The Case for Early Testing
by Tom Lecklider, Senior Technical Editor
Before an electronic product can be sold, it must comply with EMC standards. In general,
EMC specifications attempt to ensure that a product's
performance is compatible with its environment. Emitting signals
that could disrupt operation of nearby equipment
is not allowed. It's also important that
the product continues to operate correctly when subjected to
emissions from other equipment.
The various power levels and frequency bands
used to describe allowable performance are established in an
effort to achieve compatibility. It is not the intention that
there should be no EMI generated, but rather that it should be
minimized. Obviously, emissions that could disrupt
communications channels must be controlled, but there are many
other areas such as industrial control circuits where excessive
electrical noise also can cause problems.
Because of the well-documented large cost
increase accompanying fault correction late in the
design-manufacturing-distribution cycle, EMC problems must be
identified as soon as possible. Precompliance testing is a means
of finding EMC-related issues at the design prototype stage.
Although your design eventually will have to
pass compliance testing, that procedure is different from
precompliance testing. Both are based on the same national and
international standards, but compliance testing must be done
very accurately, on a properly prepared test site with equipment
that complies with CISPR specifications. In contrast,
precompliance testing is an investigatory process.
You don't know if the product will meet the
emissions or susceptibility limits so you need to find out.
Testing approximates compliance test conditions, but rather than
being accurate to 0.5 dB, it is intended to help you understand
why some performance areas are marginal. At which frequencies
and orientations is the product failing or close to failing?
Precompliance testing can be very interactive
and iterative given the test- fail-troubleshoot-redesign-retest
cycle inherent in prototype performance verification.
Accordingly, ease of use has a different emphasis for
precompliance test software than for compliance test
applications. Total automation is much less important for
precompliance work than having straightforward control of only
the necessary functionality.
Precompliance Software Programs
Precompliance testing must be reasonably
accurate but may be an infrequent activity in a small company.
Test setups often will be ad hoc arrangements made without the
benefit of a proper screened room, anechoic chamber, or open-air
test site (OATS). And, engineers may lack detailed familiarity
with test instruments as well as test specifications.
These are some of the factors Laplace
Instruments' products address. "Test-site calibration almost
invariably is the major source of measurement error," commented
David Mawdsley, Laplace managing director. "Because they need a
low-cost solution, engineers want a precompliance system that
can be used in parking lots, loading bays, a spare conference room, or
an odd corner of the lab. Of course, they have to deal with
ambient signal interference in such an environment."
The company's precompliance test software is
dedicated to the Laplace range of analyzers and, when used in
conjunction with a calibrated reference source, will
automatically measure the characteristics of any site. The
software applies appropriate correction factors to make the site
produce measurements approximating those that would be measured
in a 3-m OATS. In addition, ambient cancellation effective under
most conditions is provided.
The user selects the type of test being
conducted, but the software does not allow other detailed
inputs. Instead, it automatically sets variables such as
resolution bandwidth, frequency step, and sweep rate to ensure
that no measurement errors result from user inexperience or
simple mistakes.
Further, a TestDirector button accesses an
expert system to guide the user through the common CE directives
leading to selection of the appropriate standard to suit the
product and explanations of the test techniques and test
parameters. A step-by-step demo is available from Laplace
Instruments.1
Rohde & Schwarz (R&S) markets ES-SCAN, a
software tool suitable for all EMI tests prior to final
certification. It is a 32-b Windows application that facilitates
precompliance testing to commercial standards and works with the
company's ESPI3 and ESPI7 Test Receivers. Capabilities not
required, such as remote control for turntables, antenna masts,
or absorbing clamp slideways, are not included.
According to Karl-Heinz Weidner, product
manager for EMI and signal analysis test and measurement
products, "ES-SCAN provides quick and reliable recording,
evaluation, and documentation of disturbance voltage, power, and
radiation as well as storage of all test parameters and
measurement data. Capabilities are tailored to the needs of
development labs, and the user interface is clearly structured.
The software only offers functions that are needed for
precompliance or precertification testing."
A predefined library of standard limit lines,
transducer factors and settings, and scan tables simplifies test setup.
ES-SCAN supports frequency scans with up to 1,000,000
measurement points for each of two traces. A large frequency
range from 9 kHz to 7 GHz can be swept using two different detectors
simultaneously. For example, you could use the new CISPR-Average
(Amendment A1:2002) and RMS-Average (Amendment A2:2007)
detectors available from R&S (Figure 1).
Click here to view Figure 1
Figure 1. Prescan Display Showing Peak and Average Measurements With Marked Local Maxima
Courtesy of Rohde & Schwarz
The benefits of ES-SCAN include the
following:
• Ease of use and a short learning phase
stemming from a clear structure and simple user interface.
• Reduced test time and increased measurement
efficiency resulting from predefined standard measurement setups
and quick programming.
• Reliable test reproducibility based on
storage and management of all measured data, settings, and test
parameters.
• Flexible and quick test-report generation.
• A built-in operation guide that prompts
step-by-step procedures and avoids incorrect measurement
settings.
As a result of a management buyout late in
2006, Schaffner's Test Systems Division became Teseq. The new
company specializes in EMC test
solutions and has retained former Schaffner employees with
extensive EMC testing experience.
Emipak 3 Precompliance RF Emission Software
addresses both conducted and radiated forms. Based on the company's well-established full-compliance software, Emipak 3
streamlines emissions testing. You can choose among a number
of predefined tests that measure emissions relative to the
limits you have selected.
Through a series of simple forms, you can
customize test setups, input details of the EUT, select the
frequency range, and make detailed changes to the type of
detector, bandwidth, and dwell time. Failed points automatically
are retested, and you also can add or delete points to be
remeasured. The graphical and tabular data is formatted as a
report, but both graphical and textual elements can be copied to
a more detailed report written in Word.
Emipak 3 supports all Schaffner measuring
receivers and a large number of Advantest spectrum analyzers. If
you don't have the relevant Advantest or Schaffner instruments,
you can't use Emipak 3. Similarly, unless you have an R&S
receiver, ES-SCAN isn't for you, and likewise, Laplace software
requires Laplace instruments. Instead of these products, you may
need a more generic precompliance software tool that works with
a range of test equipment from different manufacturers.
The Model SW1006 Radiated Susceptibility,
Conducted Immunity, and Emissions Test Software from AR
Worldwide may be what you want. It was developed under the
National Instruments LabVIEW environment that provides access to
a library of more than 500 instrument drivers from 45 different
manufacturers. Chances are that this program is compatible with
your spectrum analyzer.
Capabilities include full compliance testing
to several standards as well as the flexibility to select test
parameters and change the test threshold for susceptibility
testing. In addition, closed-loop leveling can be performed. The
program is intended for precompliance emissions testing with a
spectrum analyzer and either a preamp or LISN.
Similar to SW1006, EMITest also is a full
compliance program that can be used for precompliance testing.
EMITest was developed and is used by CKC Laboratories, an EMC
test lab. According to the company website, because the software
is a key part of the lab's testing operations, revisions and
improvements are constantly being introduced by CKC programmers.
A long list of spectrum analyzers, signal generators,
turntables, antenna masts, and GPIB interface cards is
supported.
According to Randy Clark, an EMC engineer at
the company, "EMITest can gather data from the spectrum analyzer
in either tabular or graphical format. Using the built-in recall
functions, it's easy to configure a variety of common setups,
allowing quick identification of known compliance problems. A
list of frequencies can be added to the graphical data for easy
comparison of clock frequencies or known trouble frequencies.
"The graphical data also can be compared with
other data for A-B comparisons over a range of frequencies." He
explained, "This tool is especially helpful for seeing
deviations from ambient conditions or picking out EUT vs. host
emissions. Graphical data can be captured either automatically
for simple configurations or semi-automatically, allowing signal
manipulation while using the software to control the frequency
ranges being investigated. When comparing data to various spec
limits, the software can recalculate the data against a new spec
limit with just a few clicks of the mouse."
Although the recent change to a 6-GHz upper
frequency obviously affected EMC test hardware, EMITest was not.
Only a slight modification of the spec limit was needed. Using
the EMITest spec limit editor, it is as simple as editing an
Excel spreadsheet.
An extensive library of current test limits
is essential, and EMITest has predefined limits for all the
popular commercial emissions standards. A partial list includes
EN 55011, EN 55022, FCC Part 15, EN 61000-4-3, and MIL-STD-461.
Preprototype Testing
If EMC testing at the engineering prototype
stage is worthwhile, what about during design simulation? In
addition to circuit models used to determine basic operation,
the modeling process today includes thermal performance as well
as EMC characteristics. The usefulness of different types of
simulation depends on the level of detail you include in the
various models.
At low frequencies, circuits comprising
discrete components have almost ideal behavior. Raise the
frequency, and you start to see deviations caused by stray
capacitance and inductance not included in the original circuit
simulation description.
Crosstalk and other layout-dependent effects
also will become problems at higher frequencies in the actual
circuitry. More complex component models and accurate layout
information are necessary if software simulations are to exhibit
the kinds of second-order effects seen in physical products.
Similar reasoning applies to EMC simulation
software. EMI Analyst™ is a precompliance design tool
produced by EMI Software LLC. To determine a product's EMI
characteristics at the box level, calculations involve design
parameters such as nonideal circuit components, cabling
construction, wire routing and twisting, shield construction and
termination, grounding, circuit impedance and balance,
filtering, and field distributions and propagation.
EMI Analyst avoids repeating the detailed
analyses associated with these many parameters by working at a
much higher level. For example, engineers can use current and
voltage time-domain waveforms to define the noise source.
The program's designer and company founder
Steve Newson explained: "It's the voltage and current waveforms
generated in a circuit that produce conducted and radiated
emissions. We define the noise source by using these waveforms,
which represent information readily available during the circuit
design phase of the project."
Four elements affect a circuit's EMI
performance: the source, any filtering that may be applied, the
conductors connecting sections of circuitry, and the load at the
end of the cable. The software's built-in utilities help you
define circuit parasitics, design suitable filters, describe
wiring configurations, and calculate load impedances. An
integral schematic capture capability accurately models the EMI
effects caused by interaction of these factors.
The MicroStripes EMC analysis program from
Flomerics accepts simulation data from several sources. For
example, results can be based on a thermal model of the
enclosure design. In this case, as explained by Paul Duxbury, a
senior EM engineer at the company, "The simulation mimics a
shielding effectiveness or radiated emissions test in a lab by
calculating the electric field strength on a 3-m or 10-m radius
cylinder around the model.
"Simulation not only determines the amount of
radiation, but it also establishes the path by which it escapes
from the enclosure. In a typical example, the simulation might
show that most of the emissions escape through a particular
seam. Placing gaskets along this one seam would increase the
shielding effectiveness at a much lower cost than installing
gaskets throughout the entire enclosure," he concluded.
The behavior of complex sources is modeled by
the compact source interface (CSI). MicroStripes works with the
high-level data provided by CSI without the need to explicitly
describe the details of a PCB, antenna, or noise source. Compact
models also can be based on near-field data obtained from other
PCB simulation and measurement tools. For example, Flomerics is
collaborating with tool vendors such as SimLab and Sigrity to
provide data-translation utilities to support the use of field
data.
In contrast to other general-purpose EM
simulation software, MicroStripes uses the transmission line
matrix method of solving Maxwell's equations. Mr. Duxbury said,
"This method provides major advantages when performing EMC
simulations because it solves for all frequencies of interest in
a single calculation. As a result, it captures the full
broadband response of the system in one simulation cycle. This
characteristic is very useful in EMC analysis where potential
resonances and emissions vary over a wide frequency spectrum"
(Figure 2).
Figure 2. Emissions From a Noise Source Within an Electronics Enclosure With a Slotted Front Panel
Courtesy of Flomerics
Summary
Precompliance EMC testing really does have
objectives and constraints that are distinct from full
compliance testing. Testing should be accomplished as
economically as possible, but because precompliance tests may be
repeated several times, cost can become a major factor. Also,
test facilities typically will be very different for
precompliance testing. Unless a company has a large number of
products to test on a continuing basis, it can't justify
investing in a permanent EMC test facility and associated staff.
When deciding what software best suits your
situation, it's important to remember the reason that
precompliance tests are run. Will the proposed software deliver
the information you need to ensure that the product passes full
compliance testing on the first try? That's clearly the
objective, whether you test conventionally with a physical
prototype or via EMC simulation.
Reference
1. Laplace Instruments,
http://www.laplace.co.uk/ERSCAL/sitecorr.htm
| FOR MORE INFORMATION |
|
Click below |
| AR Worldwide |
SW1006 EMC Software |
Click here |
| CKC Laboratories |
EMITest EMC Software |
Click here |
| EMI Software |
EMI Analyst EMI Simulation Software |
Click here |
| Flomerics |
MicroStripes EMC Simulation Software |
Click here |
| Laplace Instruments |
EMCEngineer Precompliance Test Software |
Click here |
| Rohde & Schwarz |
ES-SCAN EMC Precompliance Software |
Click here |
| Teseq |
Emipak 3 EMC Precompliance Software |
Click here |