As shown in the comparison chart that accompanies this article (see EE October 2000 issue), precompliance test software relates to a variety of EMC requirements. Since the markets of the electronics industry are global, they involve a multitude of agencies and many standards. EMC test software is available to address various ANSI, BS, CISPR, EN, FCC, IEC, SAE, VCCI, VDE, and VG requirements plus some military specs.
Conventional wisdom says that the only users of EMC test software are product design engineers. Supposedly, they use it for precompliance testing at their facilities, detecting problems relating to emissions or immunity so improvements can be installed before the equipment goes to the official certification laboratory.
Sure, most applications relate to in-house precompliance tests, but some software goes far beyond this to encompass full-blown certification using a receiver or spectrum analyzer and an anechoic chamber, an open-area test site (OATS), or a Gigahertz Transverse Electromagnetic (GTEM) cell. The full power of such software seldom is unleashed, even for a demanding requirement.
Desirable Features
Good EMC test software should have prepackaged test setups and the capability to apply corrections for accessories such as the antenna, amplifiers, and cables to reduce measurement uncertainty. It also must compensate for ambient signals.
The user should be able to store traces and do before-and-after comparisons. Ideally, frequency and amplitude measurements are saved as pairs. Test sequences must be run automatically for greatest efficiency.
Setup
With a versatile package, a user can control external equipment such as the antenna and mast plus a variety of instruments. For example, SW1004 Software from Amplifier Research offers drivers for more than 500 instruments from 45 vendors.
Setup software should recognize several specifications and provide a library section in the database for each. The Electrometrics EM-3379 Software meets this requirement by offering separate libraries for IEC 1000-4-3, IEC 1000-4-6, MIL-STD-461A/B/C (broadband or narrowband), MIL-STD-461D, MIL-STD-1541, and SAE-J 551 applications.
Once a setup sequence has been defined, there must be a provision for easy storage and recall. The operator should be able to define macros for acquiring raw data, adding correction factors, and graphing prescans and have the capability to average measurements at specific frequencies.
“For automated emissions test, the minimum acceptable software should support all of the basic parameters involved in standard conformance tests,” said Volker Janssen, EMC product manager at Rohde & Schwarz. “This means that the software for a precompliance test analyzer or a full compliance receiver must adjust the start frequency, stop frequency, step size, resolution bandwidth, and type of detector (peak, quasipeak, average, or rms).
“Settings should be stored for future setup, report generation, and export,” he continued. “For manual, semiautomatic, or fully automatic operation, the software must have a manual reset, default setup, and initiation of abbreviated or full calibration under remote control.”
Some suppliers tailor the test functions by offering several specific modules. For example, the CKC Laboratories EMITest™ Software features modular configuration to cover a variety of applications. Two modules relate to radiated and conducted emissions. The other routines include a specification editor for the user who wants to redefine test limits, a calibration program to define transducer correction factors, and a MIL-Spec/DO-160 module.
Site Consistency and Data Collection
To be truly successful with EMC test software, it is vital that the test site be exactly the same for each test run. Tom Hoppin and Dennis Handlon, product managers at Agilent Technologies, advise that the DUT and antenna placement are critical for repeatability. Also, time-of-day, temperature, and all other test site conditions should be noted for each test sequence.
Several software packages allow an operator to tune to an area of interest and listen to what is there before initiating a measurement. This enables you to identify ambient signals such as TV and radio stations and avoid wasted time and unnecessary tests.
The Agilent Technologies E7415A prompts you to define test limits. “Corrections for cable loss, antenna factors, and preamplifier gain are automatic,” according to Ken Silk, a customer support engineer. “The user can make a test sweep across the frequency range of interest to identify suspects before information processing begins.”
Processing
Precompliance software must be able to handle unique conditions. For example, to ensure that precompliance tests in a small semi-anechoic chamber are nearly equivalent to the official certification tests that will be run at an OATS, the software should have a way to correlate the two.
“Ambient conditions can be compensated by well-planned EMC software,” noted Roger Southwick of EMC Consulting. “First, the DUT is operated in an ambient-free screen room to develop a frequency list. Then at the OATS, the software just measures the frequencies on this list. No ambient sorting is necessary.”
If the receiver in an EMC test overloads, the readings during that overload will be invalid. Some software packages are programmed to recognize the overload flag from the receiver and make adjustments to restore in-range reception. This prevents the inaccuracies that would make such tests meaningless.
Displays and Reports
Real-time display is a critical element in data gathering and analysis. Simultaneous presentation of measurements helps evaluate the validity of test data in real time. The complete spectrum of emissions is displayed by some software and is a valuable tool in controlling the sequence of tests.
The typical software package will divide data into subranges so the peak in each category can be identified. Graphics can show both data and limits, but in different colors. Data can be sorted in tables for meaningful viewing. When the displayed results are satisfactory, all the information can be stored on a disk, output via the Internet, and sent to one or more networked receive-only computer stations.
Even if the software is used solely for precompliance testing, reports can be generated in suitable form for submittal to the appropriate agency. “Our EMITest™ Software converts test data to different regulatory specifications automatically,” according to Todd Robinson, marketing manager at CKC Laboratories. “You may have data with FCC limits but need it revised and annotated for submittal to CISPR. We can do that.”
Mr. Janssen of Rohde & Schwarz agreed that test-report generation by the software is very important. “Various graphic presentations and scan table settings can be generated, and transducer descriptions and preamplifier settings must be saved. Operator comments regarding DUT behavior plus serial numbers of hardware devices and software versions will be very helpful in the analysis of test results. The typical report may consist of 50 or more pages with just a few having actual data,” he said.
Operator Interface
Software suppliers are searching for ways to help the equipment designer who knows a new product in incredible detail but is not comfortable with EMC test requirements. To this end, the typical test package runs under the Windows 95/98/NT operating system, offering a friendly operator interface with context-sensitive help and online documentation.
Running an immunity test is relatively easy; however, it does involve operator participation. Just how much will depend on the application. The EMI-GTEM-01 Test Software from EMC Automation can be configured for extremely simple operation, or the user can interact with the test process every step of the way.
To the consternation of test-software suppliers, some design engineers seem to think that an EMC test consists of pushing a button and watching the lights flash. It doesn’t work that way, of course. Mr. Janssen of Rohde & Schwarz recommends at least one day of intense hands-on training for the engineer performing precompliance testing and three to five days for one preparing to do compliance testing.
Amplifier Research recognizes the wide gap between inexperienced and sophisticated users and offers a package for the relative novice. At the other end of the experience spectrum, the company also provides a package for the sophisticated user.
Published by EE-Evaluation Engineering
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October 2000