AWR eBooks

Best Practices for Efficient and Effective Planar EM Simulation 2 www.cadence.com/go/awr Chapter 1: Ports Best Practice Tip #1: Use the autoports feature, which will usually choose a reasonable port ground reference by consid- ering potential ground planes and nearby metal. The AWR AXIEM simulator has many port options, which differ based on their grounding type, their ability to be de-embedded, and their grouping with other ports for mutual de-embedding schemes. Note that the term "de-embedding" in the EM simulation community is commonly used to mean calibration, as well as the traditional meaning of de-embedding to change the reference plane position of the port. This large number of port setting choices can make it difficult for the user to choose the best option. When first introduced, AWR AXIEM software shipped with the default setting of implicit grounding for all ports, which is not usually ideal. Thus, autoports were developed to give the designer nearly optimal port settings for most geometries without having to be an EM expert and/or spend valuable design time manually setting up the simulation. The most commonly used port in AWR AXIEM software is the edge port, where a port is placed at the end of a trace. To function properly, the ground definition of the port must be correctly defined, as well as reasonable settings for de-em- bedding. Figure 1 shows an edge port at the end of a line with two different grounding types. The technology used is microstrip on Teflon substrate with a ground plane on the bottom of the board. Figure 1: Explicit ground reference (left) with ground strap and implicit ground reference (right) with no strap The left picture shows an explicit ground reference in which a reference ground strap connects the line to the ground plane below the port. This reference strap, made of metal, is added automatically by AWR AXIEM software. The ground symbol annotation next to the port in the 2D view shows it is using an explicit ground reference. The strap is meshed with the rest of the metal in the circuit, and a voltage source in series with a 50-ohm resistor, not shown in the figure, is placed at the end of the strap where it connects to the ground plane below. The advantage of this type of port is that its ground is well defined and is normally the preferred ground reference choice in the simulator. There are situations, however, where the port doesn't work well because its operating assumptions have been violated. A common issue is that some other metal is too close to the port. The most serious case is a via, or another port with a reference strap. Vias like to talk to one another in microstrip circuits. The explicitly grounded edge port needs to be de-embedded, because the strap will affect the simulation results. It will add phase delay, because of parasitic inductance and capacitance, which must be removed from the S-parameters. This is accomplished by calibrating the port behind the scenes. The specific method used is similar in concept to the widely known through-reflect-line (TRL) method for on-chip calibration of probes. The right picture in Figure 1 shows the other grounding option available for edge ports: the implicit ground. Notice there is no strap to the ground plane; the ground for this type of port is at infinity, not the metal on the bottom of the board. At first sight, this seems absurd. However, when carefully used, this type of port can give accurate results. These ports work best when used in pairs where the current is flowing out of one port and into the other, for example two ports across a gap in the line, or two ports in a differential pair of lines. The errors introduced by one port tend to be cancelled by the other port.

• Cover