Understand Ports: The Key To Mastering EM Simulation

This is the second in a series of guest columns by Dr. John Dunn of AWR Corporation.

If you use an EM simulator, you use ports. No kidding! Ports are the way the EM simulator "talks" to the outside world and are the way you measure S-parameters — the "high-frequency" data that can be used in a circuit simulator. Analogy: Using probes in the laboratory; without them, you have no way of making a measurement.

The difficulty with ports is that they are often misunderstood, and therefore improperly used inside of EM simulators. To again make an analogy with the laboratory, imagine you use the wrong type of probe for your measurement of interest...

• A low-frequency probe cannot be expected to give a good result at microwave frequencies.
• An improperly calibrated network analyzer probe cannot be expected to give accurate answers to -70 dB.
• A two-port network analyzer is going to have limitations when looking at differential line issues. Here you can use transformers and get some useful information, but you had better understand their limitations.

All ports (port types) have limitations, assumptions, and approximations. If you don't understand what they are doing, you may well fall victim to the topic I covered in my January article — the simulator gives an answer, but it's NOT the one you want. So, I'd like to devote the next couple of articles in this series to better understanding, appreciating, and using ports successfully within EM simulators. I guarantee that time spent understanding how ports work is time well spent. You will be rewarded with more accurate results and attain confidence when using your EM simulator of choice.

We'll get started by looking at two important concepts that all ports must address: grounding (next issue) and deembedding.

• Grounding. As electrical engineers, we work with the notion of ground everyday. Ground gives a circuit its voltage reference. Without it, we can't even talk about voltage in a sensible way. What isn't as well known is that every port has its own ground reference. The S-parameters you get at a given port use the "local" ground. If you don't have it properly defined, you can get completely confusing results. I like to remind engineers that not understanding ground can be hazardous to your health! Think of someone taking the back off of a tube TV (yes, they are still around) and measuring the voltage with a two-point probe. They get a few volts. Then, they reach their hand in and find out about not understanding ground! Fortunately, not understanding ground in your simulator will not land you in the hospital, but it certainly can mess up your simulation results.
• Deembedding. This is the process of removing the parasitics associated with the port — for example, its parasitic capacitance and inductance. Incidentally, not all ports can be deembedded. Does that make them useless? Not necessarily. To again refer to the laboratory, should you not use hand held probes because they can't be calibrated? Of course not, but you had better understand their limitations.

I invite you to join me over the coming months and issues as we explore the wonderful world of ports in EM simulators, with all their assumptions, pitfalls, and limitations.

About The Author
Dr. John Dunn, a recognized expert in EM modeling and simulation for high-frequency and high-speed circuit applications, is a senior applications engineer at AWR and develops and presents AWR training material to customers world-wide. Before joining AWR, he was head of the interconnect modeling group at Tektronix and a professor of electrical engineering at the University of Colorado, Boulder, where he led a research group in EM simulation and modeling. Dr. Dunn received his Ph.D. and M.S. degrees in applied physics from Harvard University and is a senior member of IEEE. You can contact him at jdunn@awrcorp.com.

About AWR
AWR is the innovation leader in high-frequency EDA. Its software solutions quicken the pace at which high-tech products like cell phones and satellite systems are developed. When AWR software is part of the design process, engineers can deliver cutting edge, affordable products faster, more reliably, and at a lower cost. Headquartered in El Segundo, CA, AWR is a privately-held, growing company with thousands of users world-wide. Learn more: www.awrcorp.com