Design Feature: February 17, 1994

EMC-design tools

Doug Conner,
Technical Editor

The problem facing designers of high-speed circuits is not just to make sure that products work by themselves. Designers must also make sure that their products fit into a larger community of electronic systems. The issue is electromagnetic compatibility (EMC).

As electronic systems increase in speed, designs may begin to encounter signal-integrity problems—excessive ringing that delays signal settling, crosstalk between traces, and ground bounce, to name a few. What your designs are experiencing are the first symptoms of electromagnetic-compatibility (EMC) problems. If your designs have suffered from these types of problems, you've probably looked for tools to help you predict and avoid such EMC difficulties.

What is EMC? The IEEE defines it as the ability of a device, equipment, or a system to function satisfactorily in its environment without introducing intolerable electromagnetic disturbances to anything in that environment.

Broadly speaking, when designing for EMC, you have to consider three major questions. First, is your system compatible with itself? Obviously, a system that jams or degrades its own performance is a problem. Signal-integrity problems usually fall into this group. Signal-integrity tools can help solve this problem.

Second, does your system introduce intolerable electromagnetic disturbances to anything in its environment? What constitutes "tolerable" varies from country to country, and standards organizations in the country where the system will operate define allowable limits. For example, in the United States, the FCC sets the requirements. In Europe, the IEC has instituted a new set of strict EMC standards that will go into effect in 1996. Compliance testing is usually required on most systems to verify that a system doesn't violate the standards. Newly emerging EMC tools attack the emissions problem.

Third, does the surrounding environment adversely affect your system? If so, you have a susceptibility problem, which is typically more difficult to solve than are the other two types because it's difficult to know what the surrounding environment will be for every application. One of the reasons that standards organizations are developing emission standards is to specify an environment for which you can design. Specifying the maximum allowable emissions for a system can also tell you that another system in the vicinity will have emissions less than or equal to the maximum. Currently, none of the EMC-analysis tools effectively addresses the susceptibility problem.

The methods of dealing with the first two EMC considerations differ, depending on which of the issues you are addressing.

The problems of an electronic system's compatibility with itself typically manifest themselves without any special testing when you attempt to operate the system. Signal-integrity tools help you at the design stage by predicting such problems before you build a system. This gives you a chance to change the design and avoid the problem.

Currently, the most common method of handling electromagnetic emissions is through compliance testing of the final product. Although emission testing is necessary and will remain so, finding EMC problems by testing a production system or prototype is undesirable. Product testing usually happens late in the development cycle, and problems may delay your product's completion date. The unit cost of the product may also increase because you typically don't have as many options available for correcting an EMC problem late in the development process.

To make their systems pass emission tests, engineers often try to seal a radiating mess inside a shielding enclosure. Although this technique is often necessary for passing emission testing, a well-designed system inside may reduce the cost of the enclosure.

Correcting an EMC problem earlier in the design stage may be as simple as moving components and traces around on a pc board. Table 1 lists EMC-design tools that are or soon will be available. EMC tools that can help you at the design stage are of two types.

Table 1—EMC-design tools
Manufacturer	Product	Price	Availability	Description
Ansoft	Maxwell SI Eminence	$49,900	March, 1994	Simulates emissions of multiple pc boards, cables, and enclosures
Aries Technology	MSC/Aries EMC	$19,950 $950/month	Q1, 1994	Simulates emissions of multiple pc boards, cables, and enclosures
Contec	ContecRadia	$70,000	January 15, 1994	Simulates emissions from a pc board with out an enclosure
Quad Design	Quiet	$60,000	Beta: Q1, 1994	Simulates emissions from a single pc board without an enclosure
Racal-Redac	EMC Advisor	$15,000	Now	Expert system evaluates compliance to EMC-design rules

Let an expert system help you

The first is an analysis tool that checks your adherence to rules that minimize EMC problems. As this article goes to press, the rule-based EMC tool called EMC Advisor is the only commercial tool that is actually available. EMC Advisor is a part of Racal-Redac's CAD Expert EDA-layout-design System but is an independent application. The tool is an expert system that evaluates a design to measure its relative compliance to a set of EMC-design rules. It is not a simulator, although you might want to use it as a complementary tool with an EMC simulator.

EMC Advisor evaluates a design against a set of 15 rules that focuses on the two categories of emissions: differential mode and common mode. Differential-mode emissions result from a current flowing around a loop, such as a signal and its return path. Common-mode emissions result from currents on the power-distribution system, such as transients in the ground plane. Poorly designed power-supply decoupling or improper component placement often results in common-mode emissions.

To evaluate a design, EMC Advisor analyzes layer stack, track shielding, impedance profiling, component placement, partitioning, power-supply decoupling, power-plane impedance, and power-plane overlap. You can also add your own rules. Because the tool uses data normally available in the design system, you can run the tool periodically as you perform a layout. Furthermore, you don't have to be an EMC expert to use EMC Advisor. After the tool completes its analysis, it highlights problem areas of the design in red and marginally problematic areas in amber. It also suggests improvements.

Although a rule-based EMC tool still leaves you guessing as to just what your systems electromagnetic emissions are, such a tool can effectively minimize potential EMC problems. For designers with limited knowledge of how to avoid EMC problems, the rule-based tools can help you create a better design that has a much better chance of passing emission tests without problems. Even if you use an EMC simulator on the design, EMC Advisor may help you create a better design. Another advantage: EMC Advisor runs much faster than analysis tools that perform complete electromagnetic simulations.

Simulators help analyze your design

The second type of analysis tool for EMC design is a simulator. The benefit of simulators is well known: You get to see how your design behaves without actually building and testing the hardware. The major concerns with most simulators is how well they represent reality and how difficult it is to obtain or develop the models to use in the simulation. (See box, "The ABCs of EMC".)

The ABCs of EMC Zoltan Cendes, PhD, President, Ansoft Corp
Many designers may think of electromagnetic compatibility (EMC) as a black art and view its problems as mysterious, counterintuitive, or strange. Solutions to EMC problems tend to be heuristic, based on years of experience with trial-and-error methods. Yet, to understand EMC, you must first understand that the source of the phenomenon is electromagnetic. However, many designers find EMC confusing because they use inappropriate models or simple circuit models to represent complex electromagnetic phenomena. Maxwell's equations govern electromagnetic fields. These equations predict that time-varying magnetic fields induce electric fields and that time-varying electric fields induce magnetic fields. At low speeds—with leading-edge rise times less than 1 nsec, for example—the induced fields are small, and you may overlook them. In these cases, using traditional lumped-circuit parameters, such as capacitance, inductance, and resistance, you can accurately model the voltages and currents in the components. However, as computer speeds increase so that signals exhibit subnanosecond rise times, induced fields become significantly larger, and frequencies become higher. In these cases, ordinary circuit theory breaks down. As a result, there is no alternative to full-wave solutions—solving the full set of Maxwell's field equations. A spiral inductor's field The spiral inductor in Fig 1 illustrates how a simple RLC model breaks down at high frequency. This spiral inductor has a three-to-one metalization pattern; that is, the conducting turns are three times as wide as the space between them. As a result, current is readily induced in neighboring turns. A low-frequency solution, assuming the absence of induced currents in the conductors, provides a lumped-parameter response that rises monotonically with frequency (see the green line in Fig 2). Measuring the circuit and providing a full-wave solution of Maxwell's equations determines the actual circuit behavior. This is a much more complicated solution (see the red and blue lines in Fig 2). In this case, the low-frequency model is accurate to about 5 GHz but breaks down at higher frequencies. Clearly, the low-frequency, lumped-parameter circuit model is useless at high frequencies. Although 5 GHz appears to be a very high frequency, it may not be. For example, a digital circuit operating with a 1-nsec rise time has a significant amount of leading-edge harmonics at this frequency. What is going on here? Why does the performance of the spiral inductor depart so radically above 5 GHz from the behavior that circuit theory predicts? There are three reasons. First, as the frequency rises, the current no longer flows in the familiar stream-function patterns common to low frequency. Instead, the current crowds toward the edges of the conductors due to skin effect. Second, eddy currents are induced in the ground plane below the spiral. These ground-plane eddy currents further alter the inductance and resistance values of the inductor. Third and most important, high-frequency electronic components set up and radiate electromagnetic waves. These waves establish resonances and standing wave patterns in the inductor, dramatically altering the voltage and current patterns and radiating energy away from the system. In fact, at high frequency, the spiral inductor in Fig 1 operates like and exhibits behavior similar to that of a classical dipole loop antenna. Because the radiation from a loop antenna grows with the square of the frequency, inductor performance deteriorates rapidly following the onset of radiation. Fig 3 shows the far-field radiation pattern for the spiral inductor at 10 GHz. At this frequency, 10 to 15% of the energy input to the inductor radiates away. Solving EMC problems, therefore, involves determining the underlying electromagnetic fields. For low frequencies, you can solve the quasistatic field equations for LCR, and you can use the resulting values and quasi-transverse-electromagnetic analysis to simulate signal-integrity phenomena, such as crosstalk, ringing, and ground bounce. At midrange frequencies, you must model the eddy currents in the conductors by solving the electromagnetic-diffusion equation. However, at leading-edge rates lower than 0.1 nsec, there is no alternative to solving the full set of Maxwell's equations. The full-wave solution is difficult to compute. However, once you compute it, you can generate a full-wave Spice model to interface directly with circuit simulators. Fortunately, finite-element methods to compute full-wave solutions have emerged in recent years. In many cases, these methods provide easy and highly accurate solutions to EMC problems. Consider, for example, the layout of the pc-board power and ground traces in Fig 4a. Ansoft's Maxwell SI Eminence simulated the layout. The first step in simulating the radiated emissions is to enter the geometry of the structure by using the software's solids-modeling system or by reading the geometry from a CAD file, such as GDSII, Gerber, or AutoCAD. Next, define the materials in the structure and the input signals. Finally, the software creates and refines the finite-element mesh, solves the resulting matrix equations, and determines the frequency response. After the software completes the solution, you can test the solution in many ways, including having the software compute inductance and reactance as a function of frequency, determine resonances and standing wave patterns, and plot near- and far-field patterns. Fig 4a shows a plot from the pc board of the near field at 100 GHz. The plot shows that placing the power and ground leads on opposite ends of the pc board creates a large current loop that emits excessive radiation. To dramatically reduce the radiated emissions from the pc board, the designer rerouted the ground trace to lie directly under the power trace. Fig 4b shows the resulting near field at 100 GHz. Thus, solving EMC problems is not a black art after all. The electromagnetic fields in electronic components causes these problems. Using full-wave finite-element methods to solve these fields directly, you can perform a precise simulation of EMC phenomena and minimize electromagnetic coupling and emissions.

The ABCs of EMC

Zoltan Cendes, PhD, President, Ansoft Corp

Many designers may think of electromagnetic compatibility (EMC) as a black art and view its problems as mysterious, counterintuitive, or strange. Solutions to EMC problems tend to be heuristic, based on years of experience with trial-and-error methods. Yet, to understand EMC, you must first understand that the source of the phenomenon is electromagnetic. However, many designers find EMC confusing because they use inappropriate models or simple circuit models to represent complex electromagnetic phenomena.

Maxwell's equations govern electromagnetic fields. These equations predict that time-varying magnetic fields induce electric fields and that time-varying electric fields induce magnetic fields. At low speeds—with leading-edge rise times less than 1 nsec, for example—the induced fields are small, and you may overlook them. In these cases, using traditional lumped-circuit parameters, such as capacitance, inductance, and resistance, you can accurately model the voltages and currents in the components.

However, as computer speeds increase so that signals exhibit subnanosecond rise times, induced fields become significantly larger, and frequencies become higher. In these cases, ordinary circuit theory breaks down. As a result, there is no alternative to full-wave solutions—solving the full set of Maxwell's field equations.

A spiral inductor's field

The spiral inductor in Fig 1 illustrates how a simple RLC model breaks down at high frequency. This spiral inductor has a three-to-one metalization pattern; that is, the conducting turns are three times as wide as the space between them. As a result, current is readily induced in neighboring turns. A low-frequency solution, assuming the absence of induced currents in the conductors, provides a lumped-parameter response that rises monotonically with frequency (see the green line in Fig 2). Measuring the circuit and providing a full-wave solution of Maxwell's equations determines the actual circuit behavior. This is a much more complicated solution (see the red and blue lines in Fig 2).

In this case, the low-frequency model is accurate to about 5 GHz but breaks down at higher frequencies. Clearly, the low-frequency, lumped-parameter circuit model is useless at high frequencies. Although 5 GHz appears to be a very high frequency, it may not be. For example, a digital circuit operating with a 1-nsec rise time has a significant amount of leading-edge harmonics at this frequency.

What is going on here? Why does the performance of the spiral inductor depart so radically above 5 GHz from the behavior that circuit theory predicts? There are three reasons.

First, as the frequency rises, the current no longer flows in the familiar stream-function patterns common to low frequency. Instead, the current crowds toward the edges of the conductors due to skin effect.

Second, eddy currents are induced in the ground plane below the spiral. These ground-plane eddy currents further alter the inductance and resistance values of the inductor.

Third and most important, high-frequency electronic components set up and radiate electromagnetic waves. These waves establish resonances and standing wave patterns in the inductor, dramatically altering the voltage and current patterns and radiating energy away from the system. In fact, at high frequency, the spiral inductor in Fig 1 operates like and exhibits behavior similar to that of a classical dipole loop antenna. Because the radiation from a loop antenna grows with the square of the frequency, inductor performance deteriorates rapidly following the onset of radiation.

Fig 3 shows the far-field radiation pattern for the spiral inductor at 10 GHz. At this frequency, 10 to 15% of the energy input to the inductor radiates away.

Solving EMC problems, therefore, involves determining the underlying electromagnetic fields. For low frequencies, you can solve the quasistatic field equations for LCR, and you can use the resulting values and quasi-transverse-electromagnetic analysis to simulate signal-integrity phenomena, such as crosstalk, ringing, and ground bounce. At midrange frequencies, you must model the eddy currents in the conductors by solving the electromagnetic-diffusion equation.

However, at leading-edge rates lower than 0.1 nsec, there is no alternative to solving the full set of Maxwell's equations. The full-wave solution is difficult to compute. However, once you compute it, you can generate a full-wave Spice model to interface directly with circuit simulators.

Fortunately, finite-element methods to compute full-wave solutions have emerged in recent years. In many cases, these methods provide easy and highly accurate solutions to EMC problems. Consider, for example, the layout of the pc-board power and ground traces in Fig 4a. Ansoft's Maxwell SI Eminence simulated the layout.

The first step in simulating the radiated emissions is to enter the geometry of the structure by using the software's solids-modeling system or by reading the geometry from a CAD file, such as GDSII, Gerber, or AutoCAD. Next, define the materials in the structure and the input signals. Finally, the software creates and refines the finite-element mesh, solves the resulting matrix equations, and determines the frequency response.

After the software completes the solution, you can test the solution in many ways, including having the software compute inductance and reactance as a function of frequency, determine resonances and standing wave patterns, and plot near- and far-field patterns. Fig 4a shows a plot from the pc board of the near field at 100 GHz. The plot shows that placing the power and ground leads on opposite ends of the pc board creates a large current loop that emits excessive radiation.

To dramatically reduce the radiated emissions from the pc board, the designer rerouted the ground trace to lie directly under the power trace. Fig 4b shows the resulting near field at 100 GHz.

Thus, solving EMC problems is not a black art after all. The electromagnetic fields in electronic components causes these problems. Using full-wave finite-element methods to solve these fields directly, you can perform a precise simulation of EMC phenomena and minimize electromagnetic coupling and emissions.

The simulators in the table predict a system's frequency-dependent electric-field intensity at arbitrary points in space. The listed simulators simulate various types of systems.

For example, Ansoft's Maxwell SI Eminence simulates full 3-D systems, including multiple pc boards, cables, and enclosures. The simulators from Contec and Quad Design do not simulate the effects of enclosures. The EMC tools read physical-design data from a variety of compatible design tools. Make sure the tools you want to use are compatible with your physical-design tools. Such physical-design tools can include tools for pc-board layout, mechanical design, multichip-module (MCM) design, and IC-package design.

Ansoft's Maxwell SI Eminence uses a full-wave finite-element solver to solve the full, unsimplified Maxwell's equations in the frequency domain for characterization of arbitrary 3-D structures. The tool has an adaptive mesh-refinement capability to simplify mesh creation for the analysis of 3-D structures. The software is applicable to EMC and EMI problems in ICs, pc boards, cables, loops, cabinets, apertures, and radiating structures, such as microstrip, wire, slot, and horn antennas. The tool generates a Spice model of a system that runs on Spice simulators.

Aries Technology's EMC tool also analyzes 3-D designs using finite-element analysis. The software automatically generates the finite-element mesh from 3-D physical-design data. It predicts magnetic and electric fields and current densities, induced and applied voltages and currents, and power losses for frequencies from dc to RF. The analysis program provides results for any number of frequencies or in the time domain.

In June, Quantic Design will offer the EMC Greenfield EMC-analysis tool, but the company has not yet set a price for the product. The software models the radiated electromagnetic fields from multiple pc boards within an enclosure, including internal and external cables, using time- and frequency-domain analysis. The tool offers a choice of an approximate-but-fast solution or an exact analysis. Quantic will integrate EMC Greenfield with the company's other Greenfield transmission-line-analysis tools.

Contec's ContecRadia and Quad Design's Quiet model the radiated electromagnetic field of a pc board, MCM, or backplane with no enclosure. As the tools become commercially available, it will be interesting to watch Radia and Quiet. Although less ambitious than some of their competition, will they also prove to be less capable, or more realistic for real-world problems?

ContecRadia calculates radiated-noise and electric-field intensities, including electrical effects, reflection, and crosstalk of transmission lines in high-speed, high-frequency digital, analog, and mixed-signal systems. After extracting the electrical models from layout files, the ContecSpice circuit simulator calculates time-domain current and voltage distributions in the conductor traces. It uses the current distribution on the conductive traces to calculate the radiated electric-field intensities, based on loop-antenna principles.

Quad Design's Quiet uses transmission-line simulation to model the network waveforms and then sums the radiation from each net segment to get the radiation from the entire net relative to one or more specified points in space (the receiving antenna location). By summing the radiation from all nets, the software obtains the full board radiation.

Quiet simulates networks in the time domain using linear and nonlinear driver, receiver, and terminator characteristics. By accurately modeling the network-signal characteristics of all modern logic families, the software can accurately model the radiated energy. The software lets you specify the characteristics of each signal on every net or just the clock frequencies. It also lets you use pseudorandom signals to simplify the definition of signal activity.

Although the Radia and Quiet simulators handle only pc boards and do not include the effects of an enclosure, don't count them out as useful tools to help you stay out of EMC problems. The shielding effects of a conductive enclosure are useful and often necessary, but reducing radiated energy at its source is usually best.

An analysis tool that lets you work on a single board at a time can help you maintain low emission levels. However, such a tool does not let you obtain accurate data on the radiated energy of the entire system outside its enclosure—something you may want to know if you anticipate problems when you run emissions tests on the completed system.

Also, remember: Tools that help reduce transmission-line and signal-integrity problems typically result in lower emission levels.

Kellee Crisafulli, president of HyperLynx, a manufacturer of signal-integrity-analysis tools, reports that some of the company's customers find that when they clean up their transmission lines, they also reduce EMI and RFI.

The future for EMC-analysis tools for radiated emissions looks promising and competitive. Many of the companies that offer transmission-line-analysis tools expect to have EMC tools available within the next year. With the ever-increasing clock rates of many electronic systems, EMC is likely to become a more familiar acronym.

PICTURE

You can reach Doug Conner at (805) 461-9669.

Manufacturers of EMC-design tools
For free information on the EMC-design tools in this article contact any of the following manufacturers directly. Please let them know you read about their products in EDN.
Ansoft Corp Pittsburgh, PA (412) 261-3200	Aries Technology,a division of MacNeal-Schwendler Corp Lowell, MA (508) 453-5310	Contec Microelectronics USA San Jose, CA (408) 434-6767
HyperLynx Redmond, WA (206) 869-2320	Quad Design, a Viewlogic Co Camarillo, CA (805) 988-8250	Quantic Laboratories Inc Winnipeg, Manitoba, Canada (204) 942-4000
Racal-Redac Mahwah, NJ (201) 848-8000