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Designers Day Preview: Rick Hartley on Fighting EMI

In the run-up to IPC APEX Expo 2010, we're going to focus on a few of the workshops taking place on Designers Day, April 8, 2010. This week, we'll cover Rick Hartley's class EMI: High Frequency/High-Speed Design.

Many of you know Rick from his high-speed design courses and the numerous technical articles he's written over the years. Rick is a senior principal engineer with L-3 Communications, Avionics Systems, and founder of Hartley Enterprises. We e-mailed a handful of questions to Rick, and here are his responses.

ANDY: Rick, give us a preview of your Designers Day workshop.

RICK: The presentation will discuss the power distribution system (PDS) portion of the PC board. The PDS is the foundation around which all things work in the circuit. If not designed correctly the entire circuit is at risk from increased common-mode noise, noise that will impact signal integrity and can severely increase EMI. Low impedance in the power bus across the range of harmonic frequencies of a digital circuit is critical. How to accomplish that goal is greatly dependent on both frequency of operation and layer count of the board. The presentation will cover the major components of the power bus, decoupling capacitor concerns, the importance of IC pin assignments, placement of decoupling, real performance of capacitors (verses myth), how many different capacitor values are best, analog decoupling vs digital decoupling, the importance and the real performance of power/ground planes and the importance of board stack-up, as well as special high-capacitance, low-inductance planes and their impact on noise and EMI.ANDY: Why is it so important to design PCBs to control EMI? RICK: Most circuits and systems must meet government controlled levels of electromagnetic emissions and susceptibility (EMI) to legally sell their products in their market slot. Some systems, such as military and other avionics, medical devices, etc., have to meet very stringent requirements to satisfy these government mandates. Once was the time when turning on any household appliance would affect the performance of everything else in the house--the TV, radios, etc. This same problem, with the advent of the digital era, spread to other, more critical areas (i.e.,communications and medical), forced the FCC and other agencies around the world to enact controls, hence the beginning and growth of the mandates. When the TV doesn't work correctly due to EM interference, it's annoying. When a mission-critical system fails, it can be and often is life-threatening.ANDY: Where do most EMI problems begin? RICK: Most of the time when assisting in the resolution of an EMI issue, the problem is found to be in the design of the PC board itself. Circuit design issues, such as improper filtering of I/O, improper decoupling values, etc., can certainly impact EMI, and they do, but even a perfect circuit design will often have EMI issues if the PC board is NOT properly designed.ANDY: What's the biggest mistake you see designers making regarding EMI?RICK: Not controlling and containing E&H fields. Two things impact this the most: Not taking adequate care of return paths of signal lines and improper design of the power bus. Of course there are many other things that affect fields and EMI, but these are the two that I've seen to have the greatest impact. Also, too many designers and engineers assume that circuit frequency, hence field strength, is associated with the clock period and they design accordingly. Circuit frequency is really related to IC output edge rate and must be the deciding factor when determining how to setup the entire PC board layout. Appliances, elevator controllers, and other similar systems operating at 1-5 MHz can and do fail EMI testing in the range of several hundred MHz, simply because the PCB wasn't designed to take proper care of E&H fields. Again, current return paths and the power bus have the biggest impact.

ANDY: So designing for signal integrity doesn't necessarily mean that you're controlling EMI?RICK: Many of the things we do to control SI will improve EMI and visa versa, but the two are not necessarily linked. The engineer and PCB designer can set up a circuit and PCB to strongly control SI, yet the board may still have large common-mode currents that lead to severe EMI problems. On the other hand, all the right stuff can be done to control and contain EMI but the circuit, if proper impedance control and termination are not implemented, can and often will have signal integrity problems. The power bus, coincidently, is one of the areas of the board where proper design can often improve both signal integrity and EMI.

Rick Hartley's class EMI: High Frequency/High-Speed Design is scheduled for 3:30-5:00 pm on Designers Day, Thursday, April 8, 2010, in Mandalay Bay Convention Center in Las Vegas, Nevada.To contact Andy, click here.Follow I-Connect007 on Twitter here.