AWR eBooks

Best Practices for Efficient and Effective Planar EM Simulation 3 www.cadence.com/go/awr The implicit ground reference port also works better when there is an infinite ground plane on the bottom of the board, even if that plane is several signal layers down from the port. This is because the simulation behaves the same way as if there is an image port with the opposite polarity on the other side of the board. Thus, a virtual pair of differential ports is obtained, which helps cancel errors. Normally, this type of port is not de-embedded (calibrated). It is commonly used when there is no ground plane to attach a reference strap, or there are nearby vias that will interfere with the accuracy of the explicitly grounded port. The autoport setting will choose which of these two ground reference settings is best to use for a given port. Figure 2 shows the autoport feature and how it is set in the port's menu. The letter "A" appears next to the port to show an autoport is being used. Figure 2: 2D layout view of an autoport (top left), autoport settings in AWR AXIEM menu (bottom left), and preview geometry with an autoport (right) When using the autoport setting, it is important for the designer to check the final port settings, which can be previewed by going to the EM project in the project browser on the left of the software, right clicking on the project name, and selecting "preview geometry." The planar 2D view of the AWR AXIEM layout now shows the grounding type selected and any possible de-embedding length added to the line. The right picture in Figure 2 shows that Port 1 in the preview geometry has an explicit ground type with a length of de-embedding added. The 3D view of the previewed geometry shows the metal ground reference strap, along with an apparent 3D volume surrounding the port. This volume is a visual aid to the designer, meant to emphasize that the port is assumed to have no other metal in close proximity to it other than the line to which it attaches. If autoports create an implicit ground for the port, there is no strap and no keep-out volume shown. The autoport setting helps the designer make reasonable choices for port settings, but it is not infallible. AWR AXIEM software does not know if a metal polygon is the right ground reference for a port or even if the piece of metal is a ground at all. It is possible for the wrong ground to be selected, particularly in complicated multi-layer modules. The autoport setting is making decisions about how close other metal can be to a given port before something must be changed in the port type. These decisions can be wrong, of course. In the options menu shown in Figure 2, the user can change the settings for how autoports choose a port type. This menu can be reached by right clicking on the EM project in the viewer and selecting Options. Then, under the AXIEM tab, there is a section of options under the heading Auto Port Setup. (Note that many of these options are secondary options and are not visible in the menu unless the "show secondary settings" button is active.)

• Cover