Microwave Engineering Europe - December 2008 - (Page 13) EM MODELING 13 Figure 1 shows the mathematical operation that is performed. In electrical terms, a 1:1 transformer has been added to each port, so you are forcing a differential mode at the ports. This can be useful but is often misunderstood because it is easy to assume we are looking at the “local ground” of the port and that we can measure the resistance of the original lossy ground in the EM simulation by placing an ohm meter across the two new “ground” ports. That is not the case. The original S-parameter file did not have this information because it assumed the local grounds were at the same voltage. These ports were created by the mathematical operation of adding transformers. So how can we model an imperfect ground? First, we’ve decided that the various ports’ local grounds must be connected by a very good ground to make sense of the S-parameter file, so the imperfect ground cannot be treated as a ground but must be explicitly simulated as another conducting signal path with appropriate ports. Then you can explicitly address the ground’s properties. In other words, there is at least one very good ground for port references, and any number of imperfect power planes with their own ports. The three examples we alluded to earlier illustrate the ground concept. First, consider a simple gap in a ground plane, as shown in Figure 2 along with Sparameters. The gap adds inductance to the line and can be easily modeled by a lumped inductor. A coplanar to microstrip transition line transition is shown in Figure 3. The ground of the coplanar waveguide (CPW) has been connected to the ground of the microstrip with two vias and optimized to give a good transition to 20 GHz. Figure 4 shows a board-to-chip transition. In this simulation, two signal lines and their grounds are attached to a chip with bond wires. There are five bond wires in a GS-G-S-G configuration, so the chip ground is not the same as the board ground. This effect is modeled by explicitly simulating the local chip ground as another signal plane. It is not attached to “node 0” but rather is attached to the grounding bond wires through the ports of the EM simulation. Figure 2: A gap in a ground plane. The ground transition is modeled explicitly as another signal net. Global ground is used only at one end, in this case the board. The ports in the package are referenced only to local ground. We hope of course that the local package ground is electrically close to the global ground. Using a more sophisticated approach to working with a local ground [4], the local ground is accounted for by performing an EM simulation from the global ground to the local ground, and the resulting imperfect ground structure is de-embedded. The resulting S-parameter file is corrected for the local ground being different from the global ground. Summary The concept of ground, simple though it may seem, can confound novices and veterans alike when applied to high-frequency design tools, which is not surprising since circuit simulators require a global ground and EM simulators do not consider ground at all. So it’s our job to ensure that EM results (S-parameters) are properly interpreted by the circuit simulator. It is important to remember that all ports in the EM simulator assume that something close by is the ground. If different grounds for different ports are used, care must be taken to properly interpret the results of the simulation. References [1] Fields and Waves in Communication Electronics, Ramo, Whinnery and Van Duzer, Wiley, 3rd Edition, 1994. [2] “A general waveguide circuit theory”, R. B. Marks and D. F. Williams, NIST Journal of Research, 97(5), pp. 533-562, 1992. [3] “EM simulation: A look under the hood, Parts I, 2, 3”, J. M. Dunn, 2005 – 2006, archived in the Knowledgebase on the website: http://web.awrcorp.com. [4] “De-embedding the effect of a local ground plane in electromagnetic analysis”, J.C. Rautio, Microwave Theory and Techniques, 53(2), Feb. 2005, pp. 770 – 776. Figure 3: Coplanar-tomicrostrip transition. Figure 4: A board-tochip transition. About the author Dr. John Dunn is a senior engineering consultant at AWR where he is in charge of customer training and university program development. He is an expert on electromagnetic modeling and simulation for high-speed circuit applications. Prior to joining AWR, he was head of the Interconnect Modeling Group at Tektronix and earlier was a professor of electrical engineering at the University of Colorado where he lead a research group in electromagnetic simulation and modeling. Dr. Dunn received his B.A. degree in physics from Carleton College and his M.S, and Ph.D. degrees in applied physics from Harvard University. He is a senior member of the IEEE. Microwave Engineering ● December 2008 ● www.mwee.com http://web.awrcorp.com http://www.mwee.com
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.