U.S. patent application number 09/784849 was filed with the patent office on 2001-12-13 for electromagnetic radiation shield for attenuating electromagnetic radiation from an active electronic device.
Invention is credited to Pulver, Lee J..
Application Number | 20010050175 09/784849 |
Document ID | / |
Family ID | 25392226 |
Filed Date | 2001-12-13 |
United States Patent
Application |
20010050175 |
Kind Code |
A1 |
Pulver, Lee J. |
December 13, 2001 |
Electromagnetic radiation shield for attenuating electromagnetic
radiation from an active electronic device
Abstract
A shield for attenuating the strength and density of
electromagnetic radiation emitted by electronic components
comprising two parallel conductors separated by an insulator is
disclosed. The shields are installed on the surface of electronic
components by means of an adhesive.
Inventors: |
Pulver, Lee J.; (Los Gatos,
CA) |
Correspondence
Address: |
LYON & LYON LLP
633 WEST FIFTH STREET
SUITE 4700
LOS ANGELES
CA
90071
US
|
Family ID: |
25392226 |
Appl. No.: |
09/784849 |
Filed: |
February 15, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09784849 |
Feb 15, 2001 |
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08887959 |
Jul 3, 1997 |
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Current U.S.
Class: |
174/32 ;
257/E23.114 |
Current CPC
Class: |
Y10T 428/12535 20150115;
H01L 2924/0002 20130101; H01L 2924/0002 20130101; H05K 9/0088
20130101; H01L 2924/00 20130101; Y10T 428/12528 20150115; H01L
23/552 20130101 |
Class at
Publication: |
174/32 |
International
Class: |
H05K 009/00 |
Claims
I claim:
1. An shield for attenuating electromagnetic radiation comprising:
a first conductive layer; a second conductive layer; and an
insulating layer disposed between said first conductive layer and
said second conductive layer.
2. The shield of claim 1 wherein said first conductive layer
comprises aluminum alloy.
3. The shield of claim 1 wherein said second conductive layer
comprises aluminum alloy.
4. The shield of claim 1 wherein said insulating layer comprises an
adhesive which acts as a semiconductor.
5. The shield of claim 4 wherein said adhesive comprises
paraffin-based glue.
6. The shield of claim 5 wherein said insulating layer further
comprises a fibrous material.
7. The shield of claim 6 wherein said fibrous material comprises
sixty pound paper.
8. The shield of claim 6 wherein said fibrous material comprises
one hundred twenty pound paper.
9. The shield of claim 6 wherein said insulating layer comprises a
first layer of paraffin-based glue, a second layer of fibrous
material, and a third layer of paraffin-based glue.
10. The shield of claim 9 wherein said first conductive layer is
affixed to said insulating layer by said first layer of
paraffin-based glue and wherein said second conductive layer is
affixed to said insulating layer by said second layer of
paraffin-based glue.
Description
SPECIFICATION
[0001] 1. Field of the Invention
[0002] The present invention relates in general to reduction of
stray electromagnetic radiation emissions produced by electronic
systems and more specifically to apparatus for shielding stray
electromagnetic radiation.
[0003] 2. Background of the Invention
[0004] Electronic systems (e.g., electrical circuits, etc.) which
operate on alternating current ("AC") or direct current ("DC")
voltage generate an electromagnetic field. An electromagnetic field
is comprised of waves. Such fields are created because of the
electrical energy being used in the devices. In most countries,
agencies like the Federal Communications Commission (FCC) in the
United States, Industry Canada (ICAN) in Canada, and Bundesamt Fur
Post Und Telekommunikation-Zulassen Und Testen (BZT) in Germany
regulate maximum amplitudes for radiated and conducted
electromagnetic energy permissible at specific frequencies.
[0005] Unintentional radiators include electronic systems such as
personal computers and computer networking hardware. Intentional
radiators include electronic systems such as transmitters and
regenerative receivers. This is by no means an exhaustive list of
unintentional and intentional radiators, as various other
electronic systems produce unwanted stray electromagnetic radiation
emissions. Such unwanted electromagnetic radiation includes radio
frequency, electrostatic and magnetic fields. As the operating
speeds of these electronic systems increase, the strength of the
electromagnetic fields increase. For example, present personal
computers can operate at speeds above two hundred megahertz
("MHz"). These operating speeds will only increase in the
future.
[0006] Stray electromagnetic fields are highly undesirable, as they
can cause interference with other electronic devices, radio
frequency communications systems, etc. Because of this,
governmental agencies of many agencies have issued regulations
which specify the maximum amplitude of stray unwanted
electromagnetic radiation that an electronic system can emit and
still be used in that country. If a product does not meet these
specifications, it cannot be sold in that country.
[0007] Each of the government agencies has specific testing
methodologies. In general, the stray electromagnetic radiation of
an electronic system is measured using an antenna placed at least
three meters from an electronic system on a calibrated test site.
The field strength per meter of the stray fields is measured. In
general, stray fields have strength with an order of magnitude in
the one hundred microvolts range when the frequency is between
thirty MHz and eighty-eight MHz. When recorded field strength
exceeds the limit imposed by agencies such as the FCC, the
electronic system must be redesigned so that it complies with those
specifications. Electronic system redesign and implementation into
production can delay product introduction by several months.
[0008] At present, a common method to reduce the stray
electromagnetic energy produced by an electronic system is to
encase the electronic system in a continuous metal enclosure. If
there are no seams in an enclosure, then electromagnetic energy is
contained since a Faraday Cage has essentially been built around
the electromagnetic energy source. When designing such an
enclosure, the amplitude and frequency of the stray electromagnetic
field determines its thickness. Regardless of the thickness,
however, the shielding properties of the enclosure are compromised
if system being shielded has any input/output ports, data cables,
displays, etc. that cannot be shielded. Because of this, additional
shielding must usually be added to the enclosures, cables,
displays, keyboards, etc. to assure compliance to the FCC
recommendations. These additional technologies add significant
product cost and development time, and detract from the cosmetics
of a final product.
[0009] Summarizing, while effective, these enclosures are heavy,
add cost, and reduce the appeal of the product, especially if it is
intended for the consumer market. Furthermore, it is entirely
possible that this shielding might not even be necessary. Thus, a
product could be designed with such shielding, thereby greatly
adding to its cost, when the shielding was not even necessary.
[0010] In addition, in an attempt to make products that are more
attractive and/or less expensive, the electronics industry also
uses plastic enclosures. If an electronic system operating inside a
plastic enclosure exceeds electromagnetic interference limits
allowed by the regulatory agency, a metallized surface may be
applied to the plastic enclosure in attempt to transform the
interior of the plastic enclosure into a metal enclosure. The cost
of this technology can be prohibitive, however, because the top and
bottom sections of the enclosure must exhibit electrical continuity
at the seams, while maintaining a uniform thickness over the entire
enclosure surface. This is difficult to achieve.
[0011] Another common method for reducing stray electromagnetic
radiation is to change the location of components in an electronic
system. For example, a designer might change the location of the
system's crystal oscillator to place it closer to the circuit
receiving the signal. This will reduce the length of the signal
path (i.e., the connecting wire or printed circuit board trace) and
therefore might reduce the stray electromagnetic radiation emitted.
The problem with this particular method is that it requires a
redesign of the electronic system. This increases the cost of the
product and significantly delays the product from entering the
market. In addition, there is no way of knowing if the redesign
actually worked until it is retested. When it is retested, the
redesigned product may still exceed the specifications for maximum
stray electromagnetic radiation. Many times, a third redesign must
be undertaken. This costs time and money.
[0012] Examples of the specifications for the maximum stray
electromagnetic radiation that a product must satisfy to be sold in
the United States include:
1 dB microvolt/meter FREQUENCY (MHz) (dB .mu.V/m) microvolt/meter
(.mu.V/m) 30 to 88 40.00 100 88 to 216 43.50 150 216-960 46.00 200
960-1000 54.00 500
[0013] These specifications, which have been issued by the FCC as
of the filing date of this application, are measured from a
position three meters from the electronic system undergoing
test.
[0014] Examples of specifications that are in force in Europe
include:
2 dB microvolt/meter FREQUENCY (MHz) (dB .mu.V/m) microvolt/meter
(.mu.V/m) 30-230 40.46 105.44 230-1000 47.46 236.05
[0015] These are also measured at a distance of three meters from
the electronic system undergoing test. These three meter limits are
mandated in the specification EW 55022/CISPR 22, which is entitled
"Limits and Methods of Measurement of Radio Disturbance
Characteristics of Information Technology Equipment."
[0016] Furthermore, according to the newest European criteria, many
products must continue to operate and be less susceptible to a
field of three volts per meter from twenty-seven MHz to five
hundred MHz to be allowed entry into the marketplace for sale. This
is known as susceptibility. These criteria for susceptibility is
best represented by the following product standards:
[0017] IEC 801-2 Electromagnetic Compatibility for
Industrial-Process Measurement and Control Equipment. Part 2:
Electrostatic Discharge Requirements. 1984
[0018] IEC 801-3 Electromagnetic Compatibility for Industrial
Process Measurement and Control Equipment. Part 3: Radiated
Electromagnetic Field Requirements. 1984
[0019] IEC 801-4 Electromagnetic Compatibility for Industrial
Process Measurement and Control Equipment. Part 4: Electrical Fast
Transient. 1988.
[0020] An example of an electronic system that is particularly
prone to producing stray fields is a laptop computer system. Prior
to being marketed for sale or being sold, a laptop computer system
must be tested by a FCC registered testing laboratory. The product
must comply with the Code of Federal Regulations, Title 47, Part 2
entitled Frequency Allocations and Radio Treaty Matters; General
Rules and Regulations; and Part 15 entitled Radio Frequency
Devices, Oct. 1, 1996 edition. If the product does not pass this
testing, it cannot be marketed for sale or sold. Laptop computer
systems represent a fast paced electronic system technology that
must be introduced into the market quickly. If the laptop computer
system fails the testing described above, it must be redesigned
prior to sale, thereby resulting in delay. Delay in releasing the
product to the market can result in obsolescence before the first
unit is sold. The testing and redesign stage of product compliance
can sometimes be the critical path for product or company
success
[0021] Compounding the above-described problems is that when
designing electronic systems, it is difficult to predict the type
and amount of stray fields that will be created; Because of this,
designers either design their product without these radiation
reducing features and accept the risk of product delays should the
product fail testing, or implement potentially unnecessary field
control structures (e.g., a metal enclosure). However, product
delays can be fatal for many products, as time-to-market is
extremely important in the modem marketplace. This is especially
true in the electronics market. For example, technology and
consumer tastes for personal computers generally change within a
matter of months. If a product must be redesigned before it can be
sold, it probably will never be sold. On the other hand,
unnecessary field control structures increase the product's cost,
thereby increasing the chance for the product's failure in the
marketplace.
[0022] Thus, there has been a long felt need for an inexpensive
apparatus that reduces stray electromagnetic radiation of products
without requiring a product redesign or heavy shielding.
SUMMARY OF THE INVENTION
[0023] The present invention overcomes the problems and
disadvantages of the prior art through a unique electromagnetic
radiation shield. A shield or tab is disclosed. The shield of the
present invention comprises a first conductive layer and a second
conductive layer separated by an insulator. In a preferred
embodiment, the first conductive layer and second conductive layer
comprise a flexible aluminum alloy. The insulating layer of the
preferred embodiment comprises a fibrous material that is bonded to
the first conductive layer and second conductive layer by
paraffin-based glue. The second conductive layer has an adhesive on
the exterior thereof for affixing the shield or tab to a component
that emits unwanted electromagnetic radiation. If desired, the
first conductive layer can have a material applied to the exterior
thereof that allows printing images thereon.
[0024] The various embodiments of the present invention are
installed on the surface of electronic devices that are emitting
unwanted electromagnetic radiation. If during testing, a product
emits unacceptable amplitudes and/or frequencies of eletromagnetic
radiation, measurements can be taken in the vicinity of the
electronic components installed therein. The shields of the present
invention can be installed on the surface of selected electronic
components of the apparatus in the field. Then the product can be
immediately retested.
[0025] Selection of the electronic components that will have the
shields of the present invention installed therein varies from
apparatus to apparatus. In some applications, the shields can be
placed only on those components exhibiting unacceptable
electromagnetic radiation performance. In other cases, the shields
of the present invention will be placed on several components of
the apparatus.
[0026] The above and other preferred features of the invention,
including various novel details of implementation and combination
of elements will now be more particularly described with reference
to the accompanying drawings and pointed out in the claims. It will
be understood that the particular methods and circuits embodying
the invention are shown by way of illustration only and not as
limitations of the invention. As will be understood by those
skilled in the art, the principles and features of this invention
may be employed in various and numerous embodiments without
departing from the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] Reference is made to the accompanying drawings in which are
shown illustrative embodiments of aspects of the invention, from
which novel features and advantages will be apparent.
[0028] FIG. 1 is a perspective, section view of a stray
electromagnetic shield of the present invention.
[0029] FIG. 2 is a perspective view of a stray electromagnetic
shield of the present invention being affixed to an electronic
device.
[0030] FIG. 3 is a perspective, section view of a presently
preferred stray electromagnetic shield of the present invention
constructed in accordance with the teachings of the present
invention.
[0031] FIG. 4 is a simplified drawing showing an effect of the
stray electromagnetic shield of the present invention.
[0032] FIG. 5 is a simplified drawing showing an effect of the
stray electromagnetic shield of the present invention.
[0033] FIG. 6 is a simplified drawing showing an effect of the
stray electromagnetic shield of the present invention.
[0034] FIG. 7 is a simplified drawing showing an effect of the
stray electromagnetic shield of the present invention.
[0035] FIG. 8 is a simplified drawing showing an effect of the
stray electromagnetic shield of the present invention.
[0036] FIG. 9 is a simplified drawing showing an effect of the
stray electromagnetic shield of the present invention.
[0037] FIG. 10 shows an example of a printed circuit board having
electromagnetic shields of the present invention installed on
several of the electronic components located thereon.
[0038] FIG. 11 shows an example of a printed circuit board having
electromagnetic shields of the present invention installed on
several of the electronic components located thereon and the field
created by the installation of the shields.
[0039] FIG. 12 is a simplified drawing showing an effect of the
stray electromagnetic shield of the present invention when in the
vicinity of electronic components located thereon.
[0040] FIG. 13 is a simplified perspective drawing showing a shield
of the present invention installed in a curvilinear fashion.
[0041] FIG. 14 is a simplified perspective drawing showing a shield
of the present invention installed in a curvilinear fashion.
DETAILED DESCRIPTION OF THE DRAWINGS
[0042] Turning to the figures, the presently preferred apparatus
and methods of the present invention will now be described.
[0043] With reference to FIG. 1, sectional view of an exemplary
shield 10 of the present invention is shown. The shield 10 of the
present invention is comprised of a first conductive layer 15, an
insulating layer 20, and a second conductive layer 25. In use, the
shield 10 is affixed to the top of a package of an electronic
component 30, as shown in FIG. 2 and as will be described below.
Such a component could be an integrated circuit (e.g., a
microprocessor), an oscillator, a power supply, etc.
[0044] With reference to FIG. 3, a sectional view of a presently
preferred embodiment 100 of the present invention is shown. The
topmost layer 105 of the shield 100 can comprise a polyester
material which can have information printed thereon. This topmost
layer 105 can also be a lacquer type material facing outward toward
the operating environment. Information can be printed on the shield
with any type of ink compatible with the material used on the
topmost layer. This information could display, for example,
identifying information such as the part number and manufacturing
location of the component which will have the shield affixed
thereto. In the presently preferred embodiment, layer 105 comprises
a one hundred eighty degrees Centigrade, 0.0102 millimeter thick,
gold-colored liquid lacquer coating that is applied directly to the
first conductive layer 110 (see discussion of first conductive
layer 110 below). This material is chosen because ink can be
applied thereon during a high speed printing process without
smearing. It is noted that powder-type lacquers will not provide
satisfactory results.
[0045] Below the outermost layer 105 is a first conductive layer
110. First conductive layer 110 can be constructed of any material
that can conduct the electromagnetic field radiated by the
electronic device which has the shield 100 mounted thereupon. In
the presently preferred embodiment, the conductive layer 110
comprises a 0.0254 mm thick aluminum alloy comprised of 99.51%
aluminum, 0.35% silicon, 0.60% iron (as fume), 0.10% copper (as
fume), 0.05% manganese (as fume), 0.05% magnesium, 0.01% chromium,
0.10% zinc (as fume), and 0.03% titanium. Such a layer can be
obtained from the Consolidated Aluminum Corporation (Part Nos.
1050, 1100, 1145, 1200, 1235, 1250, 1270, 1285, and 1350) or the
Alcan Aluminum Corp. (Part Nos. 1025 to 1500).
[0046] Aluminum alloy is used in the presently preferred embodiment
for the following reasons. First, it has low cost. Second, it can
be purchased in large sheets. For example, in a presently preferred
method of manufacturing the present invention, these sheets have
dimensions of twenty-eight inches wide by one thousand feet long.
Third, it is flexible. Fourth, it is highly resistant to
environmental conditions typically seen by electronic components.
Fifth, it can be easily cut to specific sizes. Sixth, aluminum
alloy has performance characteristics that are well suited for this
application. Specifically, a typical stray electromagnetic field
that exceeds the specifications discussed below radiates when the
circuit is operating above thirty MHz. The aluminum alloy used in
the presently preferred embodiment is conductive to stray fields at
frequencies above approximately thirty MHz. It is noted that other
conductive materials may be used. The only limiting factor is that
the first conductive layer must conduct electromagnetic fields.
[0047] In a presently preferred embodiment, the insulating layer 20
is actually comprised of three different layers 115, 120, 125.
First insulating layer 115 is comprised of a semiconductor
material. In a preferred embodiment, this semiconductor material is
paraffin-based resin glue. This material is chosen because it is
possible to align the molecules of the semiconductor material such
that when it is exposed to an electromagnetic field, the charge of
the dielectric will increase, which will increase the electrostatic
potential of the electromagnetic radiation between the conductive
plates 110, 130. The molecules of this material are aligned as
follows: During the manufacturing process, the paraffin-based
material is subjected to magnetic field while the material is in a
semi-fluid state. When the material solidifies, the molecular
structure tends to be aligned and corresponds to the previous
polarized magnetic field. Second insulating layer 120 is comprised
of a fibrous material. In presently preferred embodiments, this
fibrous material can be sixty-pound paper, one hundred twenty-pound
paper, or ten-point paperboard. Third insulating layer 125 is
comprised of the same material as the first insulating layer
115.
[0048] It is important to note that the insulating layer 20 can be
comprised of materials other than those described. The purpose of
the insulating layer is to electrically isolate the first
conductive layer 15 from the second conductive layer 25. Any
material that is capable of doing this can be used for the
insulating layer 20.
[0049] Below insulating layers 115, 120, 125 is second conductive
layer 130. Second conductive layer 130 is preferably comprised of
the same material as first conductive layer 110. Below the second
conductive layer 130 is an outer layer 135. Outer layer 135 is
preferably constructed with the same materials as outermost layer
105. Below outer layer 135 is an adhesive layer 140. Adhesive layer
140 is used to affix the shield 100 to the electronic component
upon which the shield is mounted. Adhesive layer 140 is preferably
comprised of model no. 966 adhesive available from the Minnesota
Mining & Manufacturing Corporation. However, any adhesive that
is not water-soluble, has high resistance to heat, and will
maintain adhesion over the life of the product would provide
satisfactory performance. Furthermore, the surface energy of the
electronic component surface must be compatible with the adhesive
characteristics of layer 140. A peel-away layer (not shown) covers
the adhesive layer 140 until it is time to install the shield 100
on the electronic component. When it is desired to install the
shield 100 of the component, the peel-away layer is removed from
the adhesive layer 140 and discarded.
[0050] The method of manufacturing the presently preferred shield
100 will now be discussed. The shield is constructed using sheets
of the materials that comprise layers 105, 110, 115, 120, 125, 130,
135, 140, which result in the manufacturing of strataform panels.
The desired size of the strataform panels is determined and the
individual sheets of the materials that form each layer are cut to
the selected size. For example, as discussed, the various layers
can be purchased in rolls of 1000 feet by 28 inches. These rolls
are cut into press sheets depending on the nominal size of the
shield. Press sheets could be eight and one-half by eleven inches,
for instance, or ten by twelve inches, depending on how many
shields can be extracted from each sheet. The layers are placed
together, back-to-back, with the paraffin-based glue which will
form the first insulating layer 115 and second insulating layer
spread evenly over the interior portions of the aluminum alloy
sheet forms. The sandwich is laminated using any typical laminating
machine or laminating station. The purpose of the laminator is
merely to combine the sheet material into a strataform panel
containing multiple layers. It is noted that if outermost layer 105
and outer layer 135 do not comprise the polyester material
discussed above, it can be applied by merely spraying the surface
of the outer portion of the sheet forms which will form the first
conductive layer 110 and second conductive layer 130 with lacquer,
resin or PTFE (such as Teflon.RTM. brand PTFE). The result of this
process is that strataform panels are created.
[0051] An adhesive sheet is then placed on the side of the
strataform panel opposite the printed surface of the outermost
layer 105. This adhesive sheet has adhesive on both sides. One side
attaches to the outer layer 135 while the other side is used when
installing the shield 100 onto an electronic component. As
discussed above, the finished shield product has a peel-away-layer
on the adhesive that is removed at the time of installation. The
final product with the polyester on one side and the adhesive on
the other is in sheet form
[0052] After the strataform panels are created, artwork is printed
onto the topmost layer 105. The artwork includes the actual shield
outline and information that will remain on the shield surface
(e.g., part number and manufacturer's name). In the presently
preferred method, the artwork will be silk screened onto the
surface. A laminate (not shown in the Figures) can be placed on the
printed surface after the printing is completed. A typical laminate
would be polyester film, measuring 0.001 in. This film virtually
eliminates the possibility of the artwork being scratched. Although
used primarily for cosmetics, such a laminate also contributes to
the reduction of unwanted electromagnetic emissions. This is due to
the dielectric properties of the polyester film. A laminator known
to those skilled in the art is used to place this polyester film on
the surface.
[0053] The sheet forms are cut into the actual shields 100 by using
either a steel rule die or Class A die mounted in a clam shell
press. Cutting of the shields 100 from the sheets is a critical
step, as they must be cut in a fashion that will prevent the first
conductive layer 110 from being in physical and electrical contact
with second conductive layer 130. This is important because the
first and second conductive layers 110, 130 interact with each
other to either absorb or reflect the stray electromagnetic fields
(as will be discussed below). If the first and second conductive
layers 110, 130 were to contact each other, a short circuit would
be created, which would prevent the shield 100 from working
properly. In the presently preferred method of cutting the shields
100 from the sheet, a clam shell press is used. Such a press
ensures that the first conductive layer 110 and the second
conductive layer do not physically contact each other.
[0054] The shape of each shield 100 is important to its
performance. Preferably, the shield will not have any corners.
However, the shield can have any shape and provide satisfactory
performance. It can be oval, rectangular, curvilinear and
circular.
[0055] With reference to FIGS. 4-8, the manner in which shield 10
attenuates unwanted electromagnetic emissions will be discussed.
Shield 10 actually induces several different mechanisms for
collapsing the stray electromagnetic field created by an electronic
device. It is noted that in FIGS. 4-8, the dielectric layer 20 is
not shown, while the first conductive layer 15, the second
conductive layer 25, and the electronic device 30 are shown
separated from each other. This is done for illustrative purposes
only and is not intended to limit the scope of the invention. The
first and most basic of the mechanisms the present invention
utilizes to attenuate the stray electromagnetic field emitted from
electronic device 30 is shown in FIG. 4. When an electronic device
30 is emitting stray electromagnetic fields, many of the waves
comprising the field are directed perpendicular to the package. The
arrows in FIG. 4 represent the waves being emitted perpendicular to
the electronic device 30. In the basic mechanism shown in FIG. 4,
both the first conductive layer 15 and the second conductive layer
25 absorb some of the waves of the electromagnetic field emanating
from the electronic device 30. Thus, each conductive layer 15, 25
attenuates the strength of the unwanted electromagnetic field.
[0056] The second mechanism of the invention for collapsing the
stray electromagnetic field is shown in FIG. 5. As seen in FIG. 5,
in addition to acting as a shield, both the first conductive layer
15 and second conductive layer 25 act as antennas and reflect some
of the waves of the field back towards the source of the unwanted
electromagnetic radiation. Thus, second conductive layer 25
reflects some of the waves of the field from electronic device 30
back towards electronic device 30. The electromagnetic field
reflected by second conductive layer 25, through interference,
cancels out some of the unwanted emissions. Likewise, first
conductive layer 15 reflects some of the waves of the field that
passed through second conductive layer 25 back towards insulating
layer 20 (not shown in FIG. 6) and the second conductive layer
25.
[0057] The second conductive layer 25, the insulating layer 20 and
the first conductive layer define a wave guide. This causes
electromagnetic field to emit perpendicular to the original
unwanted emissions. The field emitted perpendicular to the original
unwanted emissions is seen by the arrows in FIG. 6. The reason for
this is as follows. The two conductive planes of the shield allow
electromagnetic energy to reflect between them. Plane transverse
electromagnetic mode waves result in a higher order transverse
electric mode. The transverse electric mode wave will not be
transmitted unless the wavelength is sufficiently short. Their
critical wavelength, at which the transmission is no longer
possible, is called the cut off wavelength. The cut off wavelength
as a function of sheet spacing is the wavelength of the
electromagnetic waves between the first conductive layer 15 and
second conductive layer 25 divided by four. However, the longest
wavelength which can be transmitted between the sheets in a higher
order mode (i.e., a higher order harmonic of the electromagnetic
waves between the first conductive layer 15 and second conductive
layer 25) is determined by Lambda=2 B, where B is equal to the
distance between the two conducting planes. Because the two
conducting planes create a resonating cavity, the energy from this
resonance is directed to the edges, since there is more energy in
the center of these two planes than there is on the edges.
Furthermore, it is not just the parallel planes themselves that
cause electromagnetic energy to emit from the sides of the shield
10, but also the dielectric sheet wave guide effect, whereby the
transverse electromagnetic wave is launched into the sheet from
reflections between the two conductive layers.
[0058] In addition to directing the waves of the field
perpendicular to the original unwanted emissions, the waves of the
field reflected off of the first conductive layer 15 reflect back
and forth between the first conductive layer 15 and second
conductive layer 25, i.e., the waves resonate. With the field
resonating between the first conductive layer 15 and the second
conductive layer 25, the magnitude of the electrostatic potential
of the field that passes through the first conductive layer 15 and
into the environment attenuates. This is because the waves of the
field resonating between the first conductive layer 15 and the
second conductive layer 25 interfere with, i.e., cancels, the waves
of the stray electromagnetic field that emanate from the electronic
device 30.
[0059] Electronic devices also emit a magnetic field that envelops
the sides of the electronic devices. This is seen in FIG. 8. The
shield 10 of the present invention also reduces this field.
However, it is important to properly install the shield 10 onto a
device because the shield 10 actually creates a field of its own.
If the field created by the shield 10 becomes too great, it could
adversely affect the operation of the electronic device upon which
it is mounted.
[0060] The various methods in which the shields 10 of the present
invention are installed will now be discussed. When a digital
device such as a computer system is being tested on a FCC
registered test site, the tests discussed above are performed. This
test site has a receiving antenna that is connected via special
cable to a spectrum analyzer. The digital device undergoing testing
is placed in the center of a rotating table that is located three
meters from the antenna. The rotating table is rotated by remote
electronic control until the antenna receives the maximum amplitude
of specific frequencies of the stray electromagnetic radiation that
is generated by the equipment under test. This information is
displayed on the spectrum analyzer. If the amplitudes received by
the antenna exceed the FCC limit, the shields 10 of the present
invention are installed on specific electronic components (i.e.,
integrated circuits) of the equipment being tested. The test is
then repeated to determine the effect that installing the shields
10 had on the unwanted emissions. If the amplitudes of the unwanted
electromagnetic emissions continue to exceed the FCC limit,
additional shields 10 may be installed to reduce the amplitudes to
a value below the FCC limits.
[0061] To determine which electronic components to install the
shields 10 of the present invention on, it must be determined where
the unwanted stray electromagnetic emissions are being generated on
the printed circuit assembly and how they are being distributed
throughout the equipment. This is done by reviewing schematics,
layout drawings and other technical references for the product
undergoing testing. By doing this, the location where the excessive
electromagnetic radiation is being produced can be located.
Examples of "noisy" components that are likely sources for the
unwanted emissions include multi-vibrators, crystal oscillators,
collpits oscillators, and other oscillating components. Shields 10
of the present invention are then placed on all of the circuits
closest to the likely source for the excessive emissions.
[0062] Another method of determining which specific electronic
components should have the shields 10 of the present invention
installed on is by the equipment's orientation. For example, the
data cables that are connected to the input/output ports of the
product undergoing testing may very well be located closer to the
antenna at the time the maximum amplitude of the unwanted
electromagnetic radiation is measured (as discussed, the product
undergoing testing is mounted onto a turntable). To resolve this
type of failure, the shields 10 of the present invention may be
installed onto the electronic component used to interface with the
input/output ports.
[0063] On the other hand, even if data cables are oriented such
that they are facing the antenna, review of the schematics might
show that the unwanted electromagnetic radiation is being generated
by a microprocessor and/or associated circuitry on the printed
circuit board driving the input/output port. In this situation, the
microprocessor and associated interface circuitry will have a
shield 10 of the present invention installed thereon.
[0064] The result of installing a shield 10 of the present
invention onto a specific electronic component will be seen on the
spectrum analyzer, as the reduction of the unwanted emission
amplitude will be measured again by the calibrated antenna.
[0065] Typically, to access the electronic components PC boards of
the product undergoing testing, a portion of the enclosure is
removed. Typically, one or two shields 10 of the present invention
are installed on electronic components in the vicinity of the
determined sources of unwanted electromagnetic radiation. After the
shields 10 are installed, portion of the enclosure which was
removed is re-attached to the product. The RF energy would then be
measured to see what the effect was. The reason that only one or
two shields 10 are installed at a time is that installing too many
shields 10 in a product may cause more unwanted emissions.
Therefore, a step by step procedure must be used to ensure that the
overall unwanted emissions of the device are reduced below
acceptable limits
[0066] Another method of determining which electronic components to
install the shields 10 of the present invention will now be
discussed. The product undergoing testing is characterized on a
calibrated radio frequency interference test site between thirty
and two hundred Mz in both the horizontal and vertical
polarization. This means that the receiving antenna connected to
the spectrum analyzer will be positioned in the vertical (with
respect to the ground surface) orientation for one set of data, and
horizontal (with regards to the ground surface) for the other set
of data. Failure data is characterized by unwanted emissions having
specific amplitudes exceeding the limit.
[0067] The product undergoing testing is then removed from the test
site. The cover of the product is removed, and a high-speed
oscilloscope (at least a 150 MHz) is adjusted so that a specific
offending frequency of unwanted emissions, such as 80 MHz, can be
seen readily on the scope face. Using the appropriate probe
attached to the high-speed oscilloscope, the areas of the printed
circuit assemblies of the product undergoing test where the
unwanted emissions (e.g., the 80 MHz signal) appear on the scope
face are located.
[0068] The spectrum analyzer connected to the calibrated antenna is
adjusted such that its resolution bandwidth set to 100 kHz, its
video bandwidth is set to 100 kHz, its frequency span set to 0 Hz,
and its center frequency set to 80 MHz (for this particular
example). The appropriate probe is connected to the spectrum
analyzer so that when that probe is brought adjacent to the
oscilloscope probe cable , the amplitude of the unwanted emission
can be seen on the spectrum analyzer scope face. The specific
electronic components where the amplitude of the unwanted emission
(in this case, the 80 MHz signal) is at its highest are then found
by moving the oscilloscope probe around the interior of the product
undergoing testing. The shields 10 of the present invention are
then installed on those specific components. If the location of the
shield 10 is suitable for the application, the spectrum analyzer
will show a definite decrease in the amplitude of the unwanted
emissions.
[0069] At this point, the cover of the product is reinstalled, and
the product is retested in accordance with the above-described
specifications. It is important to note that prior to installing
the shields 10 in the product for the first time, only the
fundamentals and associated harmonics of the unwanted emissions
should be measured. If the product undergoing testing fails again,
then it should be determined whether the unwanted emissions are
fundamentals or harmonics. If they are fundamentals, use the same
techniques described above and install the shields 10 of the
present invention on top of the electronic components and
associated conducting pathways (i.e., metal circuit traces) on the
PC board to decrease the RF energy on the PC board. Any harmonics
that still are exceeding the unwanted emission levels can be
reduced by analyzing "divide by" circuits or equivalent circuitry
and applying RF disks to the associated integrated circuits.
[0070] The level of attenuation of the stray electromagnetic field
and the density of the field created by the shield 10 varies as
function of the size of the shield 10. A larger shield will
attenuate the stray field more than a smaller shield. However, a
larger shield will create a greater field density than a smaller
shield. Thus, when installing a shield on an electronic device, it
may be necessary to use a shield of one size, determine the
attenuation, and verify that the electronic device still operates
properly. If these parameters are not met, this shield can be
removed, and a new shield of a different size can be installed on
the electronic device.
[0071] The field created by the shield 10 also varies as a function
of materials used to construct the shield. As discussed, in the
presently preferred embodiments, the first conductive layer 15 and
second conductive layer 25 are constructed of aluminum alloy. The
first conductive layer 15 and second conductive layer 25, however,
can be constructed of magnetic materials, therefore causing the
shield 10 to interact more readily with magnetic fields in an
operating environment. It is noted, however, that the aluminum
alloy used for the first conductive layer 15 and second conductive
layer 25 of the preferred embodiment interacts primarily with
electric and electrostatic fields.
[0072] In addition to preventing unwanted emissions of
electromagnetic radiation, the apparatus of the present invention
reduces an electronic device's susceptibility to external
electromagnetic radiation. This is important because external
electromagnetic radiation can disturb the operation of an
electronic device. In addition, regulatory agencies such as those
described above have either promulgated or are promulgating
regulations regarding the susceptibility of electronic products to
electromagnetic radiation. Examples of susceptibility regulations
that have been promulgated by the European Community are discussed
above. The manner in which the various embodiments of the present
invention operate to reduce an electronic device's susceptibility
to external electromagnetic radiation will be discussed with
reference to FIG. 9. As seen in FIG. 9, external electromagnetic
radiation (shown in FIG. 9 as arrows) can strike the shield 10 at
any angle The manner in which the shield 10 prevents this radiation
from reaching the electronic device 30 is similar to the manner in
which the shield 10 collapses the stray electromagnetic field
emitted by the electronic device 30. First, the first conductive
layer 15 and the second conductive layer 25 absorb some of the
waves of the external electromagnetic field. Thus, each conductive
layer 15, 25 attenuates the strength of the unwanted
electromagnetic field. In addition to acting as a shield, both the
first conductive layer 15 and second conductive layer 25 act as
antennas and reflect some of the waves of the field back towards
the source of the unwanted electromagnetic radiation, which is
shown in FIG. 9. Thus, first conductive layer 15 reflects some of
the waves of the field back towards the source of the field. This
causes some of the electromagnetic field reflected by first
conductive layer 15, through interference, to cancel out some of
the external field.
[0073] Likewise, second conductive layer 25 reflects some of the
waves of the field that passed through first conductive layer 15
back towards insulating layer 20 and the first conductive layer 15.
As described above, the first conductive layer 15, the insulating
layer 20 and the second conductive layer define a wave-guide. This
causes an electromagnetic field to be emitted coplanar with the
shield 10. For simplicity, this is not shown in FIG. 9. In
addition, some of the waves of the electromagnetic field are
reflected by the second conductive layer 25 back towards the first
conductive layer 15. Some of these waves are then reflected back
towards the second conductive layer 25, i.e., the waves resonate.
Just as described above, when the waves of the field are resonating
between the first conductive layer 15 and the second conductive
layer 25, the magnitude of the electrostatic potential of the field
that passes through the second conductive layer 25, and hence is
communicated to the electronic device 30, is attenuated. This is
because the waves of the field resonating between the first
conductive layer 15 and the second conductive layer 25 interfere
with, i.e., cancel, the waves of the external electromagnetic
field. These mechanisms act to reduce the amount of electromagnetic
radiation that reaches the electronic device 30.
[0074] FIG. 10 shows a printed circuit board 200 having various
electronic components 210, 215, 220, 225, 230 installed thereon.
When the product that utilizes this printed circuit board 200 is
tested for stray electromagnetic emissions and fails, the emissions
from each of the electronic components 210, 215, 220, 225, 230 can
be measured using an antenna that communicates with a spectrum
analyzer or other type of device, as discussed above. In addition,
whereas a spectrum analyzer can reveal specific frequencies and
specific magnitudes of the stray electromagnetic fields, it is
possible to use a receiver that tunes to a specific frequency from
which the magnitude can be derived. This allows testing of specific
frequencies, should that be necessary. Shields 10 of the present
invention can be installed on those components that are emitting
excessive levels of electromagnetic radiation. According to most
specifications (see above) most countries will not allow the
shipment, sale or marketing of digital devices without first being
tested to ensure that they have levels of electromagnetic radiation
below 40 dB microvolts per meter at 30 MHz when measured at 3.0
meters. For example, for the printed circuit board 200 shown in
FIG. 10, shields 10 where installed on electronic components 210,
215, 220, 225, but not on electronic component 230.
[0075] As in the case of printed circuit board 200, it may not be
necessary to place a shield 10 on every electronic component that
is emitting excessive amounts of electromagnetic radiation. It is
only necessary to install shields 10 on enough components such that
the emissions from the product fall within those allowed by the
regulations of the country where the product will be sold. It is
necessary, however, to carefully select which components the
shields 10 will be installed on. The reason for this is as follows.
As discussed, the shields 10 themselves produce electromagnetic
fields of their own (generally in the same plane as the shields).
These fields will interact with the fields created by the shields
10 installed on the other electronic components. Indeed, the
interaction of the fields created by the shields 10 that are
installed on the electronic components may aid in collapsing the
fields created by an electronic component that does not have a
shield installed thereon. An example of this interaction is seen in
FIG. 11, which shows the electromagnetic field created when shields
10 are installed on electronic components 210, 215, 220, 225.
Furthermore, depending upon the application, it may be desirable to
install a shield 10 on only the electronic devices that are
emitting the most electromagnetic radiation.
[0076] Shields 10 like those described herein have been constructed
and have attenuated the electromagnetic radiation emissions from an
integrated circuit by anywhere from three to twenty dB microvolts
in a frequency range of 30 to 1000 MHz.
[0077] Another embodiment of the present invention will be
discussed with reference to FIGS. 13-14. The embodiment of the
present invention shown in FIGS. 13-14 utilizes a shield 300
oriented in a curvilinear fashion. In the example of FIG. 13, a
shield 300 of the present invention is installed in a curvilinear
fashion on a printed circuit board card cage 305 to reduce unwanted
electromagnetic radiation emissions emanating therefrom. Shield 300
is constructed as described above. Shield 300, however, is
generally larger than the shields described above, in that shield
300 is designed to be installed over brackets and printed circuit
boards, not a single electronic component.
[0078] As is seen in FIG. 13, the shield 300 is installed on card
cage 305 such that it curves around card cage 305 at a
substantially ninety-degree angle. Card cage 305 contains any
number of printed circuit board assemblies 310, 315, and 320. It is
noted that redundant printed circuit boards are not shown in FIG.
13. In general, the chassis of a card cage 305 is constructed from
rigid metal or plastic of sufficient strength to hold the printed
circuit boards 310, 315, 320 in place. For illustrative purposes,
the shield 300 of the present invention is shown such that it is
slightly removed from the surface of the card cage. In a normal
application, however, the shield 300 is placed directly on the card
cage surface. By installing a shield 300 of the present invention
in such a curvilinear fashion, the unwanted electromagnetic
radiation emissions are effectively moved from the front of the
card cage to the side. In this example, the front of the card cage
consists of vertical printed circuit boards seen readily in this
view, perpendicular to a horizontal plane and perpendicular to the
invention.
[0079] In the example shown in FIG. 14, a shield 300 is installed
to reduce unwanted emissions on a cable connected to a printed
circuit board. An example of a cable that can emit unacceptable
amplitudes of unwanted emissions include a composite video cable
that is connected to a personal computer. A typical personal
computer contains a number of printed circuit boards, e.g., printed
circuit boards 310, 315, 320 (see FIG. 13), that are fastened to
the chassis with a retaining bracket (not shown in FIG. 13). These
retaining brackets are used to stabilize the printed circuit boards
and hold them in place. As is seen in FIG. 14, the metal bracket
335 traverses the entire length of the printed circuit board 325
that faces the exterior of the personal computer. In general, the
bracket 335 is connected to the printed circuit board 325 in two
places (these connections are not shown in order to simplify the
drawing). Passing through the metal bracket, however, is the
composite video port 330. The outside surface of the metal bracket
335 is usually in contact with the chassis guides (not shown) of
the personal computer.
[0080] To reduce the unwanted emissions from the cable (not shown),
a shield 300 constructed in accordance with the present invention
is installed in a curvilinear fashion such that it covers the
inside of the bracket 335 (i.e., the shield 300 is disposed between
the printed circuit board 325 and bracket 335 ). The shield 300
then turns at a substantially ninety-degree angle such that it
traverses a predetermined length of the side of the printed circuit
board 325 which has the electronic components installed thereon. In
various preferred embodiments, this predetermined length should be
at least two inches. The port 330 passes through the shield 300.
Thus, the shield 300 of the present invention must have an aperture
(not shown) disposed therein to allow the port to pass
therethrough.
[0081] It is important to note that the shield 300 may or may not
electrically conductively contact the metal bracket of the printed
circuit board 325. Spectrum analyzer measurements are taken to
determine the effectiveness of conductive contact with the
retaining bracket. If measurements determine that conductive
contact did provide satisfactory attenuation, the shield 300 can
then be installed in a non-conductive manner. Furthermore, the
shield 300 in FIG. 14 is normally in contact the electronic
components mounted on the component side of the printed circuit
board 325. It is noted that the retaining bracket 335 shown in FIG.
14 is shown removed from the printed circuit board 325 to allow
viewing for clarification of the inventive concepts shown
therein.
[0082] Thus, a preferred apparatus for attenuating stray
electromagnetic radiation emitted by electronic devices, and the
method manufacturing and using the apparatus has been described.
While embodiments and applications of this invention have been
shown and described, as would be apparent to those skilled in the
art, many more embodiments and applications are possible without
departing from the inventive concepts disclosed herein. The
invention, therefore, is not to be restricted except in the spirit
of the appended claims.
* * * * *