metallization pattern is represented by only
one “generalized” cell in the mesh. With each reduced level
an electromagnetic equivalent circuit can be build. In this
equivalent circuit, the capacitors model the charge buildup in the generalized cells and the inductors model the
current flowing from one generalized cell to another.
Starting from the solution of the lowest reduced level
allows building a series of simulation results with
increased accuracy. The concept of mesh reduction in
subsequent steps is illustrated below.
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Chapter 1 - Overview of Momentum
Substrates Definitions
Two types of layers are used to define the complete substrate:

Substrate Layers – the dielectric media, including any ground planes, covers, air
and all other layered material.

Layout Layers – Also known as Process Layers. The separate mask layers
that are applied to the top of the dielectric media. Typically, this would
include one or more conductor layer, a via layer, a resistor layer, etc.
When a substrate is defined, the Substrate Layers are specified first and the Layout
layers are mapped (assigned) to them. The substrate can be pre-computed and stored
in the database.
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Chapter 1 - Overview of Momentum
Dialog Box Used to Define Substrates
Substrates can be computed from the information in the dialog box. The dialog box has
two tab dialog entries; substrate layers and metalization layers. Substrates that are
created are stored in the database, and can be easily reused.
Substrate Layer Dialog
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Chapter 1 - Overview of Momentum
Metalization Layer Dialog
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Chapter 1 - Overview of Momentum
Specify the Mesh
The Mesh Generator creates the mesh pattern based on entries you specify in the Mesh
Setup menu. If no specific mesh is requested, a default global mesh is used. The
default global mesh is set to 30 cells per wavelength, based on the frequency you enter
– usually this is the highest simulation frequency.
Four types of meshes are available (1). Each type has its own Tab dialog box to specify
mesh density and whether to apply the Edge Mesh or Transmission Line Mesh feature:

Global Mesh – sets the mesh for the entire circuit.

Process Layer Mesh – sets the mesh for a specific layer.

Primitive Mesh – sets the mesh for a primitive.

Primitive Seed – sets the mesh for a primitive using x-y coordinate system
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Chapter 1 - Overview of Momentum
Typical Custom Mesh
The illustration below shows some of the typical features for the meshed layout.

Port (1)

Port Reference Line (defined as 0 degrees phase) (2)

Mesh cell (3)

Edge mesh (4)
The illustration below was meshed using the global mesh settings. For many structures,
using the global control for cells per wavelength and edge mesh yield accurate results.
However, custom meshes are provided for more control in the mesh process. For
example, a transmission line may not require the same type of mesh as a patch
antenna. In such cases, a specific process layer or a specific primitive shape can be
meshed with greater density.
2
4
3
1
Global Mesh 30 cells/wavelength and edge mesh on.
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Chapter 1 - Overview of Momentum
2
1

Primitive Mesh for center 50 cells/wavelength and edge mesh on.

(2) Primitive Mesh for ends 20 cells/wavelength

transmission line on

edge mesh on.
Momentum Examples
Example circuits are available in the Examples project directory. These examples have
already been solved, and are available as guides in doing similar types of simulations.
They include substrates, meshes, associated data and appropriate output.
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Chapter 1 - Overview of Momentum
Example: Spiral with Hole in Ground Plane
This is a typical example of a layout that has been simulated in Momentum. The vias
that support the air bridge are not visible but the spiral, hole and transition lines are
visible. Also, notice two ports on the ends of the circuit. Momentum requires at least
one port for a circuit to be simulated.
In order to reduce the capacitance of the spiral inductor to the backside ground, the
ground plane has been removed from beneath the inductor. This is accomplished in
Momentum by defining the ground plane layer as a slot layer. Slot layers are the inverse
of a metal layer, so anywhere a geometry is drawn on a slot layer it is removing the
metalization.
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Chapter 1 - Overview of Momentum
Spiral Mesh Pattern
This mesh was created using the following setup:
Global Mesh = default = 30 cells per wavelength
Mesh Frequency = 10 GHz (highest frequency for substrate)
S11 response of the spiral inductor and mesh pattern.
This is the end of lesson number 1
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