Wave transmission lines and networks – Transmission line inductive or radiation interference...
Reexamination Certificate
2001-05-30
2003-07-22
Bettendorf, Justin P. (Department: 2817)
Wave transmission lines and networks
Transmission line inductive or radiation interference...
C310S113000
Reexamination Certificate
active
06597255
ABSTRACT:
FIELD OF THE INVENTION
The present Invention relates to a transmission system for providing power to equipment housed in a Faraday cage, to a power source for such equipment, and to a Faraday cage provided with such a power source. Particularly, but not exclusively, the transmission system enables power to be provided to communications equipment housed in a Faraday cage which requires shielding against electromagnetic interference and leakage up to at least 5 GHz.
BACKGROUND TO THE INVENTION
Equipment which is sensitive to electromagnetic interference is often protected by isolating apparatus within a conductive shell, or Faraday cage. Faraday cages are also used to retain electromagnetic radiation which is emitted by equipment. Electromagnetic field theory describes how electromagnetic waves interact with a conductive surface shell. Essentially, the shell distributes the electrical field around the shell surface, but not through the shell surface. Accordingly, electromagnetic fields generated outside a Faraday cage cannot penetrate within the cage, and internally generated fields cannot escape outside a Faraday cage.
Any nonconductive medium in the surface of the conductive shell provides an effective “hole” through which electromagnetic waves can escape,
FIGS. 1A and 1B
of the accompanying drawings Illustrate this effect at a basic level. In
FIG. 1A
, a positive point charge
10
a
is shown outside a conductive “box”, Faraday cage
12
. Field lines
14
a
are not able to intrude substantially through gap
16
in the Faraday cage
12
. Similarly, as
FIG. 1B
shows, field line
14
b
from an internal point charge
10
b
cannot extrude substantially into the exterior of the Faraday cage
12
through gap
14
.
Reducing the size of the gap
16
, reduces the amount of external/internal field which is able to gain access to the interior/exterior of the Faraday cage
12
. Accordingly, in practice, providing a gap in a Faraday cage is less than a wavelength across, the amount of electromagnetic radiation which is able to compromise the shielding provided by the cage is small.
Faraday cages can thus have a mesh-type structure, or be perforated for cabling access etc., and still function effectively as a shield against most electromagnetic radiation. The Faraday cage will not providing any shielding, however against radiation which has a substantially shorter wavelength than any gaps or other irregularities in the cage surface structure. This provides a limit on the size of any noninductive feature on the Faraday cage surface, whether an air-gap or another type of irregularity.
Commercially, it is advantageous if communications equipment can be isolated from electrical or electromagnetic interference (EMI). EMI can be defined as undesired conducted or radiated electrical disturbances, such as other electronic equipment might generate including transients, which can interfere with the operation of electrical or electronic equipment such as electrical power supplies. EMI disturbances can occur anywhere in the electromagnetic spectrum. However, radio frequency interference (RFI), usually defined as between 24 kilohertz (kHz) and 240 Gigahertz (GHz), is often considered to be synonymous with EMI for practical purposes. For communications purposes, however, providing protection against electromagnetic pulses up to 5 GHz is usually considered sufficient.
EMI/RFI can be generated by a range of phenomena, and include potentially destructive electromagnetic phenomena, both natural and man-made. Lightning and other types of electromagnetic pulses can create very destructive EMI as the pulse fields couple to electrical radiation and generate electrical transients in power and communications systems. For example, some electromagnetic fields can generate very short high energy transients in electrical connections and cables or longer transients which have lower voltage disturbances. For example, core current transients up to around 150 Amp having full width half maximum FWHMs of 1.2 to 1.4 s, and core voltage transients of up to around 7 kV (FWHM 2.1 s to 4 s) can be induced which are known to cause damage. Similarly, transients which have lower core currents (<40 Amp) can be induced which can last for much longer (FWHM 38 to 40 s), and can accompany voltage pulses as high as 9.4 kV, which last for over a minute and so are potentially very destructive. Such transients can generate energy dumps sufficiently large to melt electrical connections and cables.
It is particularly highly desirable to shield communications equipment from such EMI/RFI due to the fact that communications equipment, even if predominantly provided by optical components, often includes electronic components such as integrated circuit (IC) chips, IC packages, and multi-chip modules along with hybrid components.
One solution to the problem of providing protection against such EMI is to filter out the disturbance at the equipment level, as shown in U.S. Pat. No. 6,034,855. However such filters can be relatively costly and expensive to maintain. Another solution is to house equipment in Faraday cage structures which act as shields against EMI.
Faraday cages can have a variety of forms and can be constructed on a variety of scales. At the circuit board level, a conductive shield can be provided by a “can” covering components and electrically grounded to a substrate such as a printed wiring board. At a larger scale, entire racks of equipment may be housed in conductive cabinets which are grounded to shield their contents from external sources of EMI. Other examples of the forms of Faraday cages include conductive housings, temporary large scale structures effectively providing a “conductive tent”, a metallic or metalised box or case. The cage surface may be smooth or may be perforated in such a way as to minimise the radiation which is able to penetrate the interior of the cage at any given frequency.
In the field of fibreoptic communications, transmission equipment is often shielded by installing the equipment in a conductive housing. The conductive housing functions effectively as a Faraday cage. Cabling conduits for the housing must be sized appropriately according to the desired cut-off frequency for the Faraday cage, i.e., the conductive surface must not be interrupted for more than a fraction of the maximum wavelength the housed equipment can tolerate. For the purposes of most communications equipment, a hole of around one tenth of the cut-off wavelength is possible without unduly compromising the efficiency of the cage.
However, a problem exists in that power must still be provided to the interior of the cage. Batteries and generators may be provided within some Faraday cages as isolated sources of power, however, these are costly to run and maintain and can take up the space within the Faraday cage. However, in the case where a cage needs to be completely sealed, an isolated power source is required.
In the case where a Faraday cage can be perforated, a direct connection with an external power supply is usually not suitable unless a low-pass filter is provided. Such a filter needs to be able to remove any undesirable power characteristics in the external supply, and has to be able to cope with a range of EMI disturbances. It is essential that such filters are capable of removing electromagnetic power surges, pulses, or spikes, whether created by human endeavour or by natural means such as lightening, electrical storms, etc., within the design tolerance required for the Faraday caged equipment. Accordingly, such filters are often a costly solution to the problem of providing power to equipment housed in a Faraday cage. Other problems include the space required by such filters and the inherent uncertainty in their performance providing protection against higher energy transients.
OBJECTS OF THE INVENTION
The invention seeks to provide a transmission system for power to equipment housed within a Faraday cage. The system transfers power from an external power source to an internal power generator and yet does not mitigate
Barnes & Thornburg
Bettendorf Justin P.
Nortel Networks Limited
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