Types of ports in AWR

Port types and their selection criterion, as a topic, is an important one for a designer using a circuit simulator, because the correct use of ports is the most important determining factor, other than drawing the structure itself, in obtaining successful and meaningful simulation results. The choice of the correct port type is not trivial, especially with the bewildering array of port choices available. This article presents useful information that will help designers make the correct decision regarding their selection of port types so that they can obtain valuable results.

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Figure 1: (a) Edge port and (b) Internal port

Figure 2: Explicit and implicit grounding

Figure 1 shows two edge ports. Both ports are at the end of a microstrip line, which is on top of a single-layer board with a ground plane on its other side. Both ports are using the bottom plane as the ground of the ports. The difference is that the left port has a grounding strap (explicit grounding) from the port to the ground plane and the right port in Figure 1 uses the ground plane as the ground reference, but there is no grounding strap (implicit grounding). Figure 2 shows the 3D view of the edge and internal ports.

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Figure 3: (a) Coplanar line with edge ports (b) Series port and its local ground.

Figure 3 (a) shows Coplanar line with edge ports. Port 1 is the signal line, and port 2 is the ground return. Notice that there are two lines that are port 2. This is a common feature used in simulator. Figure 3 (b) shows the port used in a gap in a line. A chip capacitor is going to be placed across the line by attaching its model to the two ports when the S-parameter block is used in the schematic. Therefore, implicit grounding of the ports will lead to lower parasitics, and a more accurate answer.

 In the laboratory, the process of removing the parasitic in the measurement system is called calibration. In numerical simulation, typically, the process of calibrating the port and setting the reference plane is simply called de-embedding. De-embedding is a procedure in which the port parasitics are removed. It is extremely important to note that not all ports can be de-embedded. Typically, the circuit-type ports mentioned above are not de-embedded by default.

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Figure 4: (a) De-embedding structure of a port (b) Example of where/when grouped ports and de-embedding are needed

Figure 4 (a) shows a port with the reference plane (de-embedding length) set by the arrow extending from the port. It also shows the underlying de-embedding structure, which is a line twice the length of the original de-embedding length, with one port at each end. An internal port is also placed in the middle of the two lines. There is an important assumption that is made when a port is de-embedded. The assumption is that there is no metal near the port or the de-embedding line to influence the de-embedding procedure. A good rule of thumb is that there should be no other metal within two substrate heights or two line widths, whichever is bigger. This is one of the most common errors in de-embedding ports. Circuit layouts can be dense, and de-embedding is not always possible.

Figure 4 (b) shows the FET model to be inserted at ports 2, 4, 7, and 8 after the EM simulator determines the S-parameters of the lines. Unfortunately, the ports are so close to one another that it is not possible to calibrate them individually. In group de-embedding, the calibration structure extends the lines out from the ports, with the ports placed at the end of the lines.

Table 1 below summarizes all port types with their advantages and disadvantages.

Reference: J.M. Dunn, A Plethora of Ports: Making Sense of the Different Port Types within EM Simulators, AWR Axiem White paper.

Now that you have read the article, just think about these:

• What is de-embedding and why is it important?
• What are implicit and explicit grounds?

Team SimTech

Cadence Design Systems