What to Look for in EMC Test Software

The growing trend toward a global marketplace has compelled many manufacturers to place strong emphasis on EMC compliance. This is particularly true for companies seeking to export their products to Europe. They must demonstrate compliance with the European Union EMC Directive 89/336/EEC.

In-house EMC test capability can provide some measure of assurance that products will meet the applicable test criteria. An internal EMC capability also helps evaluate prototypes at various stages of product development and determine the impact of post-production changes to products on EMC performance.

Simply acquiring the equipment to conduct the testing would not be practical without obtaining the means to automate the testing. Whether the goal is electromagnetic interference (EMI) or electromagnetic susceptibility (EMS) testing, a little research will find many software packages to help maximize testing productivity.

Software packages can offer you varying degrees of automation and flexibility. All software should provide some level of diagnostic and precompliance testing. But only a few of the more comprehensive programs will include full-compliance test capability as a part of the basic package.

Testing for Emissions

Software that is organized in a logical format and allows you to review all aspects of a particular test sequence on one screen or window will help reduce the learning curve and minimize the potential for errors (Figure 1 and Figure 2). When researching EMI software packages, consider the following features:

Capable of adequately performing all routine emissions measurements in an accurate and time-effective manner.

Radiated RF emissions using peak, quasipeak (QP), and average detectors: Qualification measurements at a particular spot frequency must be made using QP or average detectors. To do so for preliminary measurements over a wide frequency range would result in excessively long sweep times. As a result, preliminary measurements generally are performed using peak detection to quickly identify the frequency points which subsequently will be measured with QP or average detectors.

Line-conducted emissions using peak, QP, and average detectors: Internationally, cord-connected equipment must comply with both QP and average limits for interference.

Conducted disturbance power measurements using peak, QP, and average detectors: Conducted power is the measurement of the disturbing energy radiated by a product’s power cord. If products intended for international markets consist of household appliances, electric tools, or other related items incorporating motors, heating elements, or compressors, then the software must perform conducted disturbance power measurements in accordance with EN 55014-1.

Capable of accounting for user-definable system losses and correction factors. Many aspects of the measurement system external to the spectrum analyzer negatively impact the amplitude of the emissions measured. These are termed system losses and must be considered to develop accurate data. If the software does not adjust for system losses, then the measured data will be inaccurate and meaningless.

A good software package corrects for cable losses, transducer (antenna) factors, preamplifier gain or attenuation (if used), external transient limiter losses (if used), and added external attenuation.

Support automated system calibration. Spectrum analyzer calibration should be routinely performed before making any measurements. The software should track elapsed time since the last calibration procedure and prompt you when these times become excessively long. It also should perform the calibration in an automated fashion. For older-model spectrum analyzers requiring manual adjustment to complete the calibration process, the software should be intuitive enough to step you through the entire procedure.

Preselector calibration (if used): Again, if manual intervention is required, the software should step you through the entire calibration process.

Automated turntable and antenna mast control and characterization: Automatic repositioning and point-and-click targeting for table azimuth and tower height also are convenient features.

Allow user-definable or -selectable accuracy levels: Each spectrum analyzer has a finite number of display pixels or points of data, typically between 400 and 1,000, relating to one full sweep across the CRT. The EMI software will upload these points of trace data that will then form the whole emissions data.

A log periodic antenna covering the frequency range of 200 to 1,000 MHz in conjunction with an analyzer with a resolution of only 400 data points means the resultant frequency resolution would be ± 2 MHz when a plot of that frequency range is prepared. While this may be sufficient for a quick-look scenario, it would not be desirable for comprehensive precompliance data.

The frequency resolution can be significantly improved if the software allows you to subdivide that frequency span into smaller segments. Using the same parameters but specifying four subdivisions within that range of 200 to 1,000 MHz now would produce a plot with a frequency resolution of ± 500 kHz. The resultant amplitude resolution also would increase.

Another concern with respect to the accuracy of recorded data is conflicts in the spectrum analyzer settings. It is important that the software allow you to specify spectrum analyzer settings such as frequency range measured, input attenuation, reference levels, sweep speed, and bandwidth settings. The software also should be smart enough to inform you if the settings specified are in conflict. Failure to do so could result in time spent developing useless data.

Compare measured emissions data against test limit(s) and automatically flag noncompliant (over-limit) data.

Provide export of all test data, graphs, and text and test setup parameters as an aid in report generation.

Include comprehensive library functions. The software should create and maintain libraries for test setups; test data; transducer files; gain/loss files such as test cables, transient limiters, and attenuators; and test limits.

EMI software packages generally include scripted setups for most commonly performed test sequences, such as FCC Part 15, European Norms, and Japanese VCCI. Adding a library of test files permits you to gather useful data immediately while learning the software.

Some software allows custom measurement subroutines to tailor a measurement sequence to a particular product. For instance, suppose a product has intermittent functions in which the intensity of the emissions varies as a function of time. If the software simply sweeps a range and plots the resultant data, then it is likely that the data gathered would not reflect the higher emissions level of the intermittent function. As a result, the option of creating a custom subroutine to account for the periodic emissions profile is beneficial.

Testing for RF Immunity

EMS testing for conducted and radiated RF immunity presents unique concerns; the biggest is the risk of damage to test equipment. Unlike EMI software, EMS software controls the power applied to RF amplifiers to generate the fields necessary to perform the test. If the amps are over-driven by the software, then they may be damaged. Keeping this in mind, users should look for software that:

Calculates maximum safe drive levels for the particular RF amplifier equipment. The software should characterize the connected RF amps by checking the 1- and 3-dB compression points to determine maximum safe drive levels or monitoring the RF output to ensure the maximum rated output wattage is not exceeded. The software also should adjust the drive level to account for modulation of the applied signal.

Performs the testing in an automated fashion. European regulations now require many products to comply with conducted RF immunity criteria. Often, radiated and conducted RF immunity testing are offered as separate software packages, but some programs on the market incorporate both test methods.

Conducts an automated 16-point field calibration per IEC 1000-4-3 for RF immunity testing (Figure 3). The software should record drive levels needed to generate the intended field strength, perform a 16-point field calibration, sort the data to calculate new drive levels, and plot the resultant calibration data.

Performs automated calibration of coupling-decoupling networks and current clamps for conducted immunity testing per IEC 1000-4-6.

Provides library functions to store calibration data. Because the calibration is time-consuming, you will want to store calibration data to eliminate the need to repeat the process before every test.

Permits manual control of frequency and field strength for troubleshooting and establishing failure thresholds. The software should permit you to manually step forward and backward in frequency to establish a window where failures typically occur. Then it should allow you to manually increase or decrease field strengths within this window to establish a threshold level where these failures occur. The capability to cycle RF power and modulation on and off also should be supported.

Supports user-definable start/stop frequencies, dwell times, modulation factors, and test levels. Dwell times should not be fixed by the software because there may be some instances in which a long dwell time may be necessary.

User-selectable start and stop frequencies also are desirable for troubleshooting because they allow you to test over a narrow window of frequencies known to be troublesome. For example, software that forces you to perform the test from 80 MHz through 1,000 MHz would waste valuable test time if prior testing has indicated that the problems typically only occur between 200 MHz and 225 MHz.

Conclusion

Above all, whether the software is chosen for EMI or EMS testing, it should enhance productivity by reducing test time and increasing throughput without risking damage to the test equipment. There is no question that some degree of in-house EMC test capability as a developmental tool can result in substantial time and cost savings as opposed to troubleshooting at the EMC test lab. With numerous automation software packages available on the market, locating one that will satisfy complete test needs and yet be truly compatible with existing test equipment can be a real task.

Table 1 references some, but certainly not all, of the many programs available today. While Underwriters Laboratories does not endorse any of the software products listed, it provides a point of reference for beginning the search.

Some programs, by nature of their supplier, will be geared to work with equipment from only that supplier. Others will be more flexible in terms of the varied equipment supported. The software supplier should be able to help you by demonstrating—in practical terms—the features his package uses to address a specific set of products and needs.

Take the time to catalog user-specific test and hardware requirements and review the many software options before making final decisions. Also, take full advantage of options such as demos and training when they are offered.

About the Author

Eric Parent is a staff-engineering associate for Underwriters Laboratories. He has been with UL more than 20 years, the last 10 with the UL International EMC Services Group. Mr. Parent has an associate of science degree in engineering from Harper College. Underwriters Laboratories, International EMC Services Group, 333 Pfingsten Rd., Northbrook, IL 60062, (847) 272-8800, ext. 42939, e-mail: [email protected].

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Copyright 1999 Nelson Publishing Inc.

April 1999