Nature of EMC Work | Time-Domain Simulation Tools and Methods

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Article first published in EDN Magazine , March 2, 1998

The problem with electromagnetic compatibility (EMC) is not the difficulty of calculating the emissions from any particular radiating structure. The problem is much more one of recognizing the various types of radiating modes, deciding which is the worst case, and then knowing how to fix it. Identifying the worst-case radiating mode is quite important, because any problem you fix that isn't the worst-case mode makes zero difference to your compliance.

Don't expect anybody's software to tell you what is going to be the worst-case radiating mode in your design. Experts can't even do that. [135]

[135] Put any three EMC specialists in a room, have them review your design, and ask them to agree on what is the most significant flaw. Don't interpret the resulting conflagration as a reflection of their skill; merely accept it as an indication of the difficulty of the problem.

The ability to discover which is the worst offender is 90% of the battle. Once the worst source is identified and fixed, the second-worst source looms as the next problem. For a product that is 20 dB out of spec, there may be 10 different fixes, each worth 2 to 3 dB, required to fully address all the problems. It's a long, serial process.

13.8.1 EMC Simulation

Article first published in EDN Magazine , March 2, 1998

Let me give you some advice: Real live EMI problems are much too complex for even the best software tools. As much as I wish the situation weren't true, at this point the best tool is still experience. Many aspects of the EMI problem make prediction a difficult task, especially when working at the system level.

First, the process involves simulation of 3-D wave patterns over a rather large area. That is, your computer is going to spend a lot of time grinding numbers to get even the most rudimentary results.

Second, every bit of metal in the product matters: every via, trace, pad, bonding wire, connector, and cable. Many times, the system parts you choose not to include in the model turn out to be the very ones that create the worst EMI headaches . That's the nature of the problem. You seldom know beforehand what parts of a system will turn out to be the worst EMI offenders.

Third, EMI is a strong function of switching speed, data patterns, and precise data timing. That's right, data patterns and timing. If you don't believe me, find out what kind of software people ship to the EMI test range ”some companies send several versions to see which works best.

EMI problems are much too complex for even the best software tools.

The EMI problem is so complex, it's na ve to think you can just type in a few parameters and get meaningful results. For example, how should you model the split-plane zone on a mixed 3.3V/2.5V processor board? Obviously, the board stack-up, the shape of the 3.3V and 2.5V regions , and the trace layout matter. Did you realize that the placement and layout of bypass capacitors will markedly influence the result? How about power supply noise? You'd have to model the power supply noise, including phase, at all frequencies from 30 MHz through several GHz to get meaningful answers. Oops, that would be a function of the software running on the board, wouldn't it? Guess you'll have to model that too. Now add little features such as power supply wiring. It is well known that enclosing your power supply wiring in a good metal conduit can reduce radiation from the wiring. How would you like to spend your Easter break modeling the contact resistance and inductance of the screws used to hold the conduit in place?

Get the picture? You can't model everything. It's too complex. You can't leave anything out, either, because you never fully know what is going to matter. So, what can you do? Apply these time- tested , common-sense rules:

Limit your risetimes where practicable.
Use your experience. If last year's design ran at speed x , and today you are planning to do 2 x , you are likely to need another 6 dB of protection.
Run a quick EMI scan as early as possible.

The most promising new EMI tools will be more like expert systems than simulators. They will give you advice and provide reference information, but they won't make rash predictions . If these tools are worth their salt, the first three bits of advice they will provide are listed in 1, 2, and 3 above.

POINT TO REMEMBER