Like it or not, most electronic designs today are subject to formal EMI testing. So even if you are new to EMI/EMC (electromagnetic interference/compatibility), you need to understand what is involved and how to best prepare for a trip to the EMI test lab.
Like any trip, good preparations are key. We’ll look at three phases — pretest, test, and post test. Try to anticipate problems, and don’t overlook contingencies. Most EMI tests are not successful the first time. As engineers we always need to have “Plan B” ready, and maybe even “Plan C.”
Before we begin, however, a little philosophy. Too often designers take EMI failures personally. So change your mind set — think verification, not testing.The goal is not to criticize your designs, but rather to assure your designs will work in the field. Make it a positive experience. As we learn, we improve — even us grumpy old EMC consultants.
Phase 1 – Pretest
The first step is to write a plan. If you are working in the defense industry, a test plan is usually a contract requirement. We find an EMI test plan very useful for communicating among the design team, the test lab, and the customer.
But even if not required, a test plan is still a good idea as it forces one to address critical issues ahead of time. Here is a summary, which you can even use as a checklist.
Identify necessary tests. If you are not sure what tests are needed contact your test lab prior to your visit. Nothing is worse than showing up without knowing what needs to be done. You should also determine the test configuration for each test, which is usually defined in the relevant test specification.
Define failure criteria. With emissions, this is easy. Are the levels above or below the limits? But with immunity/susceptibility, however, you may need to define failures. For example, is a reset with recovery acceptable? How much perturbation can you withstand in an analog sensor?
Depending on the equipment under test, the failure criteria are already specified. Other times you have more flexibility. The different failure levels prescribed in the European Union EMI specifications are a useful place to start. Be sure to include this in your plan, and to get advance agreement on the failure criteria.
Determine failure monitors. Again, with emissions this is easy – just watch the spectrum analyzer. Immunity/susceptibility are not as easy. How will you determine a failure? Special software? Or special hardware, such as a blinking “heartbeat” detector? Or maybe just indicators on the EUT (equipment under test) via a video camera.
Determine equipment hardware. What specific equipment will you test? Are peripherals needed? What about memory or I/O configurations? Probably best to test a “worst case” configuration, which assumes that lesser configurations will have lesser EMI issues.
Determine equipment software. Will you need special test or diagnostic software? Some software may even be prescribed. For example, the prescribed emissions test software for personal computers includes reading/writing to hard drives and peripherals, along with a “scrolling H” test pattern for monitors. Not fair to let the system idle – you need to exercise the hardware.
For immunity/susceptibility, how will you monitor, recognize, and report failures? Will the standard software do it, or do you need additional special software? Will that software run on the EUT, or on remote equipment?
Determine support hardware. Passive peripherals, or active exercisers? Will you need to develop special hardware (and associated software?) Are there special power or cooling needs?
Don’t forget about cables and connectors. If shielded, make sure they are properly terminated. If necessary, how will the cables penetrate the test chamber? You may need to develop a special test fixture for this.
We’ve see too many problems with cables — check them out before going to the lab. We still recall one engineer admonishing his colleague with, “I thought we brought the good cables.”
Put together a tool kit and spares. As a minimum, you should bring spare boards. Better yet, bring an extra system or two. There is nothing worse than having equipment break during the tests, with no backup. Bring backup software too.
You may also want to include some spare parts – ferrites, small caps, EMI copper tape, and a roll of heavy duty aluminum foil. A soldering station can be useful too if you need any minor board modifications. Much of this may be available at the lab, but better to be prepared.
Consider multiple configurations. For cost sensitive designs, we often recommend three test samples (ABC method.) The A unit has minimal modifications (management’s dream); the C unit has all the EMI fixes you can think of (the EMI engineer’s dream): and the B unit is somewhere in between (the designer’s dream.).
If you’re an optimist, start with A. If you pass, life is great! If you are a pessimist, start with C. If you fail, you’re still in trouble. In most cases, you’ll be somewhere in between, which is where the B unit comes into play. But this approach lets you quickly bracket things. It also means you have spares on hand if needed, and that can be modified as needed.
Schedule your tests. With all this preparation, don’t forget to call your test lab for scheduling. Test labs can get pretty busy, so don’t expect to get in right away. The more advance time you can give the lab, the better. They will appreciate your courtesy. But for emergencies and panic situations, most labs will do their best to accommodate you. Just don’t make every test a panic.
Phase 2 – Testing
All your planning and prepping is done, and you are now at the lab. Regardless of your overall responsibilities, somebody from your company should attend the tests. Don’t just throw the design over the wall to the lab. Yes, it is done but very often is not effective, particularly if problems arise.
Many EMI tests take a week or less. In that time, not only will you learn a lot, but by being on site you’ll also save your company time and money. After all, you know the design, how it works, and how to fix it if it breaks. Here are some issues to consider.
Setup the EUT. Do the basic stuff – connect power, peripherals, ventilation (if needed), etc. Run a diagnostic to be sure everything is working as it should.
Start a test log. Note date, time, test configuration, and summary results. Keep it simple but organized, as you will get a full set of data at the end of the tests. This is very useful if you start troubleshooting. Without it, you will soon be confused as to what has been tried. Photos are a good idea too.
Baseline tests. For emissions, run an ambient test (power to the EUT off.). This is normally done anyway, but make sure it happens. For immunity, run a pre-scan. This verifies proper operation before you begin subjecting the EUT to the EMI torture chamber. This is the time to catch any glitches in the test setup. Note and record the results.
Dealing with test failures. Unless you are incredibly lucky, you will encounter test failures, particularly with initial tests. Expect two or three trips to the lab before achieving full success. Even with the best design techniques, there are always unknown factors. That is why we test – it is still the most cost effective way to assure EMC, and ultimate successful operation of our equipment in the field.
If you fail a test, don’t just stop and give up. Do some quick troubleshooting instead. If you are lucky, you may fix the problem right away. If not, at least gather enough information to narrow the possible failure mechanisms. Think like a doctor trying to diagnose an illness.
Do the simple stuff first. Add ferrites to cables, or better yet pull cables to see if emissions drop (or immunity improves.) If you think the box is leaking, wrap the EUT in aluminum foil, sealing the seams in copper tape. You did bring your ferrites, aluminum foil, and tape with you, right? If not, the lab probably has a supply, but better to be prepared.
For immunity, back off the test levels to determine the actual failure levels. How bad is the problem, anyway? If you are close, maybe a ferrite will fix things. But if you are a long way from success, more serious fixes may be needed. You need to know this.
Ask for suggestions. Your test engineers and technicians have seen a lot of problems, and may have some ideas to try. Be polite, and don’t be a hot shot trying to impress everyone. Worst of all, do NOT blame the test lab or question their equipment or abilities. Wish we didn’t have to include this last piece of advice, but we’ve seen it happen.
Verify operation. Finally, regularly check to see if the EUT is still working right. This is particularly important with immunity tests that might cause damage or subtle changes, such as ESD or power transients. But even random equipment failures can invalidate your test data.
How often to verify? The answer is how much data are you willing to discard. If you are willing to lose a day’s data, then once a day is enough. For a half day, then twice a day suffices. Test time is expensive, so we usually recommend revalidating every two to four hours, assuming the revalidation does not take a lot of time.
Phase 3 – Post Test
If all has gone well, you’ve passed the necessary tests. If not, hopefully you have gathered enough data and ideas to fix things for the next round of testing. Rest assured – eventually you will achieve test success. So what now?
Test report. It is not enough to just pass the tests — you need to document the results. For military designs, the test report is another contractually required document. As such, it can be quite formal and detailed. For commercial products, the test report can be less formal, but should still contain enough relevant data to show that you have, in fact, passed the tests.
You can have the test lab prepare the test report, or you can do so yourself. Since most engineers do not like to write reports, we usually recommend paying the test lab to provide the report. With their experience and templates, they can do so in a cost effective manner. Either way, keep the test report on file in case there are future questions about the tests.
Raw data. Before leaving the lab, it is a good idea to leave with raw data — graphs, tables, and photographs. Of course, you have your lab notes too , right?
In addition, we like to record other relevant data — test equipment, serial numbers, calibration dates, etc. That will be included in formal reports, but it only takes a few minutes to gather.
Last, but not least, thank everyone for their help. Not only is this courteous but will be very much appreciated. You will also find youself welcomed back on your next trip to the EMI lab.
We hope this makes your next trip to the EMI test lab both easier and more enjoyable. EMI testing is an important step to assure our equipment will work properly in its intended environment. The ultimate goal is a better design, which is what we all want as engineers.
|Daryl Gerke, PE and Bill Kimmel, PE
are partners in Kimmel Gerke Associates, Ltd., an engineering consulting and training firm that specialized in EMI/EMC design and troubleshooting issues. Both are degreed engineers (BSEE), iNARTE Certified EMC Engineers, and registered Professional Engineers (PE).Daryl and Bill have prevented or solved hundreds of EMI problems in a wide range of industries – computers, military, medical, industrial controls, automotive, avionics, railroad, telecomm, facilities, and more. They have also trained over 10,000 engineers through their public and in-house training classes. They just celebrated 25 years in full-time practice as EMI/EMC consulting engineers. For more EMI information, visit their web site at www.emiguru.com.