Printed Circuit Design & Fab - February 2009 - (Page 29)

HS SiMulation lossy RLGC Matrix model is often derived from a field solver and is suited for uniform geometric structures like PCB traces. An S-parameter model is the scattering matrix from the RF world, suited for frequency dependent interconnects. When selecting what type of interconnect model to use, there are really two key factors to consider: edge rates and loss. When edge rates reach a point fast enough so that an interconnect starts to look like a transmission line, more accurate models that include loss need to be used. As frequencies increase, so does the loss of different substrate materials in the interconnect. Model formats, like S-parameters that can model these affects, should be used. A third factor, the type of simulation to be performed, can be introduced to make the selection criteria easier as well, such as, whether a PCB designer is going to simulate crosstalk using coupled interconnect models. This can impact the selection of an interconnect model format. Again, a model matrix can be developed (FiGurE 6) to aid in the selection of an interconnect model type. In the example of an interconnect model matrix, the loss-versus-simulation type is displayed as a selection criteria. More advanced model matrices can be developed to include edge rates and other factors. When dealing with low frequencies, I recommend using simpler models because the loss is constant. Once you enter the medium-low frequency ranges, you can start to mix in lossy models in a hybrid approach to balance the tradeoff between model accuracy and simulation run time. At high frequencies, I recommend going with all S-parameter models to ensure the most accurate models are used. There are many trade-offs and equally valid methods to model and to simulate interconnects using multiple model formats. For example, you can model a PCB trace at high frequencies using a lossy RLGC Matrix generated from a field solver that includes frequency dependent effects for the conductor and dielectric loss. This type of model can be as accurate as a lab-measured S-parameter covering a wide frequency range. The idea of using selection criteria like a model matrix can help a PCB designer understand the multiple model types available and compare the benefits of the different model types against each other. Summary With complex interface designs like advanced SerDes, a PCB designer needs to be able to simulate and validate a design before it is taped out to make sure it will meet specification and avoid costly rework. In order to get useful simulation results, the right simulation models need to be used for the IO buffer and interconnect in an interface. By developing a selection criteria such as a model matrix, a PCB designer can easily determine the best simulation model to use and make informed design choices on how to put these models together in a simulation environment. pCd&f tiMothy coylE is principal consultant and owner at at Signal Consulting Group LLC. he can be reached at reached at tim.coyle@siconsultant.com. Exhibition: February 11, 2009 Hilton Phoenix East/Mesa Mesa, AZ • 1-day tabletop exhibition • Free technical sessions • Reception on the show floor • Great networking opportunities • And much more! www.pcbshows.com/phoenix www.pcbshows.com/phoenix FEBRUARY 2009 Brought to you by printEd CirCuit dESign & fAB 29 http://www.pcbshows.com/phoenix http://www.pcbshows.com/phoenix

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - February 2009

Printed Circuit Design & Fab - February 2009 Contents Our Line Market Watch Around the World Happenings ROI Tip Jar BGA Bulletin Interconnect Strategies Final Finsh Forum Defects Database Embedded Active Components In Multilayer LCP Packages Simulation: The Need for Speed Advanced Registration Systems The DC Design Squeeze Ad Index Do You Really Want a Better Autorouter? Designing With Conductive Materials, Part 1 Off th eShelf Marketplace On the Forefront

Printed Circuit Design & Fab - February 2009

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