Abstract:
To provide a method of pseudogrounding which is capable of protecting against EMI by pseudogrounding communication lines, etc. without necessity of expensive ferrite cores and increasing the number of steps and cost. A wiring which is connected to an LSI ( 43 ) on a package and is connected to an external wiring ( 47 ) via a connector ( 461 ), radio-frequency connected via a capacitance ( 451, 452 ) to ta housing ( 42 ) in which protection against EMI is achieved so that the radiated noise energy is not higher than EMI restricted value level. This enables said wiring to be pseudogrounded.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a technique for protecting an electronic device from electromagnetic interference (EMI), and in particular to a method and apparatus for pseudogrounding an electronic device which is connected to a plurality of cables such as communication lines, power supply lines, and interface buses. 
     BACKGROUND 
     For corporate information communication devices such as switches for PBX (private branch exchange), ATM (asynchronous transfer mode) switches, etc. which are connected to a number of communication lines, such as telephone lines of unshielded twisted pair cables which are wired simultaneously with construction of building, it is generally impossible to replace all existing indoor wires with new shielded wires which are employed for information devices such as personal computers and main frames, etc. 
     SUMMARY OF THE DISCLOSURE 
     Accordingly, protection against EMI was endeavored by passing communication lines, interface lines or power supply lines through apertured ferrite cores. However, cost for preventing EMI is tremendously high due to use of a lot of expensive ferrite cores. 
     Since the standards of measurement which are specified by CISPR22, VCCI is added with an additional requirement that the packages and cables should be added until no increase in measured radiated noise by 2 dB or more occurs when the number of packages and cables is increased in case where a plurality of packages and cables are incorporated in a device under test, a measurement should be conducted in a system in which a maximum number of packages and cables are accommodated. A stable protection against EMI, in which the energy of the radiated noise never increase even if the number of packages and cables is increased has been demanded. 
     The present invention has been thus achieved in view of the above-mentioned problem. It is an object of the present invention to provide a method and apparatus for protecting an electronic device from EMI at a low expenditure without using any expensive ferrite cores, particularly those which achieve reduction in the number of steps for EMI protection and in cost therefor. 
     It is a further object of the present invention to provide a novel method and apparatus which is capable of pseudogrounding communication lines, etc. 
     In order to achieve the above-mentioned object, there is provided a method for protecting against EMI, according to a first aspect of the present invention. In the method, a wiring is pseudogrounded by radio-frequency connecting said wiring having a level of the radiated noise energy which is reduced to a value less than that of a pseudogrounded housing to said housing via a capacitance. 
     In a method according to a second aspect of the present invention, a wiring is pseudogrounded (a) by making the level of the radiated noise energy of a package which is accommodated in a pseudogrounded housing less than that of said housing, and (b) by radio-frequency connecting said wiring connected to a circuit block on said package and connected to an external wiring via a connector to said pseudogrounded housing via a capacitive element. 
     According to a third aspect of the present invention, there is provided a structure, wherein a wiring that is connected to an integrated circuit block on a package to be accommodated in a housing, and is connected to an external wiring via a connector is radio-frequency connected to a pseudogrounded housing (particularly pseudogrounded earth position) via a capacitive element in said package. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view explaining the principle of the present invention. 
     FIG. 2 is a view explaining the principle of the present invention. 
     FIG. 3 is a view explaining a mode of embodying the present invention, FIG.  3 ( a ) is an elevational and sectional view of a housing, FIGS.  3 ( b ) and  3 ( c ) are views showing the characteristics of the radiated noise energy in a package and on the housing, respectively. 
     FIG. 4 is a view explaining another mode of embodying the present invention, FIG.  4 ( a ) is an elevational and sectional view of a housing, FIG.  4 ( b ) is a view showing the characteristics of the radiated noise energy in a package and on the housing. 
     FIG. 5 is a schematic view explaining a prior art method of protecting against EMI as a comparative example. 
     FIG. 6 is a A perspective view showing the appearance of a housing of the embodiment of the present invention. 
     FIG. 7 is a sectional view taken along the line A—A in FIG.  6 . 
     FIG. 8 is a view showing a mode of embodying the present invention. 
     FIG. 9 is a schematic view explaining a mode of embodying the present invention and a view explaining a general formula representing the strength of the electromagnetic waves. 
     FIG. 10 is a view showing a second embodiment of the present invention. 
     FIGS.  11 ( a ) and  11 ( b ) are views showing a third embodiment of the present invention. 
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
     A mode of embodying the present invention will be described. In a preferred embodiment of the present invention, a wiring is pseudogrounded by radio-frequency connecting the wire ( 47  in FIG. 4) connected to an integrated circuit block of a package to be accommodated in a housing and connected to an external wiring via a connector ( 461 ,  461  in FIG. 4) to a housing having a radiated noise energy which is reduced to the EMI restricted value or less (pseudogrounded housing) via a capacitance ( 451 , 452  in FIG.  4 ); to equalize the radiated noise energy ( 49  in FIG. 4) of the package which is protected against EMI so that the radiated noise energy is less than the radiated noise energy ( 50  in FIG. 4) of said pseudogrounded housing to the radiated noise energy of the pseudo-grounded housing via the capacitance. 
     In a preferred embodiment of the present invention, a wiring may be pseudogrounded by high frequency-connecting the wiring ( 110 A,  110 B in FIG.  10 ), connected to a circuit block ( 104  in FIG.  10 ), on said package accommodated in a molded housing and connected to an external wiring ( 108  in FIG. 10) via a connector, to a given pseudogrounded earth position on the package via a capacitive element ( 106 A,  106 B in FIG. 10) 
     In a preferred embodiment of the invention, a package to be accommodated in a housing may have a substrate, the substrate comprising, a wiring pattern layer ( 115  in FIG. 11) which is connected to a circuit block and is connected to an external wiring ( 114  in FIG. 11) via a connector ( 113  in FIG.  11 ); and 
     a pattern layer ( 112  in FIG. 11) which is connected to a pseudogrounded earth position on the package; and the wiring pattern may be pseudogrounded by radio-frequency connecting the wiring pattern layer to the pseudo-grounded earth position via an interlayer capacitance. 
     Now, the principle of the present invention will be described with reference to the drawings. 
     If a wire  12  is buried in a hole which is formed in the ground  11  of the earth as shown in FIG. 1, no electromagnetic waves are radiated from the wire  12 . 
     A housing  21  in which the level of the radiated noise energy is reduced to the level of the restricted EMI or less as shown in FIG. 2 can be considered as a pseudoground. No electromagnetic waves are radiated from the wire  22  which is screwed to the pseudogrounded housing  21 . 
     FIG. 6 is a perspective view showing the appearance of an embodiment of the housing to which the present invention is applied. Refer ring now to FIG. 6, energy of the noise radiated from the housing  61  is reduced by closing gaps in a housing  61  and by determining the dimensions of heat radiating holes according to Quine&#39;s formula and the intervals of screw pitches according to the theory of wave guide. All communication functional packages with metallic front plates  61  are loaded in the housing  61 , and communication lines  64  and connectors  63  are removed from the housing  61 . The communication lines are screwed to upper and lower screws of the metallic front plates  62 . This condition is schematically illustrated in FIG.  8 . 
     Any communication lines  806 , power supply lines  807 , interface lines  808  and other lines  809  leading from the housing  805  accommodating all packages therein are screwed to the housing  805  as shown in FIG.  8 . 
     If the energy of the radiated noise is not higher than the EMI restricted value under this condition, this housing  805  will serve as a pseudogrounded housing. 
     If all the communication lines  806 , power supply line  807 , interface line  808  and the other line  809  are removed from the housing  805  by unscrewing and the communication line circuit in which the level of the hoise energy is reduced to a level of the noise radiated from the pseudogrounded housing or less is radio-frequency connected with the pseudogrounded housing within the package ( 35  in FIG. 3) having a communication circuit and the like, no increase in the noise energy of the pseudogrounded housing would be occur. 
     FIG. 3 is a view illustrating the mode of embodying the present invention. FIG. 3 a  is a view schematically showing the structure of the housing and package. FIGS.  3 ( b ) and  3 ( c ) are views showing the characteristics of the radiated noise energy of the housing and packages, respectively. 
     In FIG. 3, the noise energy  40  of the pseudogrounded housing  33  assumes a minimum value B in the vicinity of a connector  361 . A communication line  37  is representative of all communication lines, power supply lines, interface lines leading from the pseudogrounded housing  33 . 
     Referring now to FIG. 3, the energy from an LSI  34  which is a noise source in the communication line circuit which is inserted into a BWB (backboard)  32  is reduced by means of a noise reducing component such as common mode choke coil  38  and the like so that the radiated noise energy  39  in the package is reduced to a level A at the communication line connector. In other words, the radiated noise energy is reduced in the package  35  until the radiated noise energy A of the connector  361  for the communication line circuit is reduced to a level or less of the noise energy B of the pseudogrounded housing  33  or less. 
     The communication lines  47  can be also pseudogrounded to comply the EMI restriction by radio-frequency connecting a communication line wiring pattern (section) between the common mode choke coil  48  of the communication line circuit and the connector  461  to a part of the housing (metallic front plates  62 ,  76  of the packages in FIGS. 6 and 7, respectively) in the vicinity of the connector  461  of the pseudogrounded housing  42  with the package  44  via capacitors  451 ,  452  having radio-frequency characteristics as shown in FIG.  4 . 
     Referring to FIG. 4, a step different at in the interface between the radiated noise energy  49  within the package and the noise energy  50  of the housing represents a difference between the energy A and B immediately before the connection of the capacitors  451  and  452  having radio frequency characteristics in the condition shown in FIG.  3 . 
     The energy transfer occurs from the noise energy  50  (level B in FIG.  3 ( c )) to the radiated noise energy  49  (level A in FIG.  3 ( b )) within the package after the connection of the capacitors  451 ,  452  having radio-frequency characteristics, so that the level of the noise energy  50  of the housing is reduced below the level B. 
     Radio-frequency connection of the communication line having a noise energy level lower than that of the housing with the pseudogrounded housing in such a manner makes the communication line equivalent to the wire  22  buried in the pseudo-grounded housing as shown in FIG.  2 . 
     The radiated noise energy A from the communication line is reduced below that B of the pseudogrounded housing. 
     The above-mentioned communication line is radio-frequency connected to the pseudogrounded housing. 
     Now, prior art method will be described for comparison. In the prior art method, the radiated noise energy from the communication line has been reduced, for example, by passing the radiated noise energy from the communication line to the housing  53  via Y capacitor  54  as shown in FIG.  5 . 
     The problem of the prior art resides in that the housing will be contaminated with a noise current flowing from the communication lines into the housing and when the energy of the noise current increases, the housing will serve as an antenna for radiating noise therefrom. 
     In brief, the metallic housing is a finite pseudoground unlike the earth which is an infinite ground. The amount of the absorbed energy which can be passed to the housing serving as the pseudoground is limited. 
     This problem will be described in more detail with reference to FIG.  7 . The metallic housing  71  is connected with the earth (E)  72  of the BWB via studs  81  at multiple points and is also connected to the earth of the PWB (printed wired board)  73  which is inserted into the backboard, so that all grounds are only one reference ground. Accordingly, a very small radiated noise current may flow on the surface of the housing  71 . 
     If the reduction in the radiated noise energy from the communication lines is insufficient, the radiated noise energy might be passed to the housing. In particular, if protection against EMI is conducted when a plurality (number) of packages are inserted into the housing, high radiated noise currents might flow from the communication. There occurs a problem that the housing may function as an antenna. The efficiency of EMI prevention becomes very low. 
     In the embodiment of the present invention, the capacitors  451 ,  452  for radio-frequency connection as shown in FIG. 4 play a role to make a radiated noise energy A of the communication line function package which is protected against EMI so that the energy A is lower than that of the pseudogrounded housing  42  equal to the noise energy B of the pseudogrounded housing  42 . This is based upon the theory which is reversal to a scheme that the radiated noise energy from the communication lines is passed to the housing  53  via the Y capacitor  54  in a conventional protection against EMI. 
     In other words, if the noise from a communication line circuit is for example in a common mode, the common mode noise is reduced by means of the common mode choke coil  38  which is shown in FIG.  3 . The radio-frequency connection capacitors ( 451 ,  452  in FIG. 4) which are seemed to play a role to bypass the common mode noise to the housing achieve the purpose which is totally opposite to that of the Y capacitor ( 54  in FIG. 5) for reducing the noise from the communication lines. 
     If the energy of the radiated noise of the communication lines is lower than that of the housing, it may be considered that the radio-frequency connection capacitors ( 451 ,  452  in FIG. 4) are unnecessary. However, these capacitors play an important role. 
     There are a number of kinds of packages having communication lines for each of a plurality of functions. Although the radiated noise energy of the communication line circuits which are protected against EMI in the packages is generally lower than that of the housing, the levels of the radiated noise energy of the communication lines are of course largely different due to difference in the package kinds and variation in parts. 
     If there are differences in the radiated noise energy on communication lines after the radiated noise energy has been reduced, radio-frequency currents might flow (like cross talk effect) due to differences in the levels of the radiated noise energy existing among a plurality of communication lines in bundled (and twisted or yarned) cables. 
     When a current I having a frequency of flows through a current loop having an area S in a X-Y plane in FIG. 9, the strength of the electromagnetic wave E at a point A which is far a distanced from an point 0 which is the center of the X-Y plane and the center of the current loop is generally represented by following formula: 
     
       
         E(V/m)=K×{S(m 2 )×I(A)×f 2 (Hz)/d(m)}×sinθ  (1) 
       
     
     where K is a constant and θ is an angle between a segment connecting points 0 with A and the Z-axis. 
     If the loop area S increases even when the current value I remains constant in the formula (1), the strength of the electromagnetic wave increases in proportional to S×I, so that the radiated noise energy increases. 
     In other words, the value S×I increases, so that the strength of radiated noise electromagnetic waves increases if a large current loop area S is formed by a large length of the cables due to the difference in the energy level among the communication lines of cables even if the current value I is low. 
     Accordingly, radio-frequency connecting capacitors are necessary for the purpose of equalizing the energy levels of all communication lines in the pseudogrounded housing so that there is no difference in the radiated noise energy among the communication lines if they are bundled as cables. 
     In the embodiment of the present invention, it is necessary that the position of the radio-frequency connection ( 451 ,  452  in FIG. 4) between the pseudogrounded housing and the communication lines be far by a distance from the point A, minimizing the strength E represented in FIG.  9  and equation (1) of the strength of the electromagnetic wave, that is far by a largest distance d  10  from the current loop (center) which is a radiated noise source and be on the side of the front plate ( 76  in FIG. 7) opposite to the backboard ( 72 ) which is on the noise source side. 
     It is important to radio-frequency connect the communication lines ( 78  in FIG. 7) to the side of the front plate ( 76  in FIG. 7) in which the radiated noise energy is least by fastening (e.g.,screwing) them in order not to radiate noise due to the communication lines functioning as antennae. 
     If the communication line is accidentally screw-fastened to the housing side close to the backboard having a high radiated noise energy of the housing, the communication line would serve as an antenna for radiating electromagnetic waves. An embodiment of the present invention will be described in more detail. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 6 is a view showing the appearance of an apparatus of one embodiment of the present invention. Communication circuit function packages (referred to as “package”) having metallic front plates  63  are fastened to the housing  61  by screws. The communication function packages having the metallic front plates  62  are provided with connectors  63  for connecting the packages to external indoor wirings  64 . 
     FIG. 7 is an elevational and sectional view taken along a line A—A as viewed in an arrow in FIG.  6 . In FIG. 7, the E (earth) of the back board (BWB)  72  is electrically connected to the studs  81  of the metallic housing  71  by means of screws. The communication line circuit function packages  73  are inserted into the connector  84  of the back board  72 . 
     A metallic front plate  76  is disposed in the side opposite to the back board  72  of the communication line circuit function package  73 . The metallic front plate  76  is electrically connected to the housing  71  by means of screws and also electrically connected to a solid wired pattern  75  in the package. 
     Communication wiring patterns  85 ,  86  lead from a communication line circuit block  80  on which an integrated circuit component such as LSI which is a noise source is mounted in the vicinity of the back board  72  of the communication line circuit function package  73  and in which the radiated noise energy is reduced by decreasing S, I and f in the formula (1) and increasing d. 
     If the common mode noise energy of the communication lines  85  and  86  is high, a common mode choke coil  87  is inserted between the communication lines  85  and  86 , and between  86  and  89 . 
     The communication line  88  is connected to a connector  722  and one end of the capacitor  74  having high frequency (radio-frequency) characteristics. The communication line  89  is connected to a connector  772  and one end of the capacitor  79  having radio-frequency characteristics. Both of capacitors  74  and  79  are electrically connected at the other ends thereof to the solid wired pattern  75 . 
     The twisted pair of indoor communication lines  78  is inserted into the connector  772  which is mounted on the package  73  by means of the connector  771 . 
     Another embodiment of the present invention will be described. In accordance with the present invention, the pseudo-ground is an earth  103  of the package  102  in the above-mentioned metallic housing as well as a molded (plastic) housing  101  as shown in FIG.  10 . The earth  103  of the package in which protection against EMI is achieved until the radiated noise energy from the earth  103  of the package  102  becomes not higher than EMI restricted value can be regarded as the pseudoground. 
     The distribution of the noise energy is not uniform, but in a spotted pattern also in the earth  103  of the package  102  which is the pseudoground. The radiated noise energy is minimal at the position where d in the electromagnetic wave formula (1) is high, that is at the position where the distance from the radiated noise source  104  such as LSI, etc. is furthest. It is the position (side)  109  in FIG. 10 where the communication line can be stably led out. 
     A phrase “pseudogrounded earth” means a position (part or area)  109  in which the radiated noise energy is minimal In this position, the pseudogrounded earth is radio-frequency connected to communication lines  110 A,  110 B through the capacitors  106 A,  106 B, respectively, having radio-frequency characteristics. 
     In other words, even in the molded housing, the earth (E) of the package is only required to be connected to the communication line at a given position in which the radiated noise energy is minimized. 
     Although a capacitor having radio-frequency characteristics is used as means for connecting the communication line with the pseudoground which is essential for pseudogrounding in the above-mentioned embodiment, it is to be noted that the present invention is not limited to such a structure. 
     For example, the capacitor may be formed by a distributed capacitance formed by a pattern wiring technique which requires to consider no break-down voltage as shown in FIGS.  11 ( a ) and  11 ( b ). In this case, a cheaper protection against EMI can be achieved. 
     Referring now to FIGS.  11 ( a ) and  11 ( b ), the communication line  115  is connected to the indoor communication line  114  via the connector  113 . Since a two-layered printed board is assumed, first and second layers comprise a pattern wiring of the communication line  115  and a solid wired pattern  112  which is connected to an earth (E) position of the pseudogrounded metallic housing or molded housing, respectively. 
     Radio-frequency connection between the communication pattern  115  and the solid wired pattern  112  can be achieved by the distributed capacitance (i.e., interlayer capacitance) formed by the thickness of the two-layered substrate or the interlayer thickness of the multiple-layered substrate. 
     Since radio-frequency connection for pseudogrounding can be achieved by a capacitor of 5 pF or less and deterioration of the device function performance for protection against EMI should be avoided, a capacitor having a capacitance of 5 pF to 10 pF is generally used. Accordingly, radio-frequency connection using distributed capacitance can be conducted by providing upper and lower layers having a thickness of several micrometers between layers of a PWB, with a communication line and pseudogrounding layer, respectively, by a pattern design technique. 
     The meritorious effects of the present invention are summarized as follows. 
     As mentioned above, all plural communication lines, interface lines and power supply lines can be pseudogrounded by radio-frequency connecting the communication lines, interface lines and power supply lines to a pseudogrounded housing in accordance with the present invention. Therefore, distinct advantages are obtained that no electromagnetic waves exceeding the EMI restricted value are radiated and that economical, excellent and stable protection against EMI can be achieved. 
     In other words, in accordance with the present invention a stable protection against EMI can be achieved only by subjecting communication devices in building to the inventive pseudo-grounding without necessity of adopting any expensive ferrite core, shielded cables and working of standard communication lines (non-shielded telephone lines, interface lines, power supply lines which can not be shielded for the reason of safety standards) which are wired indoors simultaneously with the construction of building. The practical value of the present invention is very high. 
     It should be noted that other objects of the present invention will become apparent in the entire disclosure and that modifications may be done without departing the gist and scope of the present invention as disclosed herein and appended herewith. 
     Also it should be noted that any combination of the disclosed and/or claimed elements, matters and/or items may fall under the modifications aforementioned.