You have just been assigned to work with the EMC department of your company and you are not really up to speed on why you need to do this. Sure you must meet the regulations to sell your product worldwide. But why are these regulations required?
What if you test your product and it fails? How do you go about fixing it to comply?
According to some sources, the only objective of an EMI test is to meet some government regulation. When the test is successfully completed and the paperwork filed with the appropriate agency, the purpose of the test is done.
It is easy to lose the focus of the purpose of the test when you are troubleshooting a problem discovered during compliance testing of a product. Let’s face it. When you are battling a harmonic of 75 MHz trying to make your product FCC Class B, your only goal is to lower the offending signal and hope the rest of the spectrum lowers to allow you to complete the test and continue with the certification or verification.
At this point, your overall goal is to meet the limit for this test—nothing else. This may be the aim of your test. But is it the actual purpose of the standard you are testing to?
The real goal is not just to comply with some set of government regulations. It also is to help ensure that the product has been designed properly to work in its intended environment without experiencing interruptions from adjacent systems or interrupting the operation of other systems.
Remember, EMI can cause serious problems in certain environments. The airliners don’t ban the use of cellular phones on airplanes just so you have to use their airphones. Nor do they prohibit the use of portable electronic devices for the first 15 minutes and last 15 minutes just to make you listen to their announcements. The reason is the possibility that your laptop computer just might interfere with the on-board avionics.
The EMI problem, however, does not have to be deadly to cause havoc. Sometimes they are just plain nuisances. A good example is something that happened to a sheriff’s department in Michigan.
A new computer system (FCC Class B-certified and with a CE Marking) failed to work properly when installed. At least three times a day, the computers would reset for no apparent reason, requiring a reboot of the system. This posed a real headache to the operators and caused a communications problem as well.
The systems integrator was getting chewed out for providing faulty computers and had replaced the units once. But the problem still existed. Another problem—a flicker on the display—also was observed.
Suspicious that it might be an EMI problem, an EMI investigation was conducted. According to the systems integrator, the computers ran well in the temporary location where they were first set up, but the performance degraded shortly after the computers were relocated. The location then became suspect.
A site survey was performed and many low-frequency problems were observed. These problems appeared at least three times a day at approximately the same time.
That was the clue that cracked the case. The communications center was located above the power controls for the jail’s electromagnetic door openers for the prison cells. Every time the doors opened, a power surge was induced into the power grid of the computers, causing the machines to reset.
The least expensive solution was to relocate the equipment. The next suggestion was a separate filtered power drop to the room for the radios and the computers. Additional filtering of the I/O cables and shielding of the computers also was suggested.
The choice was relocation. This action was the most logical, particularly since the same recommendation came from the providers of the radio equipment that also was experiencing EMI problems at the same location.
Unfortunately, most systems integrators or service people do not consult with an EMC engineer before installing equipment or performing any type of service. This could lead to EMI-related service or function problems down the road.
Nothing is more annoying than having your computer serviced and then finding that you have a new problem with it. When you take it back to the service center, the problem does not appear.
I saw this happen with a hard drive that produced read errors. It finally was tracked to a cable. The original shielded cable had been replaced with a nonshielded drive cable. The computer worked fine on the bench, but when the modem card was installed, the drive quit working. When the cable was replaced with a shielded cable, the problem disappeared. Although the machine was still in Class B limits with the unshielded cable, the immunity level of the machine had changed by a simple cable replacement.
There are many approaches to a possible EMC-compliance issue. The best way is to keep it simple—if possible.
Determine what the problem is. Not all random problems are EMC related. If the unit is failing, determine what the failure is.
Don’t be afraid to experiment. Most EMC engineers learn a bit by trial and error. It helps if you have a good assortment of caps, ferrite beads, chokes and copper tape in your EMI kit.
When you find a possible solution, the fun really begins. Remember, the product fix must be cost-effective for the customer. If possible, have several alternative solutions.
Keep a good log of the project as a reference guide.
No matter how you approach an EMC problem—whether in a lab or in the field—the objective is not just to meet some government paperwork, but also to avoid causing interference problems. It is this objective that we must focus on when performing EMI troubleshooting.
About the Author
David A. Case, N.C.E., is the worldwide compliance engineer for Aironet Wireless Communications. He holds certifications from the National Association of Radio and Telecommunication Engineers in Electrostatic Discharge Control, Electromagnetic Compatibility and Telecommunications. Case is a member of the Executive Committee of the Council of the United States EMC Labs, the Board of Directors for NARTE and the IEEE EMC Society. Aironet Wireless Communications, 367 Ghent Rd., Suite 300, Fairlawn, OH 44334-0292, (330) 665-7900.
Copyright 1996 Nelson Publishing Inc.