Ensuring Top Performance From EMC Software

You’ve got your precompliance test equipment all set up, a new version of an EMC test software program is running on a super fast laptop computer, and the UUT in your semi-anechoic chamber is performing perfectly. Measurement after measurement flows into the computer as antenna height and turntable positions are varied automatically.

Later that day, you proudly announce that the product should pass radiated immunity and emissions compliance tests with room to spare. Unfortunately, the test lab performing the compliance testing doesn’t agree. What went wrong? Surely, it had little to do with the new software.

EMC software runs the tests that prove compliance with a wide variety of immunity and emissions standards. In doing so, it provides a convenient, central resource that can control your test instruments, position the UUT and antenna, store and recall specific tests and limit lines, acquire test results, and display data (Table 1).

Because there are so many different types of test instruments, make sure the correct instrument drivers are available for the particular products you intend to use. For example, the SW1004 EMC Software from Amplifier Research is based on National Instruments’ LabVIEW and offers more than 500 instrument drivers from 45 vendors. In addition to controlling instruments, SW1004 controls the DUT through a suitable interface card.

Software also deals with correction and calibration factors that must be applied to compensate for nonideal performance, such as the RF power amplifier gain differing slightly from its nominal value or the antenna factor varying with frequency. For example, the Hewlett-Packard 8542E and 8546A EMC Receivers accept three different types of correction factors. A total of 80 separate amplitude vs frequency data pairs can be stored for antenna factors, cable losses, and an unspecified third error source. For less sophisticated receivers, the test software stores and applies the correction factors itself.

Immunity

For immunity testing, the uniformity of the field surrounding the UUT is specified in two ways. In the IEC 61000-4-3 radiated susceptibility method, 16 field strengths are measured, spaced evenly over a 1.5 m × 1.5 m area. If 75% (12) of them are within 6 dB of each other, the field is termed uniform. (See sidebar.) The IEC 801-3 approach uses closed-loop leveling which feeds back a measure of the field strength near the UUT to control the generator output level.

Having the capability to deal with both approaches is an advantage if you need to test at frequencies as low as 26 MHz. Dynamically controlling the field strength via feedback is particularly effective at lower frequencies. Although IEC 801-3 was withdrawn in 1995 and replaced by IEC 61000-4-3, it is still referenced by other standards. Typically, you would follow the IEC 801-3 method from 26 MHz to 80 MHz and use IEC 61000-4-3- above 80 MHz

Assuming that the field is being produced correctly at a particular frequency, it may not remain at the same level as you change frequency, even if closed-loop leveling is used. As you reduce the test frequency, the antenna gain may fall off significantly, requiring increased output from the power amplifier. As an example, a biconical antenna is reasonably flat from 80 to 200 MHz; but at 26 MHz, the gain typically is 25 times less than at 80 MHz.

If the amplifier’s 1-dB compression point is exceeded, harmonics will be produced as it distorts high-level signal peaks. Because the E-field probe is designed to be independent of frequency, at least over the frequency range you will be using, added harmonics simply produce a larger probe output signal.

Following this reasoning, Timothy D’Arcangelis, EMC manager at Antenna Research Associates, commented that “…most of the radiated testing claimed to have been performed at 26 MHz may have been, in all likelihood, performed at the harmonic frequencies.… Similar problems can occur elsewhere in the test frequency range. The harmonic levels may not dominate…but they could become a significant part of the indicated field strength.”

Emissions

Similarly, you may not be aware when problems arise in radiated emissions testing. A particularly large signal can saturate a preamp if one is being used, or it can increase the noise floor nearby if the receiver has become overloaded. RF input-filter preselection will bandlimit the input signal and help avoid overload.2 Make sure that there is a link between the receiver’s overload detection and the software rather than just a warning LED on the front panel of the receiver.

A pre-scan identifies areas of the spectrum with significant signal activity. EMC test software can produce a list of signals to be investigated in detail by comparing measured peak amplitude with a preset selection level. Peak detection is the fastest of the three commonly used weightings: peak, quasipeak, and average. But, when should you use which one?

According to Paul Sikora, manager of EMI/EMC Products at Electro-Metrics, answering two questions will determine which detector weighting is most appropriate. “Are devices that normally would be found operating in proximity to the UUT especially sensitive to electromagnetic energy? And would the failure of devices in proximity to the UUT be hazardous to humans or detrimental to interests of the owner/operator of the device? If the answer is yes to both questions, the peak detector probably is the proper measurement detector.”

As two examples of typical EMC problems, Mr. Sikora suggested the continuous hum on a TV receiver and an infrequent, intermittent strong signal that completely overwhelms the TV’s audio output for only a second. The nuisance potential of the continuous hum is high compared with that of the occasional complete audio interruption. A quasipeak detector will give a higher reading for the hum than for the complete interruption because the interruption is so short and infrequent.

Mr. Sikora continued, “In a battlefield situation, a hum in the output of the receiver would not rate a high level of nuisance relative to the other things going on. The momentary loss of audio, yielding a potential loss of critical information, would. The evaluation of the UUT in this situation would result in yes answers to both of the detector weighting questions. The use of a peak detector would insure detection of potentially disruptive emissions.”

If you do use a quasipeak detector or an average detector in your tests, remember that the readings you obtain will be equal to or lower than those that would have been obtained with a peak detector. This may make it easier to satisfy a specification limit. But unless there is good justification for using these types of detector, the results may not be valid.

OATS and Chambers

Although the open air test site (OATS) is the recognized standard for radiated emissions testing, obtaining consistent test results is not necessarily straightforward. Ambient signals can make testing at certain frequencies or ranges of frequencies impractical. In these cases, the UUT contribution can be determined by the method of substitution, although this adds time to the test and requires manual intervention.3 The ambient signal level simply cannot be subtracted from the level measured from the UUT plus ambient because, generally, the relative phase of the two contributions is not known.

The travel time associated with visiting an OATS in a relatively remote area, the inconvenience of taking equipment to the OATS, and the practical problem of ambient signals are reasons to use a test chamber as an alternative. Correlation of a semi-anechoic chamber to an ideal OATS to within the allowed ±4 dB is discussed by Haala and Wiesbeck.3

They pointed out that although an anechoic chamber shares the basic characteristics of an OATS—a ground plane on the bottom of the site and free space or absorbers in the other directions—chambers are not ideal. In particular, there will be reflections due to inadequate absorber performance, especially at low frequencies. Also, the antennas will not have the same characteristics as the simple dipole used for the theoretical calculations in the standards.

One solution measures the normalized site attenuation (NSA) values at an OATS using the same antenna as later will be used in a chamber. In their paper, Haala and Weisberg develop an alternative method that calculates the site attenuation of an ideal OATS using a preferred type of antenna other than a dipole. Correlation between subsequent tests in a chamber and the theoretical OATS is improved because the variation in the antenna type has been eliminated.

Another approach is proposed by Jahn and Hansen in their work with 3-m fully anechoic chambers. Eliminating the reflecting ground plane improves test time because there is no need to change antenna height—there are no reflections from the ground plane. They have developed a series of conversion factors that correlates the results between a chamber and an OATS.

Jahn and Hansen also made the point that OATS testing at a 10-m distance is not equivalent to the commonly accepted practice of testing at 3 m and subtracting 10 dB from the result. In the case of a 30-MHz horizontally polarized signal, the result was 4 dB higher than an actual 10-m measurement; for a 180-MHz vertical polarization test, the result was 6.9 dB too low.4

Summary

EMC test software can completely automate compliance and precompliance testing. A good package saves time and improves test result consistency; however, there are many sources of error that may not be corrected by the software. Among them are amplifier and receiver distortion due to overload; the effects of antenna type, nonisotropic behavior, and near-field considerations within a small chamber; incomplete absorption by the chamber lining; and high ambient signal levels at an OATS.

Once error sources have been dealt with, you can be confident that your semi- or fully automated measurements are being made correctly. This is the point at which your new EMC software package can begin to pay for itself.

Sidebar

Decibels Defined

A decibel (dB) is a unit of power ratio measurement based on common logarithms. Immunity and emissions test results can involve decibels, decibels referenced to 1 mW (dBm), and decibels referenced to 1 µV (dBµV). Although there is only one decibel, it often is defined relative to specific references.

Sidebar

Decibels Defined

You may wonder how decibels referenced to µV can be a measure of power. If two voltages are developed across two resistors of the same value, the ratio of the powers produced will be in the proportion of V12/V22. Because a decibel is defined as 10 log (Power1/Power2), if voltages are being measured, the ratio of two powers becomes 20 log (V1/V2). As Figure 1 shows, there is a big difference between the two definitions. The factor of two appears as an exponent, not as a multiplier.

For example, assume that you have measured 16 points according to IEC 1000-4-3 and discarded four of them and find that the remaining 12 are within 6 dB of each other. Because you measured E-field values in volts/meter, 6 dB is equivalent to about 2:1. In other words, if you need to test at 10 V/m, some parts of the illuminated area will see fields as high as 20 V/m.

If you develop a 10-V/m field in an imperfect chamber, reflections having random phase can reduce the field in some parts of the measured area and increase it in others. The 6-dB total deviation can be expressed as ±3 dB, meaning that an additional 3 dB of power is required to ensure all points are at least at the minimum 10-V/m field strength.

An interesting consideration relates to the power required to compensate for a 6-dB field variation caused only by a very directional antenna. The field strength at the edges of the measured area simply is lower, but reflections played no part. In this case, the deviation is entirely in one direction so an additional 6 dB of power is required, not just 3 dB. That means that the central part of the measurement area will see a 40-V/m field strength compared to the required 10 V/m at the edges.5

Decibels often are added. But remember, when adding them, you actually are multiplying. Consider an antenna gain of -14 dBi (dB relative to an isotropic radiation) and a power amplifier with a gain of 20 dB. These numbers mean that the antenna has a gain at a certain frequency of 0.04 or 25 times less than a theoretical isotropic, point-source radiator, and the amplifier has a power gain of 100. With the amplifier driving the antenna, the overall gain is a factor of 4, or 6 dB (20 dB – 14 dB = 6 dB).

References

1. O’Shea, P., “Immunity and Emissions Test-Software Features That Make a Difference”, EE-Evaluation Engineering, April 1998, pp. 78-83.

2. Vitale, P., “Making Good EMC Measurements,” EE-Evaluation Engineering, October 1998, pg. 96.

3. Haala, J., and Wiesbeck, W., “More Accurate Determination of Chamber Performance Using SA Measurements Compared to Computational Results,” IEEE Electromagnetic Compatibility Symposium Proceedings, Denver, CO, 1998, Vol. 1, pp. 300-303.

4. Jahn, T. and Hansen, D., “Are Fully Anechoic Chamber Emission Measurements in Compliance with the Standards?,” www.euro-emc-service.de.

5. D’Arcangelis, T., “Antennas and Amplifiers for Radiated Immunity Testing,” EE-Evaluation Engineering, June 1998, pp. 50-57.

Note: The EE-Evaluation Engineering articles can be accessed on EE Online at www.evaluationengineering.com. Select Article Archives from the Table of Contents.

Software Sources

These companies supply EMC test software.

Amplifier Research (215) 723-8181

Atkinson Engineering (540) 347-5716

CKC Laboratories (800) 500-4362

EMC Automation (512) 258-9478

Hewlett-Packard (800) 452-4844

Quantum Change/EMC Systems (703) 207-0344

Schaffner-Chase EMC (800) 367-5566

Tektronix (800) 426-2200

Wayne Kerr Electronics (800) 933-9319

FEATURE	DESCRIPTION
General Requirements
Control	Antenna mast, DUT turntable, test instruments
	Provides dynamic linking to hardware driver modules
	Correction/calibration factors
Test Definition	Supports international EMC regulations (pre- and full compliance)
	Libraries of predefined tests
	Allows custom tests
Measurements	Measure, store, perform analysis of data
Documentation	Report generation
	Online documentation with context-sensitive help
	Allows custom reports
	Operating information provided
Operating System	Windows 95/NT
	User-friendly operator interface
	Definition of macros, tamper-proof setup/result
Miscellaneous	Network interface for EUT monitoring and custom applications
	Modular design to accommodate expansion

Emissions
Signal Capture	Provides the complete spectrum from a device (in peak mode)
	Reduce to a signal list by comparison with limits
	Differentiate between ambient and DUT emissions
	Discrete frequency and amplitude measurements
Graphics	Simultaneous display of peak, quasipeak, average
	Limit lines and data in different colors for identification
	Limit line editing
	Overlay data from different scans and correction factors
Miscellaneous	LISN impedance switching (conducted emissions)

Immunity
Field Generation	Apply different Volts/meter over selected frequency ranges
	Perform field uniformity tests
Leveling	Define/control test levels, output levels, generator drive levels