Properly Select Ports During Circuit Simulation

FOR DESIGNERS USING SOFTWARE circuit simulators, the correct use of ports is criticalin addition to the drawing of the structure itselfto the attainment of successful, meaningful simulation results. In a white paper titled, "A Plethora of Ports: Making Sense of the Different Types of Ports in EM Simulators," AWR's John Dunn provides the reader with a fundamental understanding of ports. The nine-page document focuses on the ports common to electromagnetic (EM) simulators, which also are referred to as two-dimensional (2D), two-and-one-half-dimensional (2D), or three-dimensional (3D) planar EM simulators.

After briefly summarizing the history of ports, the author explains that the simulation community eventually realized that probe-type ports could be grouped togethermuch as red and black probe tips were used in the laboratory. Essentially, designers were creating a port and its local ground return. Depending on the port, software developers now allow the designer to specify the ground return in a flexible manner.

The most recent development to garner attention is the concept of de-embedding circuit-like ports. With designs operating at higher frequencies and in more compact geometries, circuit ports can now "see" and therefore influence each other.

Circuits typically have more than one port. As a result, whatever is being drawn out from infinity is being given back by the other port. The currents on the ground plane are therefore localized under the line as expected, despite the fact that there are no grounding straps.

The paper also delves into circuit-type ports and the methods to obtain their return ground. With differential ports, the designer can place a port's ground reference anywhere in a circuit. He or she simply has to make two ports in the customary way and then indicate that they are a differential pair, although several factors must be noted to successfully use this method.

As the paper details such concepts, the author stresses that there is no one best port for all situations. The engineer must understand the fundamental physical assumptions underlying the port. He or she can therefore appreciate its strengths and weaknesses to make the optimal and most informed choice as to port type.

In that process, it is important to know where the port's ground is. After all, the S-parameters for that port are determined with respect to the ground reference. Modern planar solvers allow the designer to choose the ground return of a port. EM planar simulators use a Green's function method to predict the mutual coupling among all current cells. Because the Green's function is zero at infinity, infinity is the ultimate ground reference.

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