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Where To Find This Example

AWR Version 13

Understanding AWR .emz Files

AWR Version 12

This example is not available for AWR Version 12 or earlier.

Design Notes

Custom Sized Wave Ports in Analyst

Wave ports in Analyst do not need to extend across the entire side of the bounding box. The width and height of the wave port can be set explicitly by the user.

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This project does not contain data sets. This example is available with data sets at: https://awrcorp.com/support/help.aspx?id=97.

Overview

Wave ports in Analyst are required to be on one of the simulation boundaries of the geometry. By default, the port extends across the entire length of the boundary. The dimensions of the port can be resized to cover only part of the boundary face. The advantages of resizing the port are:

A smaller port can give an accurate solution for the mode with fewer unknowns. This makes the port solve faster. Also, since the number of unknowns is smaller in the port, it may reduce the size of the interior 3D mesh. This is because the interior mesh must connect to the port mesh at their mutual interface.

A smaller port allows the boundary to be a perfect conductor, thereby shorting together conductors touching the boundary that are not within the confines of the wave port. In this manner, side connectors can be connected.

When multiple conductors touch the boundary, a smaller port can decrease the chance of an incorrect mode pattern being calculated, where multiple quasi-TEM modes can exist with three or more conductors.

Motivation for Custom Ports - Multiple Conductors

Analyst calculates the modal field distribution of the electromagnetic fields incident on the port. There is assumed to be one propagating quasi-TEM mode when the port is drawn in Microwave Office. (It is possible to lift the restriction of a quasi-TEM mode, for example the TE10 mode used in a rectangular waveguide. However, this must be set up in the 3D editor in Analyst, and imported into Microwave Office.) An error will result if this is not the case. A quasi-TEM mode must have at least two conductors in the port, for example a microstrip line and its ground plane. If more than two conductors are present, the number of quasi-TEM modes will be one less than the number of conductors. For example, in coplanar lines, there is a signal line and two side grounds, making three conductors. Therefore, two quasi-TEM modes exist. Analyst will pick one of them numerically, but there is a chance for error.

The coplanar mode assumes the side grounds are connected together. Commonly, this is accomplished in Analyst by adding vias to the side grounds to a bottom ground plane if one exists (this is commonly called grounded coplanar) or by attaching the side grounds to a perfectly conducting boundary, usually the side walls. The trouble with adding vias is that Analyst does not know of their presence when calculating the mode. The vias are interior to the problem, and the port is at the boundary. It will not see the side grounds as electrically connected. The problem with picking the side boundaries as perfect conductors is that they can be a long way from the line, and they can influence the rest of the circuit.

Creation of Custom Sized Ports

The port menu has a field for setting the dimensions of the port. As an example, consider the EM Structure called "Grounded_Coplanar_Window."

The Property Menu for Port 1 (opened by double clicking on the port) has the Type set to Wave Custom Size. The items on the right in this menu show the settings for the width (10 mm) and height (5.716 mm) of the port. In addition to setting the port size, the port can be offset vertically and horizontally using the two offset fields.

The resulting port is shown in the 3D view of the EM Structure.

Example of Grounded Coplanar Lines

The EM Structure, "Grounded_Coplanar_Line," consists of a simple coplanar line with one port, two side grounds, and a bottom ground plane.

The substrate has a relative dielectric constant of 3.66, and loss tangent 0.001. The height of the substrate is 716 microns. The width of the line is 1000 microns. The gaps are 381 microns. Analytic formulae, for example used by the TXLine calculator under Help, give the characteristic impedance as 55.4 Ohms.

The graph "Grounded Coplanar Impedance" shows three cases.

"Grounded_Coplanar_Line" gives a line impedance of 57.6 Ohms, which is about 2.2 Ohms higher than the theoretical prediction. Magnetic sidewalls are used for the simulation. The trace named "Grounded_Coplanar_Line_Electric_Walls" corresponds to the structure with the same geometry, but in this case the walls are perfectly conducting. The impedance is 55.1 Ohms, agreeing with the theoretical result within 0.3 Ohms. But the result becomes unstable above 4.0 GHz. This is most likely due to transverse resonances in the wide side grounds.

The resulting "Grounded_Coplanar_Window" has the port extending only 10 mm horizontally along the boundary, which has a total length of 19.56 mm. The wall to which the port attaches has been set to perfect conductors (PEC). The side grounds are therefore connected by the remaining metal on the boundary not attached to the port. The port impedance is 55.1 Ohms, in agreement with the theoretical formula. There are no problems at higher frequencies, as the port's side grounds are effectively narrower with the smaller width of the port window. Any spurious transverse effects are pushed to higher frequency.

In conclusion, the windowed port in this case has the advantages of:

Ability to easily allow the side grounds to be connected together using the PEC boundary upon which the windowed port is placed.

Reduction of the chance of spurious resonance issues in the side grounds by effectively reducing the side grounds' widths.

Example of Coplanar Lines with No Bottom Ground Plane

The EM Structure, "Coplanar Line Impedance" shows the coplanar case without the bottom ground plane, and consists of a simple coplanar line with one port and two side grounds only.

The theoretical result is 73.4 Ohms at low frequency.

The EM Structure "Coplanar_Line_Window" uses a windowed port with width 10 mm, just as before for the grounded coplanar line case.

The graph "Coplanar Line Impedance" shows the port impedance for this case.

In this case the EM Structure "Coplanar_Line" gives a result of 73.25 Ohms, in close agreement with the theoretical prediction. Both perfectly conducting sides and perfect magnetic sides were tried with no notable change in results. Note that because there is no bottom ground plane we do not see the resonance effects for the perfectly conducting case.

The port impedance is 72.9 Ohms at low frequency, about 0.5 Ohms different from the theoretical prediction. In this case the windowed port impedance differs from the theoretical result by slightly more than the case when using the entire boundary as the port face. The difference is, however, only about one percent worse. The fields for the mode spread out more in cross section without the bottom ground plane present. Therefore, the wave port needs to be larger in order to match the theoretical result. There is an assumption that the port is large enough that the energy of the mode is well contained in its cross sectional area. It is, of course, ultimately up to the designer to decide if any change in impedance is significant.

Example of Coupled Microstrip Lines

The EM Structure "Coupled_Microstrip_Lines" shows an example of two parallel lines excited differentially with a bottom ground plane.

The ports are wave ports, and are labelled as 1 and -1. This is a shortcut to excite the pair differentially. If preferred, the designer could label the ports 1 and 2, and then wire them together in a schematic using a transformer or a mode converter (MMCONV) element. This structure is differentially excited, with a custom wave port, and Analyst supports a "Ports Only" analysis.

The EM Structure "Coupled_Microstrip_Lines_Window" uses windowed wave ports, each with a width of 4 mm. Since the port windows overlap, the resulting port area is the combined area of the two ports.

The graph "Coupled Lines" shows the (differential) impedance of the compound Port 1 to be 102.2 Ohms. This is compared to the theoretical prediction of 101.0 Ohms. The case using windowed wave ports yields a differential impedance of 102.0 Ohms, very close to the case where the entire boundary is the port face.