Chapter 5 – Momentum Techniques

Lab: Momemtum Data in the Circuit Simulator

Momentum Techniques

5

This lesson introduces the basic use model of the Momentum results in a circuit simulator. By the end of this lesson, you will be able to:

·  Specify internal ports

·  Create a data element from a Momentum simulation

·  Use Momentum data in the circuit simulator

Note

At this point, some of the basic ADS operations should be familiar. For example, detailed steps in presenting plots have been covered in the previous chapters. Primarily, only the steps that are new or different from previous exercises will be explained in detail.

If needed, refer to the previous chapters for a more detailed explanation of:

Chapter 1 – overview of the Momentum process

Chapter 2 – detailed discussion of projects

Starting from schematics

Defining ports, Setting Mesh parameters, simulation

Chapter 3 – Setup Coordinate Entry

Mesh menu explored

Chapter 4 – multiple conductors, vias, Data Reuse, Far Field plotsMomentum Data in the Circuit Simulator

Lab: Designing a 3dB Splitter

This lab will test your knowledge of the Momentum interface. All of the basic interface commands and control are assumed. This means that you already should know how to use the Momentum interface, set up and solve the substrate, mesh, and analyze.

Momentum Data Used in a Circuit Simulator

A layout that has been analyzed in Momentum produces an S-parameter data file that can be used by a circuit simulator. It can be used as an N-port device in a schematic the same as any other schematic model.

Procedure

The steps used in this lab are as follows:

  1. Open the schematic and synchronize to layout
  2. Observe the Layout
  3. Meshing a curved surface
  4. Set up a schematic to include results from a Momentum Simulation
  5. Perform an S-parameter simulation
  6. Present the results
  7. Momentum setup and simulation with embedded Thin Film Resistor

STEP 1: Open the Schematic and Synchronize to Layout

·  Open the schematic file splitter.dsn from the mom_class_prj directory.

Port Layers

By default Microstrip elements have their metallization mapped to the cond layer when Msub is not placed in the schematic. If you wish to change the layer to another (not generally recommended) then you must place an MSUB in your schematic and change Cond1 to the layer you want. The default setting for the Cond1 layer is the layout layer cond as shown at the right.

Ports are inserted into a schematic using the port icon.

When ports are placed in the schematic they are mapped to the current layout layer. It is important to be certain that they are mapped to the same layer as the component that they are wired to. On our current schematic all the metallization is on the cond layout layer.

This schematic already has the ports on it. We need to inspect what layer they are mapped to.

  1. Double click on each port and verify that its layer is set to cond as shown below.

STEP 2: Create the Layout

  1. In the schematic window, generate the layout by clicking Layout>Generate Update Layout …

  1. A dialog box appears and informs you it will start creating the layout from the P1 port. Click OK to continue.
  1. A dialog box should now appear which informs you that all 19 elements were placed without any problems.

Layout

  1. Due to the scale of the ports in the layout, ports 4 and 5 may be difficult to see/understand. Therefore, select everything in the layout window and delete it.

  1. In the layout window, click on Options>Preferences...
  2. In the Placement tab change the Size of the Port/Ground to 5 layout units. Then click on OK.
  1. On the schematic page, regenerate the layout by Clicking Layout>Generate Update Layout … and following the prompts as you previously did.

Ports 1 through 5 are shown on the layout.

·  Port 1 is used for the input

·  Ports 2 and 3 are the output ports, which should have 3dB of loss compared to the input

·  Ports 4 and 5 are used for connection of a resistor for a later circuit simulation.

Note on Substrate: the default substrate definition can be used for this lab, or the dielectric permittivity can be set to 10, as is used in the illustrations in this lab.

Background: What happens without Internal Ports

The layout shown is configured with all five ports as single (calibrated) ports.


Single ports add an unseen extra transmission line to the analysis that is used for calibration purposes. The calibration line is a half wavelength long (the length changes at every frequency). As long as the extra calibration lines do not overlap any other geometry, then the circuit is valid. However, if the extra length of transmission line were to overlap other parts of the geometry, then it can no longer be used. Ports 1, 2 and 3 are valid ports for calibration (extra lengths of line would not intersect any other geometry). Extensions to Ports 4 and 5 would cause an overlap of geometries, and if an analysis is tried, a warning is issued. Momentum will automatically convert the ports to internal ports, and continue the simulation.

Internal Ports

The following geometry is the preferred setup.

Ports 1, 2 and 3 are all Single ports. Ports 4 and 5 are defined as Internal ports.

STEP 3: Meshing a curved surface

The figures below illustrate the issues of more complex mesh controls leading to a higher mesh density.

Arc Facet Angle

Experiment with setting the Arc Facet Angle.

The decrease in the facet angle to 20 degrees provides a closer approximation to the smooth curve. As the angle is further decreased, the approximation to the curve improves, but the number of cells greatly increases. When using Momentum, designers must determine to what extent to model the curvature of a geometry. If it is important to follow the curved surface closely, then the trade-off is increased number of cells and increased simulation time.

You are encouraged to try various settings of Arc Facet Angle, Edge Mesh and Cells per Wavelength. Pre-compute the mesh to view the results of changing the mesh parameters.

·  For the purpose of this lab, use edge mesh

·  The rest of this lab was done with the mesh setting shown at the right.

Simulation

·  Simulate the circuit from 1 to 10 GHz, using AFS (the steps to do this should be understood at this point).

The results of the Momentum simulation are stored in a dataset. The results are not directly meaningful until the addition of the external resistor, which is accomplished with a circuit simulation from a schematic. STEP 4: Set up a Schematic

The simulation of the power splitter is continued on a new schematic page

  1. From the ADS Main window, open a new schematic.
  2. Save this schematic to the name splitter_data. Using the command File>Save As
  1. Insert a 5-Port S-parameter File onto the schematic page.
  1. Double click on the 5-port data item you just inserted and edit it such that it pointsto the splitter_a dataset (the adaptive frequency sweep splitter data) that we just created with our momentum simulation. First change the Type to dataset. Then point the Fileto the splitter_a.ds dataset.
  2. Click OK to accept the changes.

Finish building the circuit


The S5P component should now have the dataset name entered.

Use the palettes or library browser to find and build the rest of the schematic.

The thin film resistor (TFR) is found in the Microstrip palette. Set the values:

·  W=6 mil

·  L = 12 mil

The S-parameter ports (Term) are found in the S_Param Simulation palette.

STEP 5: Set-up for the S-Parameter Simulation

When the schematic is finished, the remaining components used for simulation need to be added.

The Msub component is located in the Microstrip palette

The S_Parameters component is located in the S_Param Simulation palette.

Set the frequency range to simulate from 1 to 10 GHz, with 0.1 GHz steps.

The Msub parameters can be set as follows:

·  H = 25 mil

·  Er = 10

·  All other at default value

This completes the building of the schematic.

Start the simulation by using the Simulate button, or the menu: Simulate>Simulate.

STEP 6: Present the Results


After the simulation has completed, open a presentation window and plot the results.

The plots shown are S21 and S31 insertion losses, and S32 port-to-port isolation. The results show about 3.02 dB for S21 and S31, and about 30 dB isolation for S32 at 6.0 GHz.

Momentum Simulation with embedded Thin Film Resistor

Open the schematic and layout splitter_wtfr.dsn. This is a similar splitter with the thin film resistor to be included in the Momentum analysis.


In order to properly model a circuit that contains multiple conductor types on one layer it will be necessary to map the additional metal in Momentum > Substrate > Create/Modify
dialog box under the Metallization tab.


The AFS simulation results can now be displayed directly after a Momentum simulation.

This is the end of lesson number 5.

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