Boxes and Waveguides

When you specified the substrate definition of a circuit (from the Momentum > Substrate menu), you specified only the vertical dimension of the substrate and not the horizontal dimension. Given this definition, the substrate layers extend all the way to infinity in the horizontal direction. For many circuit designs this is not relevant and does not affect the simulation. However, there may be instances where you want to introduce horizontal boundaries. For these instances, you can use boxes or waveguides.

Boxes and waveguides enable you to specify substrate boundaries in the horizontal direction. A box enables you to set boundaries on four sides of the substrate. A waveguide enables you to set boundaries on two sides.

More specifically, for a box, you define four, perpendicular vertical planes of perfect metal as the horizontal boundaries of the substrate. These four vertical planes, or walls, form a rectangle, if you are looking at the circuit from the top, along the z-axis. A box can be used only where the top and bottom layers in the substrate definition are groundplanes or impedance termination. Thus, the four vertical metal walls, plus the top and bottom groundplanes result in a box, hence the name (the walls are the sides, and the top and bottom ground planes are lid and base of the box).

A waveguide is similar, although for a waveguide you specify only two parallel walls. The substrate is therefore only bounded in the horizontal direction perpendicular to these walls. In the horizontal direction parallel to these walls, the substrate still extends to infinity. The top and bottom layers of the substrate must also be defined as groundplanes. The two sidewalls in combination and the top and bottom covers form a waveguide.

The next section describes how to apply a box or waveguide to a circuit. For more information about applications for these items, refer to About Boxes and Waveguides.

Adding a Box

A box defines the boundaries on four sides of the circuit substrate. Either one box or one waveguide can be applied to a circuit at a time. A box can be applied to a circuit only if the top and bottom layers of the substrate definition are defined as groundplanes or impedance termination. The walls of the box are perfect metal. The ground planes can be defined either as perfect metals or a lossy metal.

Adding a box to the circuit enables you to analyze the effects of enclosing the circuit in metal, for example, to identify box resonance. Box resonance can have a significant effect on S-parameters in a small band centered around the box resonance frequency. When a simulation is performed, the resonance frequencies will be noted in the status window when the circuit is simulated, along with the frequency bands where no smooth S-parameters can be calculated.

During a simulation, all current directions on the sidewalls of the box are taken into account.

Note Calibrated ports in the circuit (i.e., Single, Differential, Coplanar and Common mode ports) must be located on and perpendicular to the box edge, or an error will occur. Internal (uncalibrated) ports may be placed anywhere in the circuit and point in any direction.

To add a box:

1. Choose Momentum > Box-Waveguide > Add Box .
2. Insert the box using one of the following two methods:
   • Position the mouse and click to define a corner of the box. Move the mouse to the diagonal corner and click.
   • From the Layout menu bar choose Insert > Coordinate Entry and use the Coordinate Entry X and Coordinate Entry Y fields to specify a corner of the box. Click Apply . Enter the coordinates of the diagonal corner and click Apply . Click Cancel to dismiss the dialog box.

The box is then displayed in the layout.

Editing a Box

Once the box is applied, you cannot change its dimensions. If you want to change the size, you must delete the current box and add a new one.

Deleting a Box

To delete a box:

1. Choose Momentum > Box-Waveguide > Delete Box .
   The box is removed from the layout.

Viewing Layout Layer Settings of a Box

The box is defined as a layout layer named momentum_box . You may review the layout layer settings, but it is a protected layer, so you should not change the settings of this layer.
To view the box layer specifications:

1. Choose Options > Layers .
2. Select momentum_box from the Layers list. The layer settings are displayed. For more information on these parameters, refer to the Schematic Capture and Layout manual.
3. Click Cancel to dismiss the dialog box.

Adding a Waveguide

A waveguide defines the boundaries on two, parallel sides of the circuit substrate. Either one box or one waveguide can be applied to a circuit at a time. A waveguide can be applied to a circuit only if the top and bottom layers of the substrate definition are defined as groundplanes or impedance termination. The walls of the waveguide are perfect metal. The ground planes can be defined either as perfect metals or metal with loss.

During a simulation, all current directions on the sidewalls of the waveguide are taken into account.
To add a waveguide:

1. Choose Momentum > Box-Waveguide > Add Waveguide .
2. Select the direction of the waveguide. To insert the waveguide parallel to the x-axis, click X-axis . To insert the waveguide parallel to the y-axis, click Y-axis .
3. Insert the waveguide using one of the following two methods:
   • Position the mouse and click to define one wall of the waveguide. Move the mouse to the position of a point on the second wall and click.
   • From the Layout menu bar choose Insert > Coordinate Entry and use the Coordinate Entry X and Coordinate Entry Y fields to specify a point on the edge of the substrate. Click Apply . Enter the coordinates of a point on the second, parallel edge of the substrate and click Apply . Click Cancel to dismiss the dialog box.

These boundaries specify the edges of the substrate and the width of the waveguide.

Editing a Waveguide

Once the waveguide is applied, you cannot change its dimensions. If you want to change the size, you must delete the current waveguide and add a new one.

Deleting a Waveguide

To delete a waveguide:

1. Choose Momentum > Box-Waveguide > Delete Waveguide .
   The waveguide is removed from the layout.

Viewing Layout Layer Settings of a Waveguide

The waveguide is defined as a layout layer named momentum_box . You may review the layout layer settings, but it is a protected layer, so you should not change the settings of this layer.

To view the waveguide layer specifications:

1. Choose Options > Layers .
2. Select momentum_box from the Layers list. The layer settings are displayed. For more information on these parameters, refer to the Schematic Capture and Layout manual.
3. Click Cancel to dismiss the dialog box.

About Boxes and Waveguides

There are a variety of reasons why you would want to simulate a circuit in a box or waveguide:

• The actual circuit is enclosed in a metal box
• Nearby metal sidewalls may affect circuit performance
• The box may resonate
• Propagating modes may be present
  These situations are discussed next.

Circuits are often encased in metal enclosures. By adding a box to your design, the metal sidewalls that are present in the real structure may be included in the simulation. This is useful is you suspect that the presence of these sidewalls will have an immediate effect on the behavior of the circuit. For example, "broad-coupled filters" are placed in metal enclosures (a box) and the sidewalls can have an significant influence on the filter characteristics.

You may want to use a box or waveguide because metal sidewalls are present in the real structure and there may be an effect from these sidewalls on the characteristics of the circuit. This can be a parasitic, unwanted effect. If the effects of the sidewalls were not taken into account while designing the circuit, you can verify any effect that the sidewalls may have on your circuit. In most cases, when the sidewalls are not too close to the actual circuit, the effect of the sidewalls on the simulation results will be marginal. There is, however a specific, significant condition, which is unique for structures with sidewalls.

In the case of a box, this is the occurrence of one or more box resonances. A box resonance is a physical effect where, under the condition of certain frequency and box size combinations, the box actually starts resonating at a certain frequency. Because a box resonance has a significant effect on S-parameters in a (small) band centered around the box resonance frequency which cannot be represented by a smooth function, no smooth adaptive S-parameters will be available in this frequency band. When simulating such a circuit, Momentum will inform you of these box resonance frequencies, of the frequency bands where there will not be smooth S-parameters available and of the quality factor (a measure for the sharpness of the box resonance, depending on the losses).

In the case of waveguide, the effect is the excitation of a waveguide mode. If your circuit will be positioned near sidewalls, you may want to add a waveguide to determine whether they have an effect on the performance of the circuit.

You may have a situation where there are no metal sidewalls in the structure, but the substrate definition is bounded by other material (for example, you have a finite-size substrate where the dielectric material abruptly ends, so you have a dielectric-air transition). Although Momentum boxes and waveguides are defined as perfect metals and not dielectric material, you may decide that defining a metallic enclosure in the simulated circuit may be more representative of the real structure than using no enclosure.

Adding Absorbing Layers under a Cover

You may want to model your box or waveguide as having absorbing layers between the covers and the layout. You can use a substrate interface layer, define its thickness, and its absorbing properties using ε and µ (make sure that ε and µ are accurate specifications for this layer). Adding absorbing layers to a box would have an effect on any box resonances that would occur by producing a weaker resonance, that is, the quality factor of the box resonances would lower significantly.

Boxes, Waveguides, and Radiation Patterns

If you have a structure enclosed in a box or waveguide and you want to calculate radiation patterns for it after the simulation, you need to set the top and bottom planes to values between 376 and 378. There are also other considerations. For more information, refer to 377 Ohm Terminations and Radiation Patterns.