Abstract:
An insulated Bayonet Neill Concelman (BNC) connector includes a replaceable non-metallic insulating shell. The non-metallic shell is attached to a metallic portion of the BNC connector such that, when mounted on an oscilloscope, or other electronic instrument, the non-metallic shell may be disconnected from the metallic portion of the BNC connector without having to gain access to the interior of the instrument housing. Thus, in the event of damage to the non-metallic shell, the damaged non-metallic shell may be quickly and easily replaced by a user, instead of having to return the instrument to the manufacturer for repair.

Description:
FIELD OF THE INVENTION 
   The subject invention generally relates to the field of coaxial electrical connectors, and specifically relates to isolated coaxial connectors. 
   BACKGROUND OF THE INVENTION 
   Bayonet Neill Concelman (BNC) coaxial connectors are commonly-used in the electronics industry for coupling low voltage signals to a measurement instrument, such as an oscilloscope. The mating halves of BNC connectors are referred to as male and female, respectively. Typically, the male portion of the BNC connector is mounted to the instrument chassis, and the female portion is attached to the end of a coaxial cable, such as a probe cable. The cable-mounted female potion of the BNC connector is secured to the panel-mounted male portion by way of a pair of metal “bayonets” mounted on the male portion of the BNC connector perpendicularly to the major axis of the connector. 
   In ground-referenced oscilloscopes, the outer shell of the Panel-mounted BNC connector is physically and electrically connected to the chassis of the oscilloscope. The oscilloscope chassis, in turn, is coupled to earth ground through the ground wire of its power cord. So long as no effort is made to interfere with this grounding system, it is safe for an operator of the oscilloscope to touch the outer shell of the BNC connector. 
   When a probe is connected to the grounded BNC connector on the oscilloscope, the metallic shield of the probe&#39;s cable is coupled to ground potential. The ground clip of the probe is also connected to the coaxial shield. Thus, all measurements being taken will be referenced to ground potential. However, there are times when it is necessary for an engineer to make a measurement that is not ground-referenced. In such circumstances, some engineers have resorted to a not-recommended and potentially dangerous practice known as “floating the scope”. The term “floating the scope” refers to the above-mentioned practice of interfering with the grounding system of the oscilloscope by breaking the connection between the oscilloscope and earth ground. In such a case, the oscilloscope chassis and the shell of the BNC connector may develop a high and potentially lethal voltage on exposed surfaces. 
   Fortunately, there is a better solution to the problem of making non-ground-referenced measurements than “floating the scope”. The THS700-series portable oscilloscopes manufactured by Tektronix, Inc., Beaverton, Oreg., employ a non-conductive case and a BNC connector having a non-conductive outer surface to prevent inadvertent contact of the user with dangerous voltages that may otherwise be present. 
   However, there is a problem that arises when such a solution is used. Safety requirements for an isolated instrument require the use of non-metallic BNC shells. The non-metallic shells necessarily require a pair of non-metallic bayonets. These have a history of failing because the bayonets are prone to being sheared from the shell. That is, the forces applied to the BNC connector when mating it will eventually cause the insulating material of the bayonets to fail. When this happens, the oscilloscope must be returned for service. In fact, the rate of replacement of damaged non-metallic BNC connectors can run as high as 300 BNC&#39;s per year for this exact problem. Without the bayonets, the mating connector will not be securely coupled to the Panel-mounted portion, and will simply fall off under normal use conditions. 
   One might think that a solution to the problem would be to change the size or shape of the bayonets to increase their strength. Unfortunately, geometry changes to increase strength are not permitted because the instrument must remain compatible with all existing BNC receptacles owned by the customer. 
   SUMMARY OF THE INVENTION 
   An insulated BNC connector assembly includes a metallic panel-mount portion and a user-replaceable non-metallic shell. The non-metallic shell is attached to the panel-mount metallic portion of the BNC connector such that, when mounted on an oscilloscope, or other electronic instrument, the non-metallic shell may be disconnected from the metallic portion of the BNC connector without having to gain access to the interior of the instrument housing. Thus, in the event of damage to the non-metallic shell, the damaged non-metallic shell may be replaced by a user, instead of having to return the instrument to the manufacturer for repair. 
   In a first embodiment of the invention, the metallic portion of the assembly has an externally-threaded portion, and the non-metallic shell potion of the assembly has a cut-away area to expose the threaded portion of the metallic portion of the assembly. The non-metallic shell slides over the metallic portion and is secured by a nut that engages with the threads on the metallic portion. 
   In a second embodiment of the invention, the metallic portion has external threads and the non-metallic shell is internally-threaded so that it can be screwed-on to mating threads formed on the metallic portion of the connector. 
   In a third embodiment of the invention, the non-metallic shell and metallic portion of the BNC connector may include features allowing a “snap fit” to retain the two portions in close association while still allowing removal of the non-metallic shell when necessary. 
   In a fourth embodiment, a non-metallic shell slides tightly over the metallic portion of the BNC connector and is prevented from rotating during the mating process by a pair of keyways formed in the shell and matching metallic keys formed on the metallic portion of the BNC connector. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  shows a side view of an insulated BNC connector shell according to a first embodiment of the subject invention. 
       FIG. 2  shows a perspective view of the insulated BNC connector shell of  FIG. 1 . 
       FIG. 3  shows a bottom view of the insulated BNC connector shell of  FIG. 1 . 
       FIG. 4  shows a side view of a metallic portion of an insulated BNC connector shell according to a first embodiment of the subject invention. 
       FIG. 5  shows a perspective view of metallic portion the insulated BNC connector assembly of  FIG. 1 . 
       FIG. 6  shows a view looking into a cable-mounted insulated BNC connector suitable for mating with the insulated BNC connector assembly of the subject invention. 
       FIG. 7  shows a front panel of for an oscilloscope, said front panel being suitable for use with the insulated BNC connector assembly of the subject invention. 
       FIG. 8  shows an insulated BNC connector according to a second embodiment of the subject invention. 
       FIG. 9  shows an insulated BNC connector according to a third embodiment of the subject invention. 
       FIG. 10  shows an insulated BNC connector according to a fourth embodiment of the subject invention. 
       FIG. 11  shows an assembly view of the insulated BNC connector of  FIGS. 1 through 5 . 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   The subject insulating BNC connector includes two parts, a non-conductive shell  100  of  FIG. 1 , and a panel-mount metallic body  400  of  FIG. 4 . Similar features in the different FIGURES have been assigned similar reference numerals. 
   Referring to  FIG. 1 , non-conductive shell  100  includes a cylindrical portion  110  and a base  120 . Non-conductive shell  100  is preferably molded in plastic. Cylindrical portion  110  has a pair of cutouts  115   a    115   b  spaced 180 degrees apart, which allow access to a threaded portion of metallic body  400  when shell  100  is installed on metallic body  400 . Cylindrical portion  110  also includes a pair of projections  130  positioned 180 degrees apart and extending radially outward. These projections are known in the art as “bayonets”, and are used to secure a mating cable-mounted BNC connector ( 600  of  FIG. 6 ) in a twist-lock arrangement. Base  120  includes a pair of cutouts  140  spaced 180 degrees apart, and used to prevent rotation of shell  100  when mating it with connector  600 . 
     FIG. 2  shows non-conducting shell  100  in perspective view for ease of understanding. Non-conductive shell  100  has an upper portion  110  and a lower portion  112 . 
     FIG. 3  is a bottom view of non-conducting shell  100 , wherein  110 ′ is the inside circumference of cylindrical portion  110 , and defines a hollow core  160 . As mentioned above, base  120  has two cutouts  140   a    140   b  spaced 180 degrees apart. Base  120  also has a substantially oblong aperture extending upward through the lower portion  112  of shell  100 . 
     FIG. 4  shows a second portion of the insulating connector of the subject invention, a panel mount metallic body  400 . Panel mount metallic body  400  includes a metallic cylindrical portion  409 , a substantially oblong portion  411 , and a pair of externally-threaded areas  417   a    417   b  on oblong portion  411 , and a pair of projections  440   a    440   b  spaced 180 degrees apart. Substantially oblong portion  411  has a major axis and a minor axis, and threaded areas  417   a    417   b  are formed at the ends of the major axis of oblong portion  411 . 
   Projections  440   a    440   b  are used to prevent rotation of shell  100  when mating it with connector  600 . Projections  440   a    440   b  are of a suitable length as to allow them to extend through a front panel  700  of  FIG. 7  and engage cutouts  140   a    140   b  of shell  100  to prevent the above-mentioned rotation of non-conductive shell  100 . Panel mount metallic body  400  also includes a non-conductive cylinder  450  surrounding a metallic cylindrical receptacle  460  used for receiving a conductive center pin  660  of mating connector  600  of  FIG. 6 . Panel mount metallic body  400  also includes a substantially circular section  426  that seats against the back of front panel  700  when installed thereon. Panel mount metallic body  400  also includes a lower portion  470  that carries the conductors  480   a    480   b  that connect metallic cylindrical portion  410  and center conductor receptacle  460  to a circuit board (not shown). 
     FIG. 5  shows a panel mount metallic body  400  in perspective view for ease of understanding. Panel mount metallic body  400  is shown with a mounting nut  518  engaging threaded areas  517   a    517   b . Mounting nut  518  is used to secure non-conductive shell  100  to panel mount metallic body  400 , and to secure the assembly to front panel  700 . 
   As noted above,  FIG. 6  shows a view looking into a cable-mounted insulated BNC connector  600  suitable for mating with the insulated BNC connector assembly of the subject invention. Connector  600  includes a non-conductive plastic covering  610  that prevents the user from coming in contact with a metallic body  620 . Metallic body  620  is in physical and electrical contact with the braid (i.e shield) conductor of the BNC cable (not shown). While normally at ground potential, in floating measurement applications, the voltage on the shield may rise to a level of several hundred volts. A substantially cylindrical aperture  630  allows the above-described generally cylindrical portion  110  of shell  100  to enter the connector  600 . A pair of substantially rectangular slots  630   a ,  630   b  extend aperture  630  to accommodate the passage of bayonets  130   a ,  130   b  of shell  100 . Slots  630   a  and  630   b  extend along metallic body  620  for a given distance and then turn substantially 90 degrees to form a twist-lock engagement with bayonets  130   a ,  130   b  of shell  100 . This twist-lock arrangement is well known in the art and need not be shown and described. Cylindrical portion  409  of panel mount metallic body  400  extends through panel  700  and engages substantially cylindrical aperture  630 . BNC connector  600  also includes a slotted metallic cylinder  640  having a plurality of segments  640   a . Segments  640   a  are spring loaded slightly into aperture  630 . Segments  640  mate with the inside surface of metallic cylindrical portion  409  and are compressed slightly by that contact to ensure good electrical connection. 
     FIG. 7  shows a plastic front panel  700  of an instrument, such as an oscilloscope, before components are mounted on it. Front panel  700  has a variety of apertures  730  formed through it for the mounting of various knobs and pushbuttons. A recessed area  735  is formed in front panel  700 , and three apertures  745 ,  745 ′, and  745 ″ are provided for the mounting of insulated BNC connectors of the subject invention. Referring also to  FIG. 11 , aperture  745  includes a pair of rectangular cutouts  740   a    740   b  to allow the mounting of panel mount metallic body  400  to the rear of front panel  700  at recessed portion  735 . Rectangular cutouts  740   a    740   b  receive projections  440   a    440   b  of panel mount metallic body  400  to prevent its rotation during mating with BNC connector  600 . 
   After panel mount metallic body  400  is attached to front panel  700  from the rear, non-conductive shell  100  is slid in place over panel mount metallic body  400 , and the assembly is locked together by applying mounting nut  518 . A decorative panel  760  snaps into place over cylindrical portion  409  via apertures  770 ,  770 ′ and  770 ″, thus isolating mounting nut  518  and protecting a user from touching it during normal use of the instrument. 
     FIG. 11  shows an assembling view of the insulated BNC Connector of the subject invention. Metallic portion  400  is inserted through aperture  745  of front panel portion  735  of front panel  700 . Projections  440   a ,  440   b  extend through cutouts  740   a ,  740   b  and thereafter prevent rotation of metallic portion  400 . Non-conductive shell  100  is then slid in place over metallic portion  400 , and pushed down so that projections  440   a ,  440   b  engage cutouts  140   a ,  140   b  of non-conductive shell  100 , and threaded portions  417   a ,  417   b  are operable through openings  115   a ,  115   b  of non-conductive shell  100 . Nut  518  slides over non-conductive shell  100  and its threads engage threaded portions  417   a ,  417   b  of metallic portion  400 , thereby locking the assembly in place on front panel portion  735  of front panel  700 . Decorative panel  760  snaps into place over cylindrical portion  110  via aperture  770 , thus isolating mounting nut  518  and protecting a user from touching it during normal use of the instrument. 
   In operation, a user may discover that bayonets  130  have been sheared off in normal use. Rather than send the entire instrument to the factory for repair, the user may change the broken non-conductive shell  100 , himself. He would first remove decorative panel  760 , then remove mounting nut  518 , slide the broken non-conductive shell off of metallic portion  400 , then replace the non-conductive shell by following the recited steps in reverse order. 
   A second embodiment of a non-conductive shell  800  of the subject invention is shown in  FIG. 8 . All portions of non-conductive shell  800  are identical to shell  100  except that cutouts  115   a  and  115   b  are not provided, and an internally-threaded portion  885  is provided. Internally threaded portion  885  is intended to screw onto externally threaded portion  417   a    417   b  of panel mount metallic body  400 . In this embodiment, projections  422   a    422   b  are shorter in length such that they engage apertures  750 , but end flush with the front side of front panel  700 , so as to not interfere with the screwing-on of shell  800 . In this case, anti-rotation fingers (not shown) are molded onto the rear of panel  760  to engage cutouts  840  of non-conductive shell  800  to prevent rotation of shell  800 . 
     FIG. 9  shows a third embodiment of the invention in which a non-conductive shell  900  slips over a metallic panel mount portion and locks in place by means of a snap fit arrangement  970 . 
     FIG. 10 , shows a fourth embodiment of the invention in which a non-conductive shell  1000  snuggly slips over a metallic panel mount portion and is prevented from rotating by the presence of two metallic keyways  1070   a    1070   b.    
   What has been described is an insulated BNC connector that allows a field service representative, or a customer, to quickly and easily replace a failed non-metallic BNC shell. One skilled in the art will realize that there are many ways to detachably connect an insulating shell to the body of a BNC connector, while three such embodiments have been described herein in detail, the following claims are not intended to be so limited, but rather, are intended to encompass other mechanical mounting techniques. 
   While the invention has been described in the environment of test and measurement instruments, such as, an oscilloscope, or the like, it is equally usable on any electronic equipment having a need to isolate the shell of the BNC connector from earth ground.