Designing early for EMC

Addressing EMC late in the design cycle is becoming less and less tenable as product complexity and densities increase and design cycles continue to shrink. The rules of thumb you commonly use to calculate EMC are breaking down at higher frequencies, and you can easily misapply them. As a result, 70 to 90% of new designs fail first-time EMC testing, translating into high late-stage redesign costs and often even higher lost sales costs if the manufacturer delays the productshipping date. Designers should consider the use of collaborative, conceptual analysis-based EMC simulation early in the design process to identify and fix problems at a much lower cost.

Higher clock speeds lead to increased challenges in meeting EMC requirements. In the gigahertz world, increased numbers of enclosure resonances enhance emissions, making apertures and seams more problematic, and ASIC heat sinks can exacerbate radiated emissions, as well. In addition, regulatory agencies are developing regulations to ensure compliance at higher and higher frequencies. To top it off, the trend toward integrating wireless capabilities, such as Wi-Fi, Bluetooth, WiMax, and UWB (ultrawideband), presents further challenges as engineers intentionally design radiators into systems.

Traditional approach to EMC design
Normally, electrical hardware designers and mechanical designers consider EMC design in parallel, communicating little with each other if at all. They often use rules of thumb during design with the hope that these rules will be sufficient for the device they are designing. Many of these EMC rules are becoming obsolete as designs reach higher frequencies, leading to failure during testing.

After the design stage, a designer builds a prototype and tests it for EMC compliance. This process all too often results in identifying EMC problems when it is too late to design in EMC compliance. Often, expensive fixes on the design are the only options available. Design changes generally increase by an order of magnitude or more as the design moves from conceptual design to detailed design to validation. So, a change that would cost only $100 at the conceptual level might exceed tens of thousands of dollars at the testing stage, not to mention the negative impact on time to market.

Challenges of EMC simulation
It has become essential to include EMC design as an integrated part of the product cycle to obtain first-pass compliance in the test chamber and to ensure on-time delivery within budget. Designers can achieve this goal with a 3-D solution of Maxwell’s equations, which provide an elegant mathematical representation of electromagnetic interactions. But EMC simulation presents challenges you do not always encounter in other areas of computational electromagnetics.

A typical EMC problem involves an enclosure that is large relative to the features—such as slots, holes, and cables—that are important to EMC performance. Accurate modeling requires that the model include large and small details. This requirement results in large aspect ratios (the ratio of the largest feature to the smallest), in turn requiring fine grids to resolve the finest details. Compact model technology can allow you to include large and small structures in a simulation without prohibitive simulation times.

Another challenge is that you must perform EMC characterization over a wide frequency range. The
time required to calculate electromagnetic fields at each sample frequency would be prohibitive. Time-domain methods, such as the TLM (transmission-line method), perform the field solution in the time domain using broadband excitation, yielding data over an entire frequency band in a single simulation run. Space is divided into cells modeled at the intersection of orthogonal transmission lines. Voltage pulses are transmitted and scattered at each cell. You calculate electric and magnetic field from voltages and currents on the lines at each time step.





EMC simulation yields accurate results. Figure 1 compares the computed radiated power in
decibels referred to 1nW (red) with the measured radiated power (blue) for three configurations of modules (one, two, and three modules)attached to a backplane (Reference 1). You can attribute the small discrepancies in resonant-peak location for the multimodule cases to difficulties in obtaining precision alignment of the modules in the measure-ments. It is interesting to note that the differences in resonant peaks and amplitude of radiated power is due solely to the layout of the system, because the input power is the same in all cases.

Range of potential applications
EMC simulation applies to examining components and subsystems, such as radiation profile versus frequency in heatsink grounding as well as assessing grounding techniques, the impact of heat-sink shape, and other factors. You can also compare the shielding effectiveness of air-venthole sizes and shapes and metal thicknesses. Recent applications in these areas include a study to evaluate the use of large-hole air vents to allow airflow and control EMC by placing two such panels closely spaced back to back to achieve shielding effectiveness.

EMC simulation is also wellsuited to EMC design and optimization at the system level to compute broadband shielding effectiveness, broadband radiated emissions, 3-D far-field radiation patterns, cylindrical near-field radiated emissions to mimic a turntable-type measurement scenario, and current and E and H field distributions for visualizations that help to locate EMC hot spots. Typical system level EMC applications include designing enclosures to ensure maximum shielding effectiveness; assessing the EMC ramifications of component location within an enclosure; computing cabling coupling both inside and outside the system; and examining the effects of radiation from the cables. EMC simulation also helps identify mechanisms for unwanted electromagnetic transmissions through chassis and subsystems, such as cavity resonances; radiation through holes, slots, seams, vents, and other chassis openings; conducted emissions through cables; coupling to and from heat sinks and other components; and unintentional wave guides inherent to optical components, displays, LEDs, and other chassis-mounted components.

EMC impact of types of joints

You can use simple, quickrunning enclosure models to perform design trade-offs of seam configurations. Figure 2 evaluates the radiation from a butted joint versus an overlapping enclosure seam. By comparing the relative shielding levels, an engineer can base his decision on the EMC budget for the enclosure and the cost of implementing a particular design configuration. Adding internal components to the simulation has only a small effect on simulation time, so the designer can easily assess the shielding of the seam in a realistic environment that accounts for coupling between slot resonances, cavity modes, and interactions with internal structures. Design rules for slot leakages do not account for these factors and can lead to costly overdesigning or underdesigning.

A typical application of EMC simulation is to evaluate the shielding of ventilation panels. Although rules exist for designing air-vent panels for EMC leakage, EMC simulation can accurately predict more exotic configurations, such as back-to-back panels with large holes, waveguide arrays, and others, keeping in mind thermal and cost constraints. The application in Figure 3 shows the computation of shielding for panels with round- and square-hole geometries and different thicknesses. The graphs show the shielding the panels provide for thicknesses (left) and hole shapes(right).


Evaluating radiation from a heat sink


The EMC-simulation application in Figure 4 examines the radiation from a heat sink. In this simple model, a broadband signal source directly underneath the heat sink excites the sink, representing electro-magnetic coupling to the heat sink from an IC to which it is bonded. The plot shows the radiated power spectrum for three configurations. Clearly, the radiation level depends on the geometry and the frequency. Although grounding the smaller heat sink provides an improvement at lower frequencies, it increases the radiation in the middle part of the frequency range.







Solving a cable-coupling problem

Figure 5 shows the use of EMC simulation to examine system level cable coupling. The geometry comprises three network hubs in a 19-in. rack. A four-wire ribbon cable connects the pc boards in the top and bottom hubs to the middle hubs. The center hub contains the only EMC source in the model. EMC simulation computes the currents coupled from the center hub to the connection at a pc board in the upper hub. The coupled current displays two strong resonances at 600 and 800MHz. A common approach to this sort of problem is to add filtering to the affected cable and then gauge the impact with simulation. The lower plot shows that adding a lowpass filter reduces but does not eliminate the magnitude of the coupled current at the resonant frequencies. It is a “Band-Aid” fix, because it does not address the problem at its source.

EMC simulation visualizes the internal physics of the cablecoupling application to find the root cause of the problem. Examining the electric-field distribution inside the center hub at 600MHz lets you identify electricfield hot spots that identify a cavity resonance that generates high field levels near the cables. Adding a metal partition to the hub suppresses the resonantcavity mode and eliminates the coupling (Figure 6).




You can use EMC simulation to identify and solve a problem that arises from a thermally driven design change. Consider an example of this technique based on a model of a controller node, essentially a dual-processor Pentium computer, for an enterprise storage system. After this design is committed to hardware, the standard Pentium heat sinks are replaced with heat pipes that occupy the same footprint as the heat sinks but are taller, with horizontal rather than vertical fins.


A broadband simulation computes the radiated emissions of the system (Figure 7). In this example, engineers are interested in isolating the emissions due to a 120MHz oscillator signal in the system, because the measurements indicated a problem. Therefore, after computing the broadband response, the engineers use an indirect excitation in postprocessing to extract the response to the desired source signal, resulting in the discrete harmonics in the figure. The radiation increases approximately 40 dB at the fundamental harmonic of the oscillator frequency, 120MHz. It’s remarkable that such an innocuous thermal-design change has such a major and alarming impact on EMC compliance.

Having identified the root cause, you can explore costeffective options. In this case, eliminating the capacitive coupling path by making a ground connection between the top of the heat pipes and the enclosure lid provides an excellent, low-cost approach. You achieve it by placing a small section of EMI gasket with a conductive adhesive on the top fin of the heat pipes, such that contact with the lid compresses the gasket and forms an electrical ground connection. Figure 8 shows the radiated emissions plot, including the results for the grounded heat pipe. The fix results in emissions that are virtually identical to the baseline case, improving thermal performance without negatively impacting emissions.


Using simulation early in the design process lets you investigate and predict key EMC phenomena and, hence, optimize electronic product design in EMC requirements and shielding effectiveness before building a prototype. Modern simulation tools enable designers to evaluate more designs than it’s practical to prototype and optimize products from an EMC perspective to a level that in the past was impossible. It’s also important to note that you cannot do EMC design in isolation, because design changes for EMC reasons frequently impact other design issues, such as thermal management. Therefore, it’s significant that some EMCsimulation tools enable designers to consider EMC with other important design constraints to optimize overall system cost and performance.

Author Information
Fred German is EMC Product Manager at Flomerics, where he is responsible for defining and developing electromagnetic simulation software for EMC design as part of an integrated design environment for electronics. He has a doctorate in electrical engineering from Auburn University (AL) and is a triathlete, avid reader, and ham-radio enthusiast.

Reference
1. Li, K, et al, “FD-TD Analysis of Electromagnetic Radiation from Moduleson-Backplane Configurations,” IEEE Transactions on EMC, August 1995.