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The I-Connect007 team recently spoke with design instructor and author Eric Bogatin about the EMI challenges facing PCB designers today. Eric is a “signal integrity evangelist” with Teledyne LeCroy, as well as an adjunct professor at the University of Colorado Boulder, and technical editor of the Signal Integrity Journal. In this interview, Eric explains why EMI is so prevalent and what designers and design engineers can do to avoid EMI from the start.
Andy Shaughnessy: Eric, we wanted to get your thoughts on EMI, a topic that you have spoken and written about quite a bit. What can designers do to avoid EMI?
Eric Bogatin: Here’s a little perspective on why I think EMI is still a big issue, will always be a big issue, and, depending on the system, will probably become a bigger issue. In FCC certification and testing, in Part 15, there are Class A and Class B. Class B is consumer electronic products, which is a more sensitive, more stringent test for radiated emissions. Just to give you a number, the test specification says that you put your product down in the chamber, go three meters away, and look at the worst-case electric field-radiated emissions. In the roughly 100 megahertz range, they say the worst case is 100 microvolts per meter, which is the maximum allowable. The important number is that this is within a 120-kilohertz bandwidth detector.
Your product has to radiate less than 100 microvolts per meter, 10 feet away within that 120-kilohertz bandwidth. You can ask, “What if I had a radio source sitting there and broadcasting on some power level. How much power is that? What is the maximum power my little radio station can transmit at and still pass the FCC tests?” Do you know what the maximum radiated power is for that little radio station to pass an FCC test?
Shaughnessy: I have no idea.
Bogatin: The answer is 10 nanowatts of power. This corresponds to a radio station broadcasting in 360 degrees within the 120-kilohertz band. The total power radiated all around from that radio station is no more than 10 nanowatts, and if more than 10 nanowatts, it will fail the FCC test. And you say, “Wait, it doesn’t take much radiated power at all to fail an FCC test. You really have to engineer your product so that it doesn’t radiate very much.”
And that’s why one of the fixes for passing a test is adding the spread spectrum clock generator. If you have a clock system, you’re radiating at the harmonics, so the spread spectrum clocking just dithers that harmonic frequency range across that 120-kilohertz bandwidth. Because it only takes a tiny amount of radiated energy to fail an FCC test, EMI has been and always will be a problem in many systems.
To read this entire interview, which appeared in the February 2020 issue of Design007 Magazine, click here.