one of the world’s top 10 information technology (it) companies recently validated the ability of flomerics’ flo/emc electromagnetic compatibility software to predict the performance of enclosure shielding at gigahertz frequencies by comparing simulation results against physical testing. "the accuracy of flo/emc in predicting shielding performance is saving time and money by reducing the need for physical testing as we move forward with a new generation of fibre channel products," said one of the company’s emc engineers (who can not be named for legal reasons).

design for electromagnetic compatibility has long been an inexact science with designs created based on guesswork and physical testing required for each iteration to determine whether it works or not. the challenge is particularly great above gigahertz frequency levels because radiated emissions launch mechanisms are not well understood above this level. "electromagnetic compatibility design tools have matured considerably over the past five years," said the emc engineer. "we saw the opportunity to minimize the test, fix, retest cycle, pass compliance, and get to market faster. so we decided to evaluate the accuracy of flo/emc."

engineers began by building a test box with horizontal and vertical slots and vertical and overlapping seams. they modeled the box in flo/emc and meshed the geometry for electromagnetic analysis. the simulation results plotted emissions seen at a point three meters from the box as a function of frequency, cylinder scans with both vertical and horizontal polarization, a delta test to determine shielding effectiveness, and surface currents to diagnose the source of emissions. the simulation results closely matched physical testing at frequencies up to 2 gigahertz.

next, the engineers evaluated the ability of flo/emc to diagnose problems and evaluate potential solutions. thermal management made it necessary to add a heat sink on the central processor unit. engineers were concerned that the heat sink might act as an antenna. they modeled the heat sink and thermal interface material and stimulated the design with an impulse peak as well as 106.25 mhz fibre channel clock emissions.

the results showed an emissions peak at a harmonic of the clock frequency. visualizing the surface currents at 2.23 ghz showed that the e field wrapped around the edge of the heat sink base and coupled over into the fins. analyzing surface currents through a vertical slice of the chassis revealed energy radiating sideways from the heat sink, beaming right into the apertures of the box.

engineers then tried adding corner supports for the heat sink and connecting them to the reference plane of the pcb in order to provide a return path for the current. the simulation showed that this approach introduced a new problem, a peak at 800 mhz. the engineers then added eight additional ground pins closely surrounding the heat sink in order to shorten the return path. this approach provided tighter control of the current, preventing it from spreading out and exciting resonances. "this validation study demonstrated the accuracy of electromagnetic simulation as well as its ability to take cost and time out of the mechanical design process," the engineer concluded.

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