Abstract:
A feed-through provides electromagnetic shielding where one or more signal leads pass through an enclosure. It comprises a frame, having at least one opening, and an assembly comprising two or more joining parts, forming one or more elongated waveguides. The joining parts are divisible along the length of the waveguides, thereby being capable of surrounding a signal lead. The assembly is adapted to be attached to the frame such that one or more signal leads can pass through an opening in the frame and through one of the waveguides. Installation is of the feed-through is made easier by the opening in the frame having a larger maximum extension than the maximum extension of a waveguide.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a feed-through for providing electromagnetic shielding where one or more signal leads pass through a cabinet or other enclosure. 
       BACKGROUND 
       [0002]    Such a feed-through can be used to prevent emission of electromagnetic waves, for example to avoid electromagnetic interference or the leakage of critical information from a network switch or server placed in a cabinet or enclosure. For the latter application, standards, usually referred to using the term “TEMPEST”, are established that require attenuation up to the order of 100 dB at frequencies up to 10 GHz at the feed-through. 
         [0003]    One example of such a feed-through, for optical cables, is described in U.S. Pat. No. 7,793,415-B2, which shows a flat member with a number of openings. An enclosure can be fitted around each cable to form a tube, and this tube can be attached to the flat member at an opening thereof. The flat member is in turn attached to an opening of a cabinet or other enclosure. This structure reduces electromagnetic interference to a desired level up to a frequency that is determined by the length and the inner diameter of the tube formed by the enclosure. 
         [0004]    One problem associated with such a feed-through is how to facilitate installation of, for example, optical cables in a cabinet or other enclosure where the feed-through is used. 
       SUMMARY 
       [0005]    One object of the present invention is therefore how to facilitate installation of, for example, optical cables in cabinets or other enclosures where the feed-through is used. 
         [0006]    This object is achieved by means of a feed-through as defined in claim  1 . More specifically, such a feed through comprises a frame, having at least one opening, and at least one assembly comprising two or more joining parts, forming one or more elongated waveguides. The joining parts are divisible along the length of the waveguides, thereby being capable of surrounding a signal lead. The assembly is adapted to be attached to the frame such that one or more signal leads can pass through an opening in the frame and through one of the waveguides. Further, the opening in the frame then has a larger maximum extension than the maximum cross section extension of a waveguide. 
         [0007]    With such a feed-through it is possible, while providing a reasonably short waveguide with a small diameter, to install, for example, a pre-terminated fiber-optic cable, i.e., one that has already been provided with a connector, which has a considerably greater cross section than a fiber-optic cable. The frame may be attached to a cabinet or other enclosure, and the user may stick the fiber-optic cable, or a bundle of cables, with a connector at its end, through the opening in the frame, which opening is large enough to allow this. The joining parts are then fitted around the cable or cables such that the cables run in an elongated cavity in the interior of the finished assembly. This can be achieved even in cases where the cavity is narrower than the connector. Means for attaching the parts together are provided. The enclosing parts are then firmly connected to the frame. Thereby pre-terminated cables can be used, which makes any installation procedure considerably more efficient. 
         [0008]    The number of cables that need to be fed through a cabinet wall may vary greatly between different applications. One way to achieve flexibility in the number of cables possible to be fed though is to let the assembly consist of a variable number of parts, with a variable number of optional middle parts between two end parts. Installation is facilitated by the completed assembly forming one solid block without extending and/or fragile parts. 
         [0009]    Machined parts allowing high precision may be used, making a tight fit between the parts possible, thus preventing the leakage of electromagnetic radiation between the parts. The frame may be welded to the cabinet to prevent leakage of radiation between the frame and the cabinet. A frame with a thickness larger than 10 mm may be used in order to facilitate welding. An electrically conductive gasket, such as a copper-beryllium gasket, can be used to prevent leakage of electromagnetic radiation between the assembly and the frame. The parts in the assembly may be made from steel, which has good conductive properties, resulting in a well-functioning waveguide. Tying the parts together using nuts and bolts is one way to assure tight electric contact between them. 
         [0010]    In general, the longer a waveguide is, the larger the attenuation will be. In addition, for a waveguide to be able to attenuate a given frequency, that frequency needs to be below the cut-off frequency of the waveguide. Standards as discussed above mandate a certain minimum attenuation, leading to waveguides needing to have a certain minimum length and maximum transversal extension. In this case, the waveguides may have length of at least 52 mm and/or a maximal transversal extension of at most 17.5 mm. 
         [0011]    On the connecting surfaces between joining parts, there may be recesses, the use of which may result in good electric and mechanical contact between the parts. 
         [0012]    Space is often tight in and around cabinets of the mentioned kind. Allowing the waveguides to extend through the opening in the cabinet is one way of saving space. In particular, more space is often available outside of the cabinet. A configuration wherein the feed-through extends further on the outside than on the inside of the cabinet will make better use of available space. 
         [0013]    Handles may be placed on any of the parts of the assembly to aid installation and removal. To aid installation, the frame may comprise one or more detachable parts. Furthermore, the frame may be comprised of two or more layers that may attach to each of the walls of a cabinet having several walls. 
         [0014]    Different waveguides may be of different lengths according to need, for example to save space and material. The signal lead going through the feed-through is typically a fiber-optic cable. 
         [0015]    The object mentioned in the first paragraph may also be achieved by means of a system, comprising the mentioned feed-through, a cabinet or enclosure, and one or more signal leads. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    These and other aspects of the invention will now be described in more detail, with reference to embodiments of the invention. 
           [0017]      FIG. 1  schematically illustrates a situation where a feed-through according to the present disclosure can be used. 
           [0018]      FIG. 2  schematically illustrates a waveguide below-cutoff. 
           [0019]      FIG. 3  is a perspective drawing of an end part of an assembly. 
           [0020]      FIG. 4  is a perspective drawing of a middle part of the assembly. 
           [0021]      FIG. 5  is a perspective drawing showing how the parts of the assembly fit together. 
           [0022]      FIG. 6  is a perspective drawing showing a view of a completed assembly and how the assembly is attached to the frame. 
           [0023]      FIG. 7  is a perspective drawing showing another view of a completed assembly. 
           [0024]      FIG. 8  is a perspective drawing showing one side of an installed feed-through. 
           [0025]      FIG. 9  is a perspective drawing showing another side of an installed feed-through. 
           [0026]      FIG. 10  is a perspective drawing showing an embodiment of the invention in which the assembly has handles and the frame has a detachable part. 
           [0027]      FIG. 11  is a perspective drawing showing an embodiment of the invention in which several assemblies are mounted on one frame and each assembly has handles. 
           [0028]      FIG. 12  is a perspective drawing showing one side of an assembly in an embodiment of the invention where the waveguides have different lengths. 
           [0029]      FIG. 13  is a perspective drawing showing another side of an assembly in an embodiment of the invention where the waveguides have different lengths. 
           [0030]      FIG. 14  is a perspective drawing showing a frame in an embodiment of the invention in which the frame has two layers. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]      FIG. 1  illustrates a protected cabinet or other enclosure  1 , which is a context where a feed-through  11  according to the present disclosure can be used. Such a cabinet or other enclosure can be used in a sensitive environment where processing devices  3  in the cabinet  1 , such as servers, electronically process unencrypted sensitive information. The cabinet  1 , which may be made of thick sheet metal, functions as a Faraday cage, containing within its enclosure any electromagnetic radiation from the processing devices  3 , thereby preventing the leakage of sensitive information. The processing devices  3  communicate with nodes  13  outside the cabinet  1  via optical fibers  5  which are terminated by means of connectors  7 ,  9  at each end. The optical fibers may convey less sensitive information or the information carried by them may be encrypted. Furthermore, the conveyed optical signals as such usually give negligible leaked radiation, preventing interception of such signals, unless the fibers are tampered with. 
         [0032]    The feed-through where the optical fibers extend out of the cabinet  1  requires special attention so that electromagnetic radiation does not escape the cabinet, risking interception by a third party nearby. 
         [0033]    In addition to the shielding situation described above, the feed-through may be useful in other situations where considerable attenuation at a connection into a Faraday cage is needed. For instance, as the effect provided is more or less reciprocal, the feed-trough could protect sensitive equipment in a cabinet from external electromagnetic interference, such as electromagnetic pulses with high energy. Further, a shielded room used for sensitive measurements could be protected from external electromagnetic interference, etc. 
         [0034]    As is well known per se, an attenuating feed-through can be obtained by means of a waveguide below-cutoff, WBCO, of which one example is schematically illustrated in  FIG. 2 . In this example the waveguide  12 , made from a conductive material, has a cavity  14 , open at both ends, with a circular cross section with diameter D and a length l. The waveguide extends through a wall in a Faraday cage  1 . 
         [0035]    Electromagnetic waves can propagate through a waveguide in a number of different modes, corresponding to different solutions to the Maxwell equations. These modes are distinguished by different configurations of the electric and magnetic fields. Each of these modes has a cut-off frequency, below which no substantial propagation in that mode is possible. Below the lowest cut-off frequency of all possible modes, no substantial propagation of electromagnetic waves is possible at all. Instead, signals suffer exponential attenuation. In a circular waveguide, the mode with the lowest cut-off frequency is the TE 11  (transverse electric) mode. The cut-off frequency f c  of that mode can be shown to be, to three significant figures, 
         [0000]    
       
         
           
             
               
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             , 
           
         
       
     
         [0000]    where v is the propagation speed of the waveguide dielectric, i.e., the nonconductive material forming the bulk of the waveguide cavity. In air, v is to a good approximation 3×10 8  m/s. Below this cut-off frequency, over a distance l, an electromagnetic wave with frequency f suffers a total attenuation of, again to three significant figures, 
         [0000]    
       
         
           
             A 
             = 
             
               32.0 
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                 l 
                 D 
               
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                   1 
                   - 
                   
                     
                       ( 
                       
                         f 
                         
                           f 
                           c 
                         
                       
                       ) 
                     
                     2 
                   
                 
               
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                 dB 
                 . 
               
             
           
         
       
     
         [0036]    It can thus easily be realized that to maximize attenuation of signals of a certain frequency requires as high a cut-off frequency as possible, and thus as narrow a waveguide as possible, as well as a long a waveguide as possible. It can also be seen that the larger the diameter a waveguide has, the longer the waveguide needs to be to achieve target attenuation. 
         [0037]    For a feed-though to function properly according to the strictest current standards, i.e. a dampening of 100 dB between frequencies 10 kHz and 10 GHz, the cross-section diameter of a waveguide should preferably be smaller than about 17.5 mm. With an example cross-section diameter of 12 mm, a waveguide should preferably be longer than about 52 mm. 
         [0038]      FIG. 6  shows the parts of a complete feed-through. The feed-though comprises a frame  43  and an assembly  41 , the latter comprising two end parts  37  and between them, optionally, one or more middle parts  31 ,  33 .  FIG. 5  shows how the parts of the assembly fit together. As the parts comprising the assembly are brought together, waveguides  53  are formed from cut-outs on those parts. A minimal configuration of the assembly  41  could consist of only two end parts  37 . Any number of middle parts  31 ,  33  can then be placed between them according to need. Middle parts  31 ,  33  can be made slightly different, e.g. with  33  or without 31 threads. Middle parts with threads  33  are on the drawings marked “C”, while those without threads  31  are marked “X”. This arrangement allows a tight fit between the parts. At the same time, the finished assembly is rugged without any extending fragile parts and not likely to break apart during installation. The completed assembly  41  as shown in  FIG. 6  fits into an opening  51  of the frame  43 . 
         [0039]    In the embodiment described here, the parts of the assembly are made from steel, which material has good electric conducting properties, allowing for a well-functioning waveguide. Machined parts are used, which allows high precision, allowing for a tight fit between the parts. However, the skilled person realizes that other fabrication methods and other materials with good electrical conductivity are also possible. 
         [0040]    An electrically conducting gasket, such as a copper-beryllium gasket  49  may be used to prevent leakage of electromagnetic radiation between the assembly  41  and the frame  43 . Copper-beryllium gaskets are rugged and easy to use. 
         [0041]    A person skilled in the art realizes that other arrangements, such as an arrangement of conductive gaskets made from, for example, silicone or rubber mixtures, lying flat on the frame  43  or running in one or more grooves in the frame, are conceivable as well. 
         [0042]      FIGS. 3 and 4  show different parts making up the assembly  41  (cf.  FIG. 6 ); an end part  37 , and one example of a middle part  33 . As mentioned above, there may be provided middle parts with holes  35  having threads, and middle parts  31  (cf.  FIG. 5 ) with corresponding holes not having threads. 
         [0043]    An end part  37  comprises a flange portion  15  and a protruding section  17  perpendicular to the flange. A middle part  31 ,  33  comprises two flange portions  15  and one protruding section  17 . 
         [0044]    The flanges  15  are devised to be attached to the frame  43  (cf.  FIG. 6 ). On each middle part  31 ,  33 , one flange portion  15  is located on each side of the protruding portion. On the flanges  15 , there are holes  19  in a direction perpendicular to the flanges  15 , extending through the flanges. These holes  19  are suitable for attaching the assembly  41  to the frame  43 . In the particular embodiment of the disclosure described here, middle parts  31 ,  33  have one such hole  19  on each flange, while an end part  37  has four holes  19  on the edge of the flange facing away from the protruding section  17  and two holes  19  on each of the two edges perpendicular to the edge facing away from the protruding section  17 . At the entrance to each hole  19 , there may be a recess  20  suitable for giving space for a nut. 
         [0045]    On the protruding section  17  on both end parts  37  and middle parts  31 ,  33 , there are connecting surfaces  24  that will be in contact as the parts of the assembly are brought together. On the middle parts  31 ,  33  there are such surfaces on each side of the protruding section  17  and flanges, while on end parts  37 , there are such surfaces only on a side of the protruding section  17  facing away from the flange  15 . The peripheries of these surfaces may be raised, such that there is provided recesses  26  there between, where holes  29 ,  35  for bolts are provided, in order to improve mechanical/galvanic contact between the surfaces of different parts by permitting higher pressure at the contact surfaces  24 . In the embodiment described here, such raised edges are present on the middle parts  31 ,  33 , as well as on the end parts  37 . 
         [0046]    At the connecting surfaces described above, there are cut-outs  21  that, when matched with corresponding middle  31 ,  33  or end parts  37 , form the waveguides. In the embodiment described here, there are two such cut-outs at each connecting surface. The cut-outs extend along the whole length of the parts  31 ,  33 ,  37 . 
         [0047]    In the embodiment described here, the cut-outs are semi-circular. However, a person skilled in the art realizes that cut-outs of other shapes, such as quadratic, rectangular, honeycomb or general polygon shaped are equally possible. A person skilled in the art would additionally recognize that it is equally possible to have a different number of cut-outs than two on the surfaces of the parts and to have cutouts that for one waveguide have different cut-outs on the two different parts forming that waveguide. 
         [0048]    Additionally, on the connecting surfaces described above, there may be pins  23  and holes  25  to aid alignment of end parts  37  and/or middle parts  31 ,  33  attaching to each other. In the embodiment described here, those pins  23  and holes  25  are located on the flanges  15 , near the end of the connecting surfaces. Just outside of the pins  23  and holes  25 , on the edge between the flange  15  and the connecting surface, there may be smaller cut-outs  27 , designed to aid disassembly, if desired, for example by inserting a screwdriver into the cut-out  27  and turning the screwdriver and/or using it as a lever. 
         [0049]    In the recesses at the connecting surfaces described above, there are through holes  29  extending perpendicularly to the surfaces, in the protruding sections  17  and the flanges  15 . In the embodiment described here, on the middle parts  31 ,  33 , there is one such hole  29  through each of the flanges  15 . On end parts  37 , there is one such hole  29  though each side of the flange  15 . On both end  27  and middle  31 ,  33  parts, there are three rows of holes  29  extending though the protruding section  17 . One such row is located on one side of the two cuts-outs  21 , one row on the other side of the cut-outs  21  and one row between the cut-outs  21 . The number of holes  29  in each row depends on the length of the protruding section  17 . In the embodiment described here, each row has three holes  29 . Needless to say, the hole configurations may be varied in a number of ways. 
         [0050]    On middle parts  31 ,  33 , the holes described in the previous paragraph may be threaded  33  or be without threads  31 . 
         [0051]      FIG. 5  shows how the assembly  41  is fitted together, here exemplified with an end part  37 , a middle part  31  without threads, and a middle part  33  with threads being attached to each other using bolts  39 ,  40 . The bolts may interact with the threads  35  (cf.  FIG. 4 ) of middle parts  33  having such threads or with nuts if the bolts penetrate the entire assembly. Thereby the parts are compressed, bringing the connecting surfaces described above into close contact. The skilled person will recognize that other means for applying this pressure than bolts and optional nuts are possible. 
         [0052]    As already has been discussed,  FIG. 6  shows a completed assembly  41 , the frame  43 , and a wall of the cabinet  1 . The frame  43  can have a rectangular cross section and an opening  51 , also rectangular in cross-section, where the narrower part of the assembly  41  fits, i.e., the part of the assembly comprised of the protruding sections  17  of the parts  31 ,  33 ,  37  making up the assembly. The frame may for example have been welded to the cabinet, although other ways of fitting the frame to the wall are conceivable. To be suitable for welding to the cabinet  1 , it may be beneficial for the frame  43  to have a thickness in a direction perpendicular to the plane of one side of the cabinet of at least 10 mm. 
         [0053]    In the embodiment described here, the waveguides  53  formed by the joining parts  31 ,  33 ,  37  are at a right angle to the plane of the frame  43  or of one side of the cabinet  1 . However, it is equally possible for the waveguides  53  and the protruding sections  17  of the joining parts  31 ,  33 ,  37  to be at an oblique angle to the flanges  15 , resulting in a feed-though with signal leads  5  entering the cabinet  1  at an oblique angle with respect to the plane of the frame  43  or of one side of the cabinet  1 . This may have an advantage of saving space inside the cabinet. 
         [0054]    When the assembly has been put together, with leads in the waveguides, the assembly is fitted in the frame and firmly attached to the frame. 
         [0055]    Threaded bolts  47  attached to the frame  43  match holes in the flanges  15  of the parts comprising the assembly  41 . These bolts may for example have been press fitted into the frame. A set of holes  45 , in the embodiment described two holes along each short side of the opening  51  in the frame  43 , extend through the frame  43  and through the cabinet  1 . 
         [0056]      FIG. 7  shows another side of a completed assembly  41 . Bolts  39 ,  40  attaching the parts together are present both through the flanges and through the protruding parts. Another set of bolts  42 , hidden in  FIG. 3 , in the embodiment described two bolts  42  on each side, are attached to the end parts on the flange  15  close to the protruding section  17 . These bolts  42  match the holes  45  in the frame described above. Attaching the assembly with bolts from both sides of the cabinet  1  makes the feed-though more tamper-resistant and make unauthorized access to the cabinet though the feed-through more difficult. 
         [0057]    The assembly  41  as illustrated has contact surfaces  28  beneath the flanges. These contact surfaces  28  are devised to come into close contact with corresponding surfaces on the frame  43 , optionally with an intervening gasket  49 . These contact surfaces as well may have recessed portions  30  as well, in order to leave space for a gasket  49 . 
         [0058]      FIG. 8  shows one side of a complete feed-through, with the completed assembly  41  attached to the frame  43 . Nuts  55  may attach to the bolts  47  attached to the frame  43 . 
         [0059]      FIG. 9  shows the other side of a complete feed-through. Nuts  57  may attach to the bolts  42  attached to the end parts. 
         [0060]      FIG. 10  shows an embodiment of the invention in which the assembly  41  has handles  61  and the frame  43  has a detachable part  63 . In this embodiment, the handles are mounted at each end of the assembly  41 , on the edges of the flanges  15  of the end parts of the assembly  41 . The handles are mounted using screws, and facilitate the handling of the assembly. Other locations of the handles and ways of mounting them are equally possible. 
         [0061]    The detachable part  63  of the frame  43  is located by one short edge of the frame and may aid installation of the feed-through. For instance it is possible to fit the frame around a pre-installed set of cables, and thereafter close the frame with the separate, detachable part. The frame is then attached to the enclosure wall. It is also possible to use a fame structure on both sides of a wall. The detachable part  63  may thus be a cut-out from the frame, for instance with stepped shape to reduce any elactromagnetic leaks. 
         [0062]      FIG. 11  shows an embodiment of the invention in which several assemblies  41 , in this case three, are mounted on one frame  43 . Each assembly  41  has handles  61 . The handles  61  are mounted on the assemblies  41  in the same way as in the embodiment depicted in  FIG. 10 . Each assembly  41  is mounted in its own opening of the frame. Removable parts  63  of the frame  43  are located adjacent to a short edge of each assembly  43  and may aid installation of the feed-through. 
         [0063]      FIG. 12  shows an embodiment of the invention in which the waveguides have varying lengths. Varying the length of the waveguides in the same assembly  41  may save space and material. As described above, a waveguide with a larger diameter  67  will need to be longer to achieve a given attenuation than a waveguide with a smaller diameter  69 . At the same time, the space for a signal lead given by a large-diameter waveguide may not be needed for all waveguides in a feed-through. Shown is an assembly  41  with middle parts  31  that form shorter waveguides and middle parts  31 ′ that form longer waveguides. Between middle parts of the two mentioned types, there are transitional parts  65 , to which further bolts and nuts can be attached to firmly fasten the parts forming longer waveguides together along the whole length of the waveguides. As in the embodiments depicted in  FIG. 10  and  FIG. 11 , the assembly  41  has handles  61 .  FIG. 13  shows another side of the same embodiment. 
         [0064]      FIG. 14  is a perspective drawing showing a frame  43  in an embodiment of the invention in which the frame has two layers  71 ,  73 . Each layer  71 ,  73  of the frame  43  has a detachable part  63 , which may aid installation of the feed-through. In a cabinet having double walls each layer of the frame may be attached to each of the walls. It is also possible to two stacked layers on one side of a single wall, or on either side of a single wall. As the detachable parts are located on opposide sides in the two layers, any electromagnetic leaks can be reduced. Needless to say, more than two layers can be provided. 
         [0065]    The present disclosure is not limited by the above example, and may be varied and altered in different way within the scope of the appended claims.