[Design Application] Conduct EMC Testing Economically In-House It Takes Some Specialized Knowledge And Test Equipment, But OEMs Can Do Much Of The EMI And Susceptibility Testing Needed To Meet EMC Regulations. Contributing Author ED Online ID #7631 |
May 13, 1998
To succeed in industry these days, most manufacturers have to
maintain a strong position in the international marketplace. In Europe,
that means products must qualify for the CE mark, which ensures compliance
with all applicable European Union directives on electromagnetic compatibility
(EMC). Designing for this stringent specification, which has been mandatory
since January 1, 1996, requires knowledge about both design practices
and test methods.
This article discusses equipment and methods electronics manufacturers
can use to qualify their products. Included is an example of how Symbol
Technologies, Holtsville, N.Y., tests its products, along with the practices
the company uses to solve emissions problems. Several problems with test
setup and configuration are also discussed. Finally, we will cover some
possible surprises that engineers should be aware of when designing a
product for EMI compliance.
From gun-shaped bar code scanners to hand-held PCs and wireless LANs,
Symbol's products all have one thing in common: electronics. More specifically,
they all incorporate embedded digital systems surrounded by analog circuitry.
Thus, the products fall into the category of information technology equipment
(ITE). The applicable standards include EN 55022 for radiated emissions
and EN 61000-4-3 for radiated susceptibility. EN 55022 requires the use
of an open area test site (OATS), a gigahertz transelectromagnetic (GTEM)
cell, an anechoic chamber, or other alternate test setup to perform electromagnetic
interference (EMI) testing.
For a number of reasons, Symbol chose to purchase a GTEM cell rather
than build an OATS. A GTEM cell is immune to ambient noise conditions,
and tests take up to 90% less time to run, making the system easier and
faster to use. GTEM cells also perform susceptibility testing, something
an OATS cannot accomplish. The equipment under test (EUT) fits inside
the cell, a convenience not all companies have. Finally, the cost of GTEM
cells versus an OATS is comparable, around $225,000, including the test
equipment. Therefore, there's much more value for the dollar with a GTEM
cell.
Despite all the advantages of a GTEM cell, Symbol is still considering
the purchase of an OATS. One large disadvantage of a GTEM cell is its
frequency limit. An OATS can measure beyond 25 GHz, while a GTEM cell
peaks at 5 GHz. For most companies, this limit is fine. However, Symbol
manufactures radio products that operate in the 2.4 GHz band, and approval
of these radios requires testing up to the 10th harmonic.
The company's GTEM setup consists of an HP 8593E spectrum analyzer with
a quasi-peak detector card, an Emco Boss manipulator, an HP 8648A
RF signal generator, an IFI SMX-100 RF amplifier, and a PC. The entire
setup is controlled by GTEM software running on the PC via the IEEE-488
(GPIB) interface (Fig. 1 ). The signal generator and
amplifier are used for susceptibility testing and the spectrum analyzer
is used for radiated emissions testing.
To perform an emissions test properly, the product (usually in prototype
plastics) is securely mounted on the manipulator table with all the appropriate
cables attached (Fig. 2 ). The test is run with the unit
operated in a user-intended mode. For a bar code scanner, this means continuously
scanning a bar code and transmitting the data to a host. To accomplish
this, a pneumatic actuator pushes the trigger on the scanner, whose exit
window aims at a bar code mounted a specified distance away. The power-supply
cable plugs into an EMI-filtered ac outlet inside the chamber, and the
data cable attaches to a host through a filtered or isolated connector.
Before the emissions test begins, the engineer enters specific standards
information into the system's test software. This software is typically
purchased from the manufacturer of the GTEM cell.
At the start of the testing, the manipulator rotates the unit to a 45°
azimuth, 120° orthogonal angle to the ground plane inside the GTEM.
The spectrum analyzer then sweeps the frequency range. When completed,
the manipulator moves to a 45° azimuth, 0° orthogonal angle,
and repeats the test. Finally, the manipulator rotates to a 45° azimuth,
-120° orthogonal angle for the final radiated measurements. These
positions, which are controlled by the software, are devised by the manufacturer
of the GTEM cell to produce the desired test results.
The GTEM software then performs calculations that correlate the data
to produce a plot of radiated emissions. The software uses many parameters
during its correlation: the height of the EUT's center from the ground
plane, the distance from the GTEM antenna to the EUT, the separation between
the ground plane and the septum, and the distance between the EUT and
the septum, just to name a few. By locating the EUT in these three positions
and performing the calculations, Symbol has consistently shown a correlation
between its GTEM and an OATS. The company can and does certify non-RF
products in-house with this setup.
A good example of EMC design and test procedures is a project currently
in the works: a cordless scanner that uses a base station for charging
and data transfer. Regulations insist that a product be tested under worst-case
conditions. One such configuration consists of the scanner mounted in
the base, charging and transferring data. Running an emissions test on
this system checks two interconnected microprocessors that transfer data
between each other and a host. Charging adds to the emissions because
the charging circuitry inside the product uses a switch-mode power supply.
In order to obtain the worst case, a completely dead battery is used to
draw the most charging current.
Symbol wants to certify the new scanner to the CISPR B standard in Europe.
Although things could have been worse, a graph of the results for
the initial prototypes of this product shows that the system did fail
to meet the CISPR B limit at a few frequencies.
<-- prev. page
[1]
2
3
next page -->
Reprints
