Knowing the ins and outs of EMC software tools can save time and money.
If you are involved in Printed Circuit Board (PCB) design, you are likely to be inundated by the many different tools now available for EMC engineers to help predict EMC performance of products before they are built. Many of these tools are excellent, but the buyer must understand what the tools can really do– and what they cannot do!
Tight schedules, tighter budgets and the rapid growth of high speed devices have made EMC software tools more attractive than ever before. Radiated emissions are always a challenge, and the lower voltage levels of very high-speed devices mean that immunity (ESD, radiated, conducted) has become even more important than in the past. Certainly, there will be no shortage of work for EMC engineers in the near future. However, there is actually a shortage of trained/experienced EMC engineers. Many companies do not have a full time EMC engineer (if they have any at all), or if there is an EMC engineer, he/she might be relatively inexperienced.
This means that product designers are looking for help to replace the shortage of experienced EMC engineers, and software tools seem to hold great promise. In recent years, many vendors have created software tools to help with EMC design. Vendors will often claim that their tools can do PCB-level analysis through system-level analysis, and they will have very impressive color 3D plots to prove it. However, the phrase “there is no such thing as a free lunch” has new meaning in this context. The tools can do some things very well, but they do not replace the need for experienced EMC engineers, basic knowledge of electromagnetics and knowledge of the various simulation techniques’ strengths and weaknesses. Actually, “free lunch” might not be the best phrase as these tools can cost $20,000 and more.
Wide ranges of automated EMI/EMC tools are available to the engineer. Automated tools include design rule checkers that check PCB layout against a set of predetermined design rules; quasi-static simulators, which are useful for inductance/capacitance/resistance parameter extraction when the component is much smaller than a wavelength; quick calculators using closed-form equations calculated by computer for simple applications; full-wave numerical simulation techniques which will give a very accurate simulation for a limited size problem and expert-system tools, which provide design advice based on a limited and predetermined set of conditions. It is clear that these different automated tools are applied to different EMI problems and at different times in the design process.
Automated EMC Design Rule Checking
The EMC performance of a printed circuit board is primarily based on the location of the various components and critical high speed and I/O nets/traces. Manually checking the various layers of today’s high speed circuit boards is too time consuming and prone to human error. Automated Rule checking software relieves the tedium and removes the human error by reading the CAD design file, taking each critical net/trace in turn, and checking that it does not violate any of the most important EMC design rules.
The usefulness of this kind of tool is largely based on the EMC design rules and whatever limits are used for each of the various design rules. Naturally, for different types of industries, some of the design rules will vary. So it is important that the automated design rule checking software allows for the creation of customer or industry specific rules.
There are many EMC design rules available from numerous sources, but most of these are in conflict with one another. So a user might reasonably ask, “Which rule is right for my products?”
Some of the automated EMC design rule checking software implement rules that are based on more detailed laboratory testing and/or fullwave simulations. Each rule should be based on solid electromagnetic physics and not on faith. Users should be very cautious before accepting EMC design rules; these rules should not only have detailed justifications but make sense with the basic fundaments of physics. Just because a rule might be commonly accepted does not mean it is right for every product or industry. Remember, it was not very long ago when it was commonly accepted that the earth was flat.
Fullwave Electromagnetic Simulation Software
Today’s full wave EM simulation software tools cannot do everything. They cannot take the complete mechanical and electrical CAD files, compute for some limited time or provide the engineer with a green/red light for pass/fail for the regulatory standard desired. An EMI and/or design engineer is needed to reduce the overall product into a set of problems that can be realistically modeled. The engineer must decide where the risks are in the product design and analyze those areas.
Vendor claims must be carefully examined. Vendors might claim to allow an engineer to include detailed PCB CAD designs along with metal shielding enclosures to predict the overall EMI performance. However, these tools are not really capable of such analysis. There are too many things that will influence the final product to make such a prediction with any level of accuracy, but these fullwave simulation tools are extremely useful to help the engineer analyze specific parts of the design in order to better understand the physics of the specific feature under study. The engineer can use this knowledge to make the correct design decisions and trade offs.
Tool Box Approach is Required
No single modeling/simulation technique will be the most efficient and accurate for every possible model needed. Unfortunately, most commercial packages specialize in only one technique and try to force every problem into a particular solution technique. The PCB design engineer and EMI engineer have a wide variety of problems to solve, requiring an equally wide set of tools. The “right tool for the right job” approach applies to EMI engineering as much as it does to building a house or a radio. You would not use a putty knife to cut lumber or a soldering iron to tighten screws, so why use an inappropriate modeling technique?
Some of the various simulation techniques require a deep level of understanding electromagnetics. The Finite Element Method (FEM) and the Method of Moments (MoM) are two such techniques. Others, like the Finite-Difference Time-Domain (FDTD) technique and the Finite Integration Technique (FIT) are much simpler to learn and to use. The reader is advised to understand how each technique works before purchasing any software.
Quasi-Static Simulation Software
When an object is electrically very small (compared to the wavelength of the highest frequency of interest) then quasi-static simulation tools can be used. The fundamental assumption is that there is no propagation delay between elements within the model.
Quasi-static tools are very useful for creating an equivalent circuit of inductance, capacitance and resistances that can be solved with circuit solvers, such as SPICE. Matrices of many elements can be used for including complex PCB connectors in signal integrity simulations.
Other Software Tools
There are a wide variety of software tools available to do specific tasks. The user must carefully consider if the software tool will do the type of analysis that is required. For example, some vendors offer simulation software that will read complete CAD files and then predict the far field emissions level based on the simple loop formed by a microstrip and the return/ground plane. This simplifying assumption is too simple for most applications, since the far field emissions are most often directly controlled by the metal shield (and the openings), as well as long attached wires not directly from the traces on the board. The metal shield and/or cables creates a dominant effect that is often ignored by these tools and can lead to dangerous and disastrous decisions when used incorrectly.
Beware of so-called ‘Expert System’ software. These software tools claim to replace the human expert and to provide advice similar to what would be received from a human expert. The basic underlying assumptions for the algorithms are sometimes very limited in scope, and when applied to a real world PCB, the algorithm ‘breaks’ without letting the user know. Tools that are in this category are often a great emotional comfort but of little actual value.
Know the Software Tool’s Assumptions!
Just as it is important to know the limitations of the simulation techniques, it is important to know and to understand the basic assumptions the vendor has used in the specific software tools. Many times, the important factors are not displayed to the user (so the tool looks easier to use and less confusing). However, these factors can have an enormous impact in the accuracy of the final results. Always remember: The tool will give you a very accurate answer to whatever question you ask it, even if the question is wrong!
Proper Simulation Validation
In the early years of EM simulation, the practitioners were experts in EM theory and simulation techniques and often wrote their own programs to perform the simulations. However, modeling and simulation are no longer restricted only to experts. The commercially available codes are diverse, easy to use and provide the user with convenient means to display results. New users can begin using these codes quickly without the requirement of being an expert.
The hidden danger is the need to validate the simulation results. It is not sufficient to simply believe a particular software tool provides the correct answer. Some level of confidence in the results are needed beyond a religious-like trust in a software tool simply because others use it, because the vendor assures their customers of the tool’s accuracy or because others have validated their results in the past.
Summary
Many different software tools are available for PCB designers to aid in meeting EMI emissions and immunity requirements. There is no one tool that can do everything, and multiple tools, often at different complexity levels, are required. Automated EMI rule checking tools can provide quick and specific analysis of PCB CAD designs, while more complex fullwave simulation tools can provide very accurate and fundamental understanding for limited portions of the overall PCB and/or system. PCD&F
Dr. Bruce Archambeault is distinguished engineer with IBM and can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..
Where to go for more information:
1. PCB Rule checking software tools; www.MossBayEDA.com
2. EM Modeling/simulation Techniques, “EMI/EMC Computational Modeling Handbook” http://www.springer.com/engineering/electronics/book/978-0-7923-7462-6
3. EM Modeling/Simulation Software; http://emclab.mst.edu/csoft.html
Model Validation; http://www.ewh.ieee.org/cmte/tc9/
