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Tabletop Shielded Enclosures
by George R. Lyman, ETS-Lindgren
T
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he space-saving table-top famIs the proposed enclosure
ily of shielded test enclosures
big enough to accommoTabletop enclosures
has gained popularity in recent years,
date the device under test
primarily because of the evolution of smaller elec(DUT) plus all of the necoffer portability and
tronic devices. Besides conserving space, these units
essary peripheral equipflexibility for
cost less than the traditional, full-size walk-in shielded
ment and interface penrooms and offer inherent flexibility and adaptability
etrations?
precompliance
in numerous applications.
This very basic quesThe tabletop shielded enclosure has evolved over
testing at a fraction
tion sometimes is overthe years into many different forms including racklooked, and unnecessary
of the cost of
mounted designs, automatic versions, and portable
testing delays can result
configurations. Tabletop enclosures are popular in
shielded rooms.
when everything doesn’t
the design and precompliance testing arenas and
fit. Consider the dimenwith small testing facilities or consultants who don’t
sions of the DUT, peripherals, and all internal
have the budget or space for a
penetrations such as feedfull-size shielded room.
throughs, filters, CCTV,
Most tabletop enclosures are used
microwave absorbers, and
for design and precompliance testelectrical outlets to detering, but many have found their
mine your overall space
way into the manufacturing envirequirement.
ronment as in-process quality-control tools. They also serve in nonOnce the overall size of the
testing applications, providing
tabletop enclosure is known,
shielding for secure communicacan it fit through the doors
tions and unmanned computing
of your building or test lab?
operations.
It sounds silly, but many
Regardless of the physical form
a door and jamb have
or function of the enclosure, a
been removed to provide
number of mechanical, electrical,
clearance for larger tableand environmental factors could
top enclosures. Take a few
apply to all of them. By reviewCopper Box Tabletop Shielded Enclosure
minutes with the tape meaing these important factors in the
sure. It’s worth taking the
beginning of your evaluation, you will have a better
time to make sure the pieces of the puzzle fit.
chance of specifying the best tool for your particular
testing requirement.
Can the enclosure be rolled into all intended test areas?
Mechanical Considerations
If the unit is a mobile design, it must be
The following mechanical considerations may
unencumbered by building structures, raised threshseem simplistic, but when overlooked, can create
olds, or other permanently fixed obstacles.
major headaches and undue testing delays.
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Is compressed air available throughout the testing area?
Some advanced enclosures use pneumatics. If
required, provide an adequate supply of compressed
air in all intended testing areas.
If the tabletop enclosure will be stationary, what physical arrangement will allow the easiest access?
Place special attention on door swing and door
location. In tight quarters, the best approach may be
to orient the door on the top of the enclosure.
Does the enclosure have to be physically secure?
These days, you have to give security some
thought, primarily to electrically and physically
protected communications or secure, unmanned,
computation applications. In some instances, the
physical-security aspect is addressed by a permanently fixed, heavy-duty safe-like structure with a
sophisticated means for shielded ventilation, filtered
power, and shielded electrical interconnection.
Do test procedures require that the DUT be manipulated in any way?
Some test procedures require that buttons or
switches on the DUT be actuated while under test.
A number of solutions can be offered, ranging from
very basic to very complex. The most cost-effective
approach uses a waveguide or series of waveguides
along with dielectric rods that traverse through the
waveguides and mechanically press a button or
actuate a switch.
In most cases, it is wise to use a fixturing system
that secures and precisely orients the DUT. This
helps to ensure a dependable, repeatable, mechanical actuation. Automated versions consisting of
elaborate electromechanical designs also are available and usually serve as in-process, quality assurance type tools. These applications normally are
custom designs, necessitating close interaction with
the manufacturer.
Electrical Considerations
While the tabletop shielded enclosures have many
mechanical components to consider, do not overlook
the electrical factors affecting design and performance, primarily including:
• The required shielding effectiveness of the enclosure.
• Electrical interconnection requirements.
• Power and signal filtration requirements.
Shielding Effectiveness and Materials of
Construction
Shielding effectiveness is influenced by a number
of items, including the basic materials of construc-
tion and the means by which the materials are
assembled. Shielding materials vary from manufacturer to manufacturer, but higher performing designs
use copper or a combination of copper and steel
alloys as the primary shielding barrier.
To assist in the attenuation of higher frequencies,
microwave absorber or ferrite tile can be applied to
the inside of the enclosure. The absorber size and
shape will be a function of the frequency or
frequency range targeted for attenuation. Once the
absorber is selected, consider the space this material
subtracts from the interior area and adjust the
enclosure size as necessary.
Superior materials of construction are important,
but your evaluation also should include a review of
the methods of construction. Higher performing
tabletop models use a fully soldered or welded
approach.
The perfect shielded enclosure provides a totally
welded, six-sided box. That would be an ideal
situation except for one detail: It would be a little
cumbersome to get your cutting torch out every
time you wanted to access your DUT. All kidding
aside, we now need to consider how you will access
the DUT while maintaining the shielding integrity
of the enclosure.
As in larger shielded rooms, a dependable door
system that allows repeated actuations while
maintaining a high level of shielding over time
is necessary. The shielded door normally is the
biggest aperture in the shield and a large
contributor to electrical performance or lack
thereof.
To maintain superior shielding over time, pick a
product that will continuously shield without the RF
seal taking a set. This induced set can create leaks,
compromising the shielding effectiveness of the
entire enclosure.
Some higher performing door systems use a
beryllium copper or phosphor bronze fingerstock
shielding system. Other designs have combination
seals consisting of neoprene or silicone elastomer
with wire mesh.
Whatever the seal design, it is critical to maintain
repeatable, dependable shielding performance over
time. Always pay particular attention to the door
system and the associated EMI shielding incorporated into that system.
Getting Connected
There are myriad interconnection options. Filtered
connectors normally are used, and they offer a wide
range of signal attenuation. A thorough review of
their performance levels will help you determine the
best connector selection for your application.
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To maximize shielding effectiveness, minimize
penetrations into the enclosure. By minimizing the
number of connectors in a connector panel, you can
help reduce leakage levels.
Ask yourself what connectors are absolutely necessary vs. the overall enclosure shielding performance required. For example, your new enclosure
by itself may provide 120 dB of attenuation at 1
GHz, but the addition of a filtered, nine-pin Dsubconnector might pull it down to 80 dB. The
connector now has become the weak link. If higher
performance is mandated, then a more creative,
high-performing interconnection must be introduced.
One popular, higher performing interconnection
uses fiber-optic modems in conjunction with highperformance waveguide feedthroughs to transfer data
through the shield. This advanced approach is more
expensive, but it will help achieve higher performance. You can help maximize interconnection
shielding effectiveness by:
• Minimizing the number of connectors per panel.
• Specifying connector types with maximum shielding.
• Covering idle connectors with metallic caps.
• Considering higher performance fiber-optic-based
interconnects.
You’re Grounded!
Protect your health and property by properly
grounding your tabletop enclosure. Proper grounding
is absolutely necessary when connecting AC to the
tabletop enclosure. A solid ground also eliminates
the possibility of circulating RF currents that could
radiate RF energy, which compromises the shielding
integrity of the enclosure.
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Shielded Window
The shielded window is a common approach to
viewing, but your overall shielding effectiveness
now is predominantly determined by the shielding
performance of the window. High-performance shielded
windows provide 50 dB to 80 dB of attenuation at 1
GHz, and these different levels of attenuation are a
direct function of the open area of the shielding
mesh.
If higher shielding is the priority, then this will
equate to a window that transmits less light.
Conversely, if better visibility is paramount, then
electrical performance will drop.
So consider what is more important to you:
visibility or shielding effectiveness. Also, depending
on what is being viewed, you may want to consider
illuminating the inside of the enclosure to improve
viewability.
In the final analysis, it comes down to the
shielded window’s electrical spec vs. your overall
shielding requirement. If the delta is too dramatic,
you may want to rethink incorporating a window.
Machined Perforations
Another basic viewing method uses a series of
machined perforations in a ferrite-tile panel affixed
to the enclosure door or wall. This perforated
window is more of a waveguide air vent that
happens to offer limited visibility.
Closed-Circuit TV
The third approach uses closed-circuit TV that
feeds images back to an exterior monitor via fiber
optics and a waveguide feedthrough. Although more
costly, the net effect will be a clearer image and
higher overall shielding effectiveness.
Power and Signal Filtering
To prevent unacceptable levels of conducted
EMI from entering or leaving the tabletop enclosure, add EMI power or signal filters. These lowpass devices attenuate unwanted EMI from conducted power and signal lines and are available in
many standard and custom configurations.
Discuss your power and signal needs with the
enclosure manufacturer. It is better to have the
filters installed during enclosure assembly rather
than retrofitting and cutting holes in the side of a
shielded tabletop enclosure as an afterthought.
Room With a View
Viewing the interior of the test enclosure can be
achieved by several methods that offer different
levels of visibility and shielding performance. The
methods include the following:
Environmental Factors
While most tabletop enclosures are used in an
ambient laboratory environment, others are subjected to more challenging conditions. Applications
that intentionally or unintentionally introduce humidity should be avoided.
The net result of humidity is condensation, which
creates oxidation and adversely affects key components such as the connectors and the door shielding
system. Keep the enclosure in a stable, dry environment to minimize the potential for oxidation.
When considering thermal factors, look at them in
two different ways: intentional introduction of temperature to the DUT for thermal evaluation purposes
and venting of heat produced by the DUT.
The first scenario is more complex because the
design must take into consideration the expansion
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and contraction of the materials of construction. A
slight movement of the shielding materials can
cause apertures that could degrade the shielding
effectiveness of the enclosure. Areas of particular
concern are the door and the venting locations.
The combination of EMI and thermal testing
usually involves inserting a tabletop shielded enclosure inside a thermal chamber. These applications
are extremely custom in nature and really help to
push the envelope regarding shielding system design. While not an impossible scenario, give careful
thought before marrying thermal and EMI testing
into one hybrid.
The venting of heat produced by the DUT is an
easier proposition. Basically, it requires that a
sufficient amount of dilution air be drawn or forced
through the tabletop shielded enclosure.
In either case, the passage of air through an
enclosure requires shielded waveguide air vents.
Two waveguide air-vent panels of the same size,
constructed from steel or brass honeycomb media,
usually are used.
While the vents can be attached in a variety of
ways, the best electrical performance occurs when
the waveguide air-vent assembly is completely
soldered to the metal skin of the enclosure. It also
is wise to make sure the waveguide air vent is
conductive in all planes. Since the vent most
probably will be the second largest aperture in your
tabletop, you want to make sure that a premium
vent is in place.
A waveguide air vent that is 100% solder fused
will guarantee maximum conductivity and offer
superior electrical performance. When specifying
waveguide air vents, factor in the pressure drop of
the venting media so that the static pressure can be
figured for proper exhaust-fan sizing.
Conclusion
The advent of the tabletop shielded enclosure has
created a world of portable and flexible testing
capabilities. The tabletop shielded enclosure has
proven to be a handy time saver for many test
professionals, and its convenient size and inherent
flexibility make this genre of shielded enclosures an
attractive solution for those who need to save time,
space, and money.
About the Author
George Lyman is the industrial business development manager for ETS-Lindgren. He has more than 20
years in EMI shielding in national sales and marketing
management, product management, and international
business development positions. ETS-Lindgren, 400
High Grove Blvd., Glendale Heights, IL 60139, P 630307-7200, F 630-307-7571, e-mail: GRLyman@ETSLindgren.com
Reprinted from EE-Evaluation Engineering, April 2003
Copyright © 2003 by Nelson Publishing Inc. • www.evaluationengineering.com