EM simulation tools only go so far

-November 07, 2013

Tight schedules, budgets, and faster devices have made EMC software tools more attractive than ever. Radiated emissions are always a challenge, and the lower voltage levels of very fast devices have made immunity (ESD, radiated, conducted) 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). If there is an EMC engineer, he or she might be relatively inexperienced.

Product designers are looking for help to replace the shortage of experienced EMC engineers, and software seems to hold great promise. In recent years, many vendors have created tools to help with EMC design. They often claim their tools can do PCB-level analysis through system-level analysis, and they have impressive color 3D plots that supposedly prove it. These tools can do some things very well, but they can't replace the need for experienced EMC engineers and basic knowledge of electromagnetics and the various simulation techniques' strengths and weaknesses. Furthermore, EM simulation tools can cost $20,000-$80,000 and more.

A wide range of automated EMI/EMC tools are available, including:

  • Design rule checkers that examine printed circuit board layout against a set of pre-determined 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 that give a very accurate simulation for a limited size problem
  • Expert system tools, which provide design advice based on a limited and predetermined set of conditions

It is clear that these different 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 mostly based on the location of the various components and the location of the various critical high-speed and I/O nets/traces. Manual checking of all the layers in 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. Naturally, for different types of industries, some of the design rules will vary, so it is important that the automated design rule-checking software allow the creation of rules for specific customers or industries.

There are many EMC design rules available from many sources. Many of these rules are in conflict with one another. A user might reasonably ask, "Which rule is right for my products?"

Some automated software implements rules that are based on more detailed laboratory testing and/or full-wave simulations. Each rule should be based on solid electromagnetic physics, not faith. Users should be very cautious before accepting EMC design rules. These rules should not only have detailed justifications but also agree with the basic fundamentals of physics. Just because a rule is commonly accepted does not mean it is right for every product or industry. Remember that, not very long ago, it was commonly accepted that the earth was flat.

On page 2, I'll offer more details on the items presented in this post's bulleted list.

Next: More details

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