Fiber optic electric switch
BACKGROUND OF THE INVENTION 
     Optical switching is known in the art wherein a lamp and a photodetector are mounted in a switch housing and a shutter is driven therebetween to block the transmission of light. Appropriate cavities may be formed in the housing for mounting the lamp and photodetector therein, and appropriate apertures are formed in certain internal portions of the housing to form light guides. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a fiber optic electric switch wherein fiber optic cables form the lightguide means. 
     Another object is to provide extremely accurate axial and lateral positioning of facing spaced fiber optic cable ends to minimize transmission losses in the gap therebetween while also affording a narrow elongated channel intersecting said gap for guiding a shutter to be driven between the cable ends. This object is fulfilled by a single mounting means which accomplishes both purposes by performing the dual functions of mounting the facing cable ends in spaced substantially colinear relation and forming a narrow elongated guiding channel perpendicular to the colinear axis of and intersecting the gap between the facing cable ends. It is because the same means performs both functions that the high degree of accuracy in cable positioning is attained to best minimize tolerance deviations in the combination of both axial gap and lateral displacement. 
     Another object is to provide a switch of the aforementioned character wherein the mounting means is a preassembled self-contained module insertable in standard switch enclosures to enable simple and economical adaptation of an electric switch to a fiber optic switch with a minimum of additional or modified parts, and to minimize tolerance deviations in the positioning of the facing cable ends since they are axially and laterally fixed before insertion into the switch housing. 
     Another object is to provide a switch of the aforementioned character which is easily convertible between normally open and normally closed. 
     Another object is to provide a switch of the aforementioned character for controlling a remote electrical switching function whereby to afford use of the present invention in intrinsic safety applications because of the absence of electric current, and also to afford electrical isolation from an electrically noisy environment where magnetic disturbances, etc., may be present. 
     Other objects and advantages will hereinafter appear. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of a limit switch constructed in accordance with the invention. 
     FIG. 2 is an enlarged cross-sectional view of FIG. 1 with the operating head removed. 
     FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 2. 
     FIG. 4 is a cross-sectional view taken along line 4--4 of FIG. 2. 
     FIGS. 5a through 5h show an exploded isometric view of the fiber optic module used in the switch of FIGS. 1 to 4. 
     FIG. 6 is a side elevational view of a pushbutton switch constructed in accordance with the invention. 
     FIG. 7 is an enlarged bottom view of the switch of FIG. 6. 
     FIG. 8 is a cross-sectional view taken along line 8--8 of FIG. 7 with the operating head removed. 
     FIG. 9 is an exploded isometric view of the fiber optic module used in the switch of FIGS. 6-8. 
     FIG. 10 is a schematic circuit diagram showing a converter circuit to provide an electrical signal in response to an optical signal. 
     FIGS. 11-13 illustrate various terms used in graphs and tables herein. 
     FIG. 14 is a graph showing the effect upon transmission loss of lateral displacement of the facing cable ends. 
     FIG. 15 is a graph showing the effect upon transmission loss of the axial gap between the facing cable ends. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1 to 5 show a limit switch constructed in accordance with the invention and FIGS. 6 to 9 show a pushbutton switch constructed in accordance with the invention. The limit switch will be described first. 
     The limit switch 10 is like those disclosed in U.S. Pat. Nos. 3,983,343; 3,882,290; 3,819,888; and 3,793,492. Operating head 12 is mounted to housing 14 by four screws 16. The operating head is provided with an operating arm 18 having a roller 20 at its swingable end for engagement by a moving machine part or the like that moves arm 18, hence operating switch 10. Arm 18 is mounted to shaft 22, rotation of which causes right-left movement of plunger 24, FIG. 2, which in turn moves through an overcenter position of a snap-action contacting mechanism to cause left or right snap-action movement of bridging contactor 26 against contacts such as 28 or 30, all as described in more detail in said Patents, which are hereby incorporated herein for further reference. 
     Housing 14 has an upper portion 14a mounted to a lower portion 14b by screws 32. A sealing gasket 15 is provided around the perimeter between the upper and lower housing portions. Upper portion 14a has an insulating enclosure 34 enclosing the snap-action contacting mechanism. This enclosure has an opening 34a in a right side wall through which plunger 24 extends, and an opening 34b in a bottom wall 34c through which a reduced end portion 36a of a lever 36, FIGS. 2 and 5e, extends upwardly to engage contactor 26. Contactor 26 and the snap-action mechanism in upper housing portion 14a are not used to make and break an electrical circuit, but rather for the mechanical snap-action movement provided thereby to impart motion to lever 36. Contacts 28 and 30 are not connected to an electrical circuit but are retained to provide stops for the left-right movement of contactor 26. It is thus seen that the actuating mechanism of prior limit switches, such as shown in said Patents, may be used in the present invention to drive lever 36, thus reducing cost by using existing standard parts. 
     Mounted to the underside of bottom wall 34c is a retaining plate 38 having a central horizontally elongated rectangular groove portion 38a, FIG. 5a, which is recessed downwardly into open-topped lower housing portion 14b. A curved portion 36b of the lever rests against the recessed top surface of groove portion 38a to afford rocking of the lever thereabout. As seen in FIG. 2, rightward movement of contactor 26 pivots lever 36 clockwise; and leftward movement of contactor 26 allows lever 36 to pivot counterclockwise under the influence of return bias means, to be described hereinafter. 
     Lower housing portion 14b is comparable to that shown in said Patents and has an open top and a threaded aperture 14c in the left side wall, FIG. 2. Mounted in a lower housing is a fiber optic module 40 which is retained therein by spring clips 42, FIG. 4, mounted to bottom wall 14d by stud 43. The module rests against spring 44, FIG. 2, which is also mounted to bottom wall 14d by stud 43. 
     Standard commercially available connectors 54, 56 and 58, 60, and hex nuts 55 and 57, FIGS. 3 and 5c, are mounted to module 40 to optically couple fiber optic cables 46, 48 to cables 50, 52, respectively. Though singular cables are shown, it is within the scope of the invention to use other types of fiber optic light guiding cable means, for example multiple fiber bundles. Each connector threadingly engages its made and a respective hex nut, the latter being snugly retained in the module by appropriate shouldered slots formed therein to prevent rotation of the nut. The connectors typically have a have a frusto-conical male portion received in a mating frusto-conical female portion of another connector to afford axial alignment of the cables coupled thereby. Cables 46 and 48 are cladded and extend from outside the housing through aperture 14c to connectors 54 and 58, respectively. Cables 50 and 52 extend rightwardly from connectors 56 and 60, respectively, and then are curved inwardly towards each other, terminating to form facing ends, 50a and 52a, defining a small gap therebetween. 
     The right side of the module has a vertical channel 40a formed therethrough, FIGS. 4 and 5c. On opposite sides of this channel, on top surfaces of the module, there are formed a pair of V-shaped grooves 40b and 40c, substantially colinearly aligned. These grooves hold fiber optic cables 50 and 52. A pair of retainer members 45 and 47, FIG. 5g, are mounted to the module by screws 49 and 51, FIG. 3, extending through apertures 45a, 40d and 47a, 40e, respectively. These retainer members have portions 45b and 47b which extend downwardly into a widened section 40g of channel 40a which is to the left of the gap between grooves 40b and 40c. In assembled condition, the retainer members thus provide a narrow section 40k of channel 40a between portions 45b and 47b. The retainer members have shouldered recesses 45c and 47c, formed on the underside of a mid portion thereof which receive wedges 59 and 61, respectively, FIG. 5f. These wedges are made of resilient material and, in assembled condition, the undersides thereof press cables 50 and 52 into the V-shaped grooves 40b and 40c to provide firm retention of facing ends 50a and 52a in spaced coaxial relation. 
     A thin opaque shutter 62, FIG. 5h, is vertically reciprocal in channel 40a. FIG. 2 shows a normally closed switch wherein the shutter is normally in an up position whereby an optical path is established between facing cable ends 50a and 52a through slot 62a of the shutter. Downward movement of the shutter brings portion 62b into the gap between the cable ends to thus interrupt the optical path therebetween. 
     The shutter is retained and guided in channel 40a by means of leaf spring 64, FIG. 5d, and a rigid template 66, FIG. 5b. The leaf spring is mounted at its left end to the underside of the module by screw 68, FIG. 2, extending through aperture 64b. The leaf spring extends rightwardly through slot 40h beneath channel 40a to engage rounded bottom portion 62c of the shutter and bias the latter upwardly. The leaf spring has an aperture 64a near its right end through which a downwardly projecting portion 62d of the shutter extends to prevent lateral movement of the shutter. Template 66 is mounted to the topside of the module by means of screw 70 extending through aperture 66b into threaded opening 40i. The template extends rightwardly to overly channel 40a and has an aperture 66a to allow clearance therethrough of upper portions 62f and 62g of the shutter and to prevent lateral movement thereof. The template stops the upwardly biased movement of the shutter when shoulders 62e strike the underside of the template overhanging the channel 40a. Left side 62h of the shutter rides in section 40f of the channel. The vertical midportion of the shutter rides in section 40k of the channel formed between portions 45b and 47b of the retainer members. The right side of the shutter, 62a and 62b, rides in the gap between the V-shaped grooves 40b and 40c holding the fiber optic cable ends. 
     Notches 66c and 66d are formed in the template through which retainer member portions 45d and 47d, respectively, extend upwardly to terminate adjacent the bottom of the upper Notches 66e and 66f allow clearance of the points of hex nuts 55 and 57, respectively. Raised portion 40j of the module extends upwardly through notch 66g to position the template. 
     Lever 36 has a straight portion 36c extending rightwardly from curved portion 36b, FIGS. 2 and 5e, beyond rectangular groove 38a of retaining plate 38. The underside of the right end of lever portion 36c engages rounded upper portion 62f of the shutter to drive the shutter downwardly against the bias of leaf spring 64 in response to clockwise rocking or pivoting of lever 36 about curved portion 36b resting in groove 38a. Leaf spring 64 biases the shutter upwardly which in turn biases lever 36 counterclockwise whereby reduced end portion 36a is biased against contactor 26, FIG. 2. Portion 36c of the lever has an aperture 36d near its right end through which upwardly projecting portion 62g of the shutter extends. 
     It should be noted that, except for portions 62a and 62b, the shutter is symmetrical about a horizontal left-right axis, FIG. 5h, whereby to afford convertibility from a normally closed to a normally open switch merely by 180° rotation of the shutter. A normally open switch would thus have portion 62b of the shutter blocking the optical path between the facing fiber optic cable ends 50a and 52a when the shutter is in its normal upwardly biased position; downward movement of the shutter, caused by rightward movement of contactor 26, FIG. 2, thus brings slot 62a into the gap between the facing fiber optic cable ends, re-establishing the optical path therebetween. 
     It is thus seen that a compact self-contained fiber optic module is provided which may be inserted in a standard switch housing to provide optical switching. The module not only mounts facing fiber optic cable ends in spaced coaxial relation but also provides a narrow channel in a plane perpendicular to the axis of the cables and intersecting the gap therebetween for guiding a movable shutter. 
     FIGS. 6 to 9 disclose a pushbutton switch constructed in accordance with the invention. A plunger housing assembly 80 is provided like that shown in U.S. Pat. No. 2,930,859 hereby incorporated by reference, except that the plunger and contacts in the left compartment of the housing 82, FIG. 8, have been replaced by a fiber optic module 84. Screws 92 mount the plunger housing assembly 80 to the face plate 90a of a pushbutton operating head 90, such as shown in U.S. Pat. Nos. 2,930,859; 2,908,776; 2,568,933; and 2,460,034. Numerous other means could be used to depress plunger 86, FIG. 8, for example rotary selectors such as shown in U.S. Pat. Nos. 2,878,368 and 2,357,971, or directly by the user. 
     Plunger 86 is biased upwardly by a frusto-conical spring 88, FIG. 8, bearing between the underside of the plunger and a bottom plate 94 which is mounted to the housing 82 by screws 96 and 98, FIG. 7. The bottom wall 94 has an aperture 94a through which a second plunger could upwardly extend from a second plunger housing assembly piggybacked therebelow, the second plunger being engaged by downwardly projecting portion 86a of plunger 86 in response to downward movement of plunger 86. 
     The left compartment of housing 82 has a closed top wall 82a, FIG. 8, and is open-bottomed for insertion of fiber optic module 84 from below. The module is mounted by screws 100 and 102, FIG. 7, extending through apertures 84c and 84d in wings 84e and 84f, respectively, of the module, FIG. 9. Front and back walls of the left compartment of housing 82 have upwardly extending slots 82b and 82c to allow clearance therethrough for cladded fiber optic cables 104 and 106. A dividing wall 82d, FIG. 8, between the compartments of the housing has an aperture 82e to allow vertical reciprocal movement of a tongue portion 108a of a shutter 108, FIGS. 8 and 9. This tongue portion fits in a notch 86b in plunger 86 to provide a direct mechanical connection therebetween. 
     The fiber optic module comprises two identical halves 84a and 84b assembled together by ultrasonic bonding and aligned by mating bosses 84g and apertures 84h. When assembled, the module has a general T-shape in top or bottom elevation, FIG. 7, with wing portions 84e and 84f forming the ends of the cross-bar of the T, and portion 84i forming the trunk portion. This trunk portion is vertically planar. Each half of the module has an externally threaded female fiber optic connector portion, 84p and 84q, formed therewith. Each half has a frusto-conical aperture, 84r and 84s, formed through the connector portion. Since the halves are identical, the apertures are aligned when the halves are assembled, with the smallest parts of the apertures being internal. Each half has a generally coplanar recessed shallow cavity, such as 84k, formed therein to thus provide an internal cavity coplanar with trunk portion 84i when assembled. 
     Shutter 108 is an integral member having a general T-shape in top or bottom elevation, with cross-bar portion 108b extending in the plane of the cross-bar portion of the T-shaped module 84, and with trunk portion 108c extending in the plane of the trunk portion of the T-shaped module 84. Tongue 108a extends rightwardly, FIG. 9, from cross-bar portion 108b. Deeper recesses 84m, FIG. 9, are formed in the module halves at the right ends of recesses 84k to thus provide a T-shaped internal cavity, in top view, in module 84 whereby to provide a guiding channel for vertical reciprocal movement of the shutter and prevent lateral movement thereof; cross-bar portion 108b being guided in recesses 84m, and trunk portion 108c in recesses 84k. Tongue portion 108a extends rightwardly from crossbar portion 108b through notches 84n in the right side of each half forming an aperture when assembled. 
     Fiber optic cables 104 and 106 are inserted and friction fitted into connector portions 84p and 84q, respectively, and secured therein by caps 114 and 116, respectively, screwed therearound. The facing ends of the cable may extend slightly into the internal cavity or guiding channel in the module and are coaxially aligned by the connector portions and apertures 84r and 84s. Trunk portion 108c has an aperture 108d formed therethrough, and a reduced width opaque portion 108e above the aperture. The width of portion 108e is less than the width of the gap between the facing fiber optic cable ends and is vertically aligned therewith. The width of trunk portion 108c may or may not be less than the width of the gap between the cable ends; this allows the width of portion 108c to be sized large enough to provide sufficient strength and rigidity thereof and to provide a snugger, though freely slidable, fit in the guiding channel in the module, thus assuring consistent vertical motion of the shutter without canting, flexing, etc. This is important for precise interruption of the optical path between the cable ends by portion 108e. 
     FIGS. 6 to 9 show a normally closed switch wherein plunger 86 is biased to a normal up position whereby an optical path is established through shutter aperture 108d between the facing ends of fiber optic cables 104 and 106. Downward movement of the plunger moves the shutter downwardly such that opaque reduced portion 108e interrupts the optical path between the cable ends. The switch can be easily converted to a normally open switch by rotating module 84 180° about a horizontal left-right axis, FIGS. 8 and 9, since the module is symmetrical about said axis. With the plunger up, the light path is blocked by portion 108e; downward movement of the plunger against the bias of spring 88 moves aperture 108d into the gap between the facing fiber optic cable ends to thus permit optical transmission therebetween. 
     FIG. 10 shows a converter circuit which could be used with the invention comprising a system for generating a train of light pulses and detection circuitry for detecting these received pulses to enable an isolated triac switch. 
     Power to LED (light emitting diode) driver 200 is derived from a zener regulated supply 202 operating from a 120 V AC source. LED 204 is driven at 2A for 2 microseconds, at a repetition rate of 1.8 kHz. The end of the fiber optics cable 206 is placed as close as practical to the LED chip. Received light from fiber optics cable 208 is coupled into photo diode 210 and the signal is amplified in a preamplifier 212. If the signal current through R1 is adequate to overcome the threshold current through R2, driver 214 is activated. However, the driver can only deliver current into the integrating network 216 during the period when enabled by the LED pulser circuit 200. When sufficient voltage has been accumulated across the integrating circuit to cause the output of gate 218 to go from high to low, a gated oscillator 220 is enabled. This signal is coupled via isolation transformer 222 into the gate of a triac 224 and causes it to become conductive. A dv/dt suppression circuit 226 may also be used. An LED indicator 228 and driver 230 may be added at the output of gate 218 if desired. It is preferred that light in the visible red (660 nanometers) region of the spectrum be transmitted through the fiber optic cables, through electromagnetic waves of other wavelengths may of course be used. 
     The converter circuit is not a constraint of the invention, but rather is included herein for completeness of disclosure. There are numerous types of well known conversion circuitry which may be used with the present invention. 
     As aforenoted, the module enables accurate axial and lateral positioning of the facing cable ends to minimize transmission losses in the gap therebetween. The following tables indicate transmission loss data achieved with the present invention. 
     Table I shows dB losses for various gaps G and lateral displacement L. G is the axial distance between the facing cable ends, i.e. the width of the gap therebetween, and L is the lateral displacement between the central axes of the cables, as shown in FIGS. 11 and 12. The dB loss is defined in terms of a ratio of power output to power input: ##EQU1## where P out is the output power in watts and P in is the input power in watts. Glass cables were used having a length of 1 meter and each having a core diameter of 200 micrometers. The cable ends had a numerical aperture of 0.27, where numerical aperture is defined as sin θ where θ is defined as the angle between the outermost edge of the dispersed cone of light and straight-through transmission, as shown in FIG. 13. The top row of the table gives G in inches and the left column gives L in inches. The loss is given in dB and is entered in the respective row and column corresponding to L and D. For example, a gap of 0.025 inch and a lateral displacement of 0.003 inch produce a loss of 5.9 dB. 
     Table II is like Table I except that cables with plastic cores are used, having a diameter of 400 micrometers. The plastic core end has a numerical aperture of 0.33. 
     Table III shows the results obtained when light is transmitted from a 200 micrometer diameter glass core to a 400 micrometer diameter plastic core. 
     Table IV is like Table III except that the direction of light transmission is reversed, i.e. light travels from the larger diametered core to the small diametered core. 
     FIG. 14 graphically illustrates the effect upon dB loss of lateral displacement of the cables by plotting dB loss against the ratio L/D where D is the core diameter of the receiving cable, and assuming a minimum axial gap between the facing ends of the cables. Curve A represents light transmission from a 200 micrometer diameter cable to a 200 micrometer diameter cable; dB loss is calculated from the relation: ##EQU2## This relation is valid for sending and receiving cables of equal core diameter. The theoretical curve A is found to fit very well with the empirical data recorded therefor. Curve B is an empirical curve for transmission from a 400 micrometer diameter cable to a 200 micrometer diameter cable. 
     
                                           TABLE I                                 
__________________________________________________________________________
L  0   .005                                                               
           .010                                                           
               .015                                                       
                   .025                                                   
                       .035                                               
                           .055                                           
                               .075                                       
                                   .115                                   
__________________________________________________________________________
00 0   -.6 - 1.6                                                          
               - 2.8                                                      
                   - 5.3                                                  
                       - 7.5                                              
                           -10.9                                          
                               --13.6                                     
                                   -17.1                                  
.001                                                                      
   - .4                                                                   
       - .9                                                               
           - 1.9                                                          
               - 2.9                                                      
                   - 5.3                                                  
                       - 7.5                                              
                           -10.9                                          
                               -13.8                                      
                                   -17.1                                  
.002                                                                      
   - 1.5                                                                  
       - 1.8                                                              
           - 2.5                                                          
               - 3.3                                                      
                   - 5.6                                                  
                       - 7.5                                              
                           -10.9                                          
                               -13.8                                      
                                   -17.1                                  
.003                                                                      
   - 2.9                                                                  
       - 3.1                                                              
           - 3.3                                                          
               - 4.1                                                      
                   - 5.9                                                  
                       - 7.5                                              
                           -10.9                                          
                               -13.8                                      
                                   -17.1                                  
.004                                                                      
   - 4.0                                                                  
       - 4.7                                                              
           - 5.3                                                          
               - 5.3                                                      
                   - 6.7                                                  
                       - 8.6                                              
                           -10.8                                          
                               -13.8                                      
                                   -17.1                                  
.005                                                                      
   - 6.3                                                                  
       - 7.1                                                              
           - 7.1                                                          
               - 7.1                                                      
                   - 8.0                                                  
                       - 8.6                                              
                           -11.1                                          
                               -13.8                                      
                                   -17.1                                  
.006                                                                      
   -11.0                                                                  
       -10.1                                                              
           - 9.3                                                          
               - 8.6                                                      
                   - 9.3                                                  
                       - 9.3                                              
                           -11.5                                          
                               - 13.8                                     
                                   -17.5                                  
.007                                                                      
   -17.0                                                                  
       -14.0                                                              
           -12.3                                                          
               -11.1                                                      
                   -10.0                                                  
                       -10.1                                              
                           -11.8                                          
                               -13.8                                      
                                   -17.5                                  
.008                                                                      
   00      -17.0                                                          
               -14.1                                                      
                   -11.0                                                  
                       -11.0                                              
                           -12.5                                          
                               -14.0                                      
                                   -17.5                                  
.009           -17.0                                                      
                   -14.0                                                  
                       -12.3                                              
                           -12.7                                          
                               -14.3                                      
                                   -17.5                                  
.010               -17.0                                                  
                       -14.0                                              
                           -13.1                                          
                               -14.5                                      
                                   -17.5                                  
.011                   -14.0                                              
                           -13.5                                          
                               -14.8                                      
                                   -17.5                                  
.012                   -17.0                                              
                           -14.0                                          
                               -15.0                                      
                                   -17.5                                  
.013                       -14.7                                          
                               -15.3                                      
                                   -18.0                                  
.014                       -15.6                                          
                               -15.9                                      
                                   -18.0                                  
.015                       -16.6                                          
                               -16.2                                      
                                   -18.6                                  
.016                       -18.0                                          
                               -16.7                                      
                                   -18.6                                  
.017                       -19.2                                          
                               -17.0                                      
                                   -18.6                                  
.018                       -21.0                                          
                               -18.0                                      
                                   -19.3                                  
.019                       -22.3                                          
                               -19.2                                      
                                   -19.3                                  
.020                       -31.0                                          
                               -20.0                                      
                                   -19.3                                  
__________________________________________________________________________
 
    
     
                                           TABLE II                                
__________________________________________________________________________
L  0   .005                                                               
           .010                                                           
               .015                                                       
                   .020                                                   
                       .030                                               
                           .040                                           
                               .060                                       
                                   .080                                   
                                       .100                               
__________________________________________________________________________
0  -.0 - .55                                                              
           -1.05                                                          
               - 1.60                                                     
                   - 2.43                                                 
                       - 3.67                                             
                           - 5.1                                          
                               7 7.97                                     
                                   - 9.6                                  
                                       --                                 
.001                                                                      
   - .10                                                                  
       - .69                                                              
           - 1.05                                                         
               - 1.76                                                     
                   - 2.61                                                 
                       - 3.67                                             
.002                                                                      
   - .43                                                                  
       - .92                                                              
           - 1.18                                                         
               - 1.91                                                     
                   - 2.80                                                 
                       - 3.90                                             
.003                                                                      
   - .79                                                                  
       - 1.19                                                             
           - 1.46                                                         
               - 2.25                                                     
                   - 3.01                                                 
                       - 3.90                                             
.004                                                                      
   - 1.18                                                                 
       - 1.46                                                             
           - 1.76                                                         
               - 2.43                                                     
                   - 3.22                                                 
                       - 4.19                                             
                           - 6.23                                         
.005                                                                      
   - 1.76                                                                 
       - 3.05                                                             
           - 2.25                                                         
               - 2.80                                                     
                   - 3.44                                                 
                       - 4.47                                             
                           - 6.40                                         
                               - 8.10                                     
                                   -10.4                                  
                                       -12.2                              
.006                                                                      
   - 2.43                                                                 
       - 2.60                                                             
           - 2.80                                                         
               - 3.44                                                     
                   - 3.90                                                 
                       - 4.77                                             
.007                                                                      
   - 3.00                                                                 
       - 3.22                                                             
           - 3.44                                                         
               - 3.92                                                     
                   - 4.17                                                 
                       - 5.09                                             
.008                                                                      
   - 3.92                                                                 
       - 3.92                                                             
           - 3.90                                                         
               - 4.47                                                     
                   - 4.77                                                 
                       - 5.44                                             
.009                                                                      
   - 5.09                                                                 
       - 4.77                                                             
           - 5.10                                                         
               - 5.44                                                     
                   - 5.80                                                 
                       - 6.23                                             
.010                                                                      
   - 6.45                                                                 
       - 6.23                                                             
           - 6.23                                                         
               - 6.69                                                     
                   - 6.70                                                 
                       - 6.69                                             
                           - 8.23                                         
                               - 8.9                                      
                                   -10.66                                 
                                       -13.4                              
.011                                                                      
   - 8.10                                                                 
       - 7.78                                                             
           - 7.31                                                         
               - 7.78                                                     
                   - 7.20                                                 
                       - 7.78                                             
.012                                                                      
   - 9.60                                                                 
       - 9.24                                                             
           - 8.23                                                         
               - 8.45                                                     
                   - 8.45                                                 
                       - 8.45                                             
.013                                                                      
   -21.4                                                                  
       -13.2                                                              
           - 9.51                                                         
               -10.7                                                      
                   - 9.24                                                 
                       - 9.24                                             
.014                                                                      
   -24.2                                                                  
       -16.2                                                              
           -21.1                                                          
               -21.17                                                     
                   -10.20                                                 
                       -10.2                                              
.015                                                                      
   -29.0   -24.2                                                          
               -23.0                                                      
                   -11.0                                                  
                       -11.00                                             
                           -10.5                                          
                               -10.5                                      
                                   -11.0                                  
                                       -13.2                              
.016       -26.6                                                          
               -35.0                                                      
                   -13.2                                                  
                       -13.2                                              
__________________________________________________________________________
 
    
     
                                           TABLE III                               
__________________________________________________________________________
L  0   .002                                                               
           .005                                                           
               .010                                                       
                   .015                                                   
                       .020                                               
                           .030                                           
                               .040                                       
                                   .060                                   
                                       .080                               
__________________________________________________________________________
0  -.15                                                                   
       - .15                                                              
           - .23                                                          
               - .31                                                      
                   - .65                                                  
                       - 1.12                                             
                           -1.76                                          
                               -3.08                                      
                                   -5.51                                  
                                       -7.55                              
.001                                                                      
   0   0   - .23                                                          
               - .39                                                      
                   - .66                                                  
                       - 1.32                                             
                           -2.11                                          
.002                                                                      
   - .07                                                                  
       - .24                                                              
           - .24                                                          
               - .57                                                      
                   - .83                                                  
                       - 1.65                                             
                           -2.37                                          
.003                                                                      
   - .23                                                                  
       - .41                                                              
           - .57                                                          
               - .93                                                      
                   - 1.12                                                 
                       - 2.24                                             
                           -2.78                                          
.004                                                                      
   - .60                                                                  
       - .96                                                              
           - 1.12                                                         
               - 1.53                                                     
                   - 1.65                                                 
                       - 2.94                                             
                           -3.40                                          
.005                                                                      
   -1.64                                                                  
       - 1.84                                                             
           - 1.88                                                         
               - 2.37                                                     
                   - 2.11                                                 
                       - 3.57                                             
                           -3.75                                          
                               -4.77                                      
                                   - 6.42                                 
                                       - 8.50                             
.006                                                                      
   -2.64                                                                  
       - 2.78                                                             
           - 2.94                                                         
               - 2.40                                                     
                   - 2.43                                                 
                       - 4.33                                             
                           -4.77                                          
.007                                                                      
   -4.33                                                                  
       - 4.39                                                             
           -  4.54                                                        
               - 5.00                                                     
                   - 5.00                                                 
                       - 5.51                                             
                           -5.25                                          
.008                                                                      
   -6.40                                                                  
       - 6.61                                                             
           - 6.40                                                         
               - 6.40                                                     
                   - 6.40                                                 
                       - 6.41                                             
                           -6.09                                          
.009                                                                      
   -9.77                                                                  
       - 8.95                                                             
           - 9.10                                                         
               - 8.52                                                     
                   - 9.24                                                 
                       - 17.7                                             
                           -7.56                                          
.010   -14.3   -11.52                                                     
                   -12.1                                                  
                       -19.4                                              
                           -8.52                                          
                               -8.01                                      
                                   -10.6                                  
                                       -10.6                              
.011   -22.6                                                              
           -18.5   -12.9   -9.77                                          
__________________________________________________________________________
 
    
     
                                           TABLE IV                                
__________________________________________________________________________
L  0   .005                                                               
           .010                                                           
               .015                                                       
                   .020                                                   
                       .030                                               
                           .035                                           
                               .040                                       
                                  .060                                    
                                      .100                                
                                          .140                            
__________________________________________________________________________
0  0   -.23                                                               
           -.48                                                           
               -1.3                                                       
                   -2.00                                                  
                       -2.8                                               
                           -4.0                                           
                               -5.5                                       
                                  -7.0                                    
                                      -12.7                               
                                          -16.7                           
.001                                                                      
   0   -.74                                                               
           -1.3                                                           
               -1.3                                                       
                   -2.00                                                  
                       -3.2                                               
                           -4.1   -7.2                                    
.002                                                                      
   -.23                                                                   
       -.74                                                               
           -1.3                                                           
               -1.3                                                       
                   -2.4                                                   
                       -3.2                                               
                           -4.3   -7.2                                    
.003                                                                      
   -.48                                                                   
       -.74                                                               
           -1.6                                                           
               -2.0                                                       
                   -2.4                                                   
                       -3.2                                               
                           -4.5   -7.4                                    
.004                                                                      
   -.75                                                                   
       -1.0                                                               
           -1.6                                                           
               -2.4                                                       
                   -3.2                                                   
                       -3.8                                               
                           -4.9   -7.6                                    
.005                                                                      
   -1.6                                                                   
       -1.6                                                               
           -2.4                                                           
               -3.2                                                       
                   -3.8                                                   
                       -4.3                                               
                           -5.3   -7.7                                    
.006                                                                      
   -2.4                                                                   
       -2.4                                                               
           -2.4                                                           
               -3.8                                                       
                   -4.3                                                   
                       -5.0                                               
                           -5.8   -8.0                                    
.007                                                                      
   -4.3                                                                   
       -2.8                                                               
           -3.8                                                           
               -5.0                                                       
                   -5.0                                                   
                       -5.8                                               
                           -6.3   -8.2                                    
.008                                                                      
   -5.0                                                                   
       -3.8                                                               
           -4.3                                                           
               -5.8                                                       
                   -5.8                                                   
                       -5.8                                               
                           -6.8   -8.5                                    
.009                                                                      
   -8.0                                                                   
       -6.0                                                               
           -5.8                                                           
               -6.8                                                       
                   -6.8                                                   
                       -6.8                                               
                           -7.3   -8.8                                    
.010                                                                      
   -12.7                                                                  
       -6.8                                                               
           -6.8                                                           
               -8.0                                                       
                   -8.0                                                   
                       -8.0                                               
                           -8.0   -9.2                                    
.011   -9.8                                                               
           -8.1                                                           
               -9.8                                                       
                   -9.8                                                   
                       -9.8                                               
                           -8.8   -9.6                                    
.012   -12.8                                                              
           -12.8                                                          
               -12.8                                                      
                   -12.8                                                  
                       -10.4                                              
                           -9.8   -10.0                                   
.013       -12.8                                                          
               -12.4                                                      
                   -12.8                                                  
                       -14.3                                              
                           -10.7  -10.5                                   
.014                   -15.8                                              
                           -12.4  -11.3                                   
.015                       -13.8  -12.0                                   
.016                       -15.8  -13.2                                   
.017                       -16.8  -13.8                                   
.018                       -19.8  -15.0                                   
.019                              -15.8                                   
.020                              -16.8                                   
.021                              -18.0                                   
.022                              -19.8                                   
.023                              -22.8                                   
__________________________________________________________________________
 
    
     FIG. 15 graphically illustrates the effect upon dB loss of the width of the gap between the facing cable ends by plotting dB loss against G, and assuming no lateral displacement. Curve C depicts transmission from a 125 micrometer diameter cable to a 125 micrometer diameter cable and curve E depicts transmission from a 200 micrometer diameter cable to a 200 micrometer diameter cable. 
     The tables and graphs cumulatively teach optimum combinations of axial gaps and lateral displacement for given applications to obtain the lowest dB losses. It is of course desirable to minimize both the axial gap G and the lateral displacement L, but this may not always be possible in view of shutter width, etc. Given the maximum dB loss which can be tolerated by a designated converter circuit, one can determine the allowable tolerance deviations in G and L for the particular application. For example, the converter circuit in FIG. 10 can tolerate a maximum dB loss of about -28 and still respond, thus establishing upper limits on G and L. Furthermore, one can determine which combinations of G and L will produce the best results, for example if a wide gap is desired in order to accommodate a thick shutter, then one can determine how small L must be in order to not exceed a given dB loss. It can thus be determined how close the tolerances must be held. 
     It has been found that G and L can best be minimized by using a single module to both mount the facing cable ends and form the guiding channel therebetween. By providing a single module which performs both functions, optimum values for the combination of G and L can be obtained, thus affording low dB losses. Furthermore, since the module is a preassembled self-contained member distinct from the switching housing, tolerances can be held to close limits because L and G are fixed before insertion into the switch housing. 
     It is recognized that various modifications of the present invention are possible within the scope of the appended claims.