Patent Publication Number: US-6217382-B1

Title: Coaxial cable ESD bleed

Description:
TECHNICAL FIELD 
     The present invention relates to a ?,(SMA) connector for a coaxial cable and more particularly to providing an electrostatic discharge (ESD) bleed path for a floating center conductor of the coaxial cable. 
     BACKGROUND ART 
     Coaxial cables and radio frequency switches that are used to leave a communications port open circuited are located inside the payload of a communications satellite and are usually well shielded. The center conductor of a coaxial cable connected to an RF switch may be electrically floating when it is not being used as a path for a RF signal. 
     The energy of electrons in a space environment is capable of causing the floating center conductor to charge to a high potential. When the cable is switched into a RF path, it is possible to induce a transient pulse from the floating center conductor onto a circuit in the RF path. The problem is that the induced pulse may cause damage to the electronics and in particular to sensitive monolithic microwave integrated circuits (MMIC) in the RF path. 
     To avoid this potentially serious problem, it is possible to insert coaxial attenuators having a low attenuation value into the paths that are susceptible to the transient pulse. However, this technique takes up valuable space due to the need for more cable roughing space, thereby adding unwanted weight to the spacecraft. In addition, the technique adds insertion loss, which is also undesirable. 
     It is also possible to continuously monitor space weather in order to determine optimum conditions for switching RF switches. RF switches are temperature dependent and simply waiting for the center conductor current potential to drop to a satisfactory level may be enough to prevent transients from entering the RF circuit. However, the monitoring method is extremely costly in that continuous, real-time space weather monitoring is required. Furthermore, scheduling problems arise because, say at 20° C., it could take up to four days for the center conductor potential to drop to a satisfactory level. The scheduling problems adversely affect customer revenues due to the fact that the switches may be out of commission for an extended period of time. 
     According to flight data the probability of damage to sensitive electronic circuits during the switching process is low. However, should it occur, the effects could be extremely costly. Therefore, what is needed is a low cost method for preventing potential damage to sensitive electronic components without adversely affecting weight, and without occupying very limited, and very valuable, space onboard a spacecraft. 
     SUMMARY OF THE INVENTION 
     The present invention provides an electrostatic discharge bleed path for the center conductor of a coaxial cable. The present invention prevents spacecraft hardware damage by preventing the center conductor from charging in an enhanced electron environment, such as in a space environment, when no other bleed paths are available. Additionally, the present invention has minimal impact on the RF performance characteristics of the SMA connection. 
     In order to accomplish the above advantages, the present invention is realized in a disk fitted over the center pin, or contact, of a SMA male connector to bleed electrostatic charge. Preferably the disk is made of a polamide material, such as Kapton® which is a registered trademark of DuPont Corporation loaded with carbon. The disk is compressed between the male and female portions of the SMA connector, thereby providing an electrical connection among the connector, the inner conductor and the outer conductor of the coaxial cable. 
     The center conductor of a coaxial cable connected to an RF switch may be electrically floating when it is not being used as a path for a RF signal. In operation, the floating center conductor has a bleed path to ground through the electrical connection of the disk and coaxial cable outer conductor. Therefore, when switching occurs, the center conductor potential is well within satisfactory limits and the risk of damage to MMIC components is eliminated. 
     It is an object of the present invention to provide an electrostatic discharge bleed path when no other bleed paths are available. It is another object of the present invention to avoid potential damage to sensitive hardware by preventing charge build-up on a center conductor of a coaxial cable. It is yet another object of the present invention to prevent an unwanted transient pulse from being introduced into a circuit in a RF path when a coaxial cable is switched into the RF path. 
     Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the invention may be well understood, there will now be described an embodiment thereof, given by way of example with reference being made to the accompanying drawings, in which: 
     FIG. 1 is a cross-sectional view of a segment of coaxial cable having a SMA plug that includes the coaxial cable ESD bleed of the present invention; 
     FIG. 2 is a graph of a plot representing the center conductor potential as a function of resistivity after a coaxial cable has been exposed to an enhanced energetic electron environment; 
     FIG. 3 is a graph of a plot representing center conductor potential as a function of temperature at the coaxial cable; 
     FIG. 4 is a schematic diagram of a test setup used to determine the size of an induced transient pulse; and 
     FIG. 5 is a graph of a plot representing a pulse induced by switching a floating center conductor into a RF path. 
    
    
     BEST MODE(S) FOR CARRYING OUT THE INVENTION 
     FIG. 1 shows cross-sectional view of a segment of coaxial cable  10  having a SMA connector  12  attached thereto. The SMA connector  12  has a plug  14  and a jack  16 . The plug  14  has a center pin  18 . The present invention is a disk  20  that fits over the center pin  18  of the SMA plug  14 . The disk  20  is typically made of a polyamide material, such as DuPont&#39;s Kapton®, loaded with carbon and provides low-level (i.e., 0.1 dB) attenuation in order to ground a center conductor  22  of the coaxial cable  10 . The disk  20  has an opening  21  therethrough that is slightly smaller than the dimensions of the center pin  18  to provide electrical contact with the center pin  18 . For example, the diameter of the opening  21  in the disk  20  may be 0.034-0.035 inches, while the center pin  18  may have a diameter of 0.036 inches. 
     The plug  14  mates with a jack  16  such that the periphery of the disk  20  is clamped between the outer conductors  23 ,  25  of the SMA plug  14  and jack  16  respectively. The clamping action provides electrical contact between the disk  20  and the outer conductor  25  of the coaxial cable. The electrical contact between the disk  20 , the center pin  18 , and the outer conductors  23 ,  25  combined with the intrinsic resistivity of the disk  20  provide and ESD bleed path for the center conductor  22  to the outer conductor  25 . Due to the high resistivity and small thickness of the disk  20 , there is minimal impact on the RF performance of the connection. 
     When a coaxial cable  10  is not being used in the path of a RF signal, its center conductor  22  is electrically floating. The energetic electrons in a space environment cause the floating center conductor  22  to charge to a high potential. 
     The disk  20  of the present invention provides an electrostatic discharge bleed path for the center conductor  22  of the coaxial cable  10 . The bleed path prevents the build up of excess charge in an enhance electron environment, thereby preventing a transient pulse from occurring when the coaxial cable  10  is switched into a RF path. Without the concern of an induced pulse, there is no concern that sensitive electronic components in the RF path may be damaged when the switch occurs. 
     Operation of the present invention will be described in conjunction with the present example along with FIGS. 1 through 5. It should be noted that the values, test procedures and methods used in the present example are for example purposes only. It is possible to use other techniques and methods and one skilled in the art is capable of substituting alternative techniques and method to accomplish similar results. 
     A coaxial cable  10  is similar to a leaky capacitor. Any charge stored on the center conductor  22  can leak to ground by conduction through the core  24 , which is typically a Teflon material. The potential of the center conductor  22  is a function of both electron flux and the resistivity of the Teflon insulation. 
     The coaxial cable  10  has a well-defined geometry and it is possible to present the cable mathematically as two concentric cylinders. The resistance (r) and capacitance (c) per unit length of a coaxial cable  10  are well known and can be found in many sources including  Foundations of Electromagnetic Theory,  J. Reitz and F. Milford, Addison Wesley, 1967. The potential (V) of the center conductor  22  as a function of time(t) is given by the equation: 
     
       
           V=jr (1− e   −1/(rc) )  (1) 
       
     
     where, 
       c= (2π kε   o )/[ln ( r   o   /r   i )]  (2) 
     and 
     
       
           r=[η ln( r   o   /r   i )/2π  (3) 
       
     
     In equations (1), (2), and (3), r i  is the radius of the center conductor  22  and r o  is the radius of the copper jacket  26 . η is the resistivity of the Teflon insulation material  24 , ε o  is the absolute permittivity, k is the relative dielectric constant of the Teflon insulation  24  of the coaxial cable  10 . For Teflon, k=2. Equation (1) can be rewritten as: 
     
       
           V=jr (1− e   −(t/kεoη) )  (4) 
       
     
     FIG. 2 is a graph  100  of a plot  102  showing the potential (V) of the center conductor as a function of the resistivity of the Teflon insulation  24  after the coaxial cable  10  has been exposed to an enhanced electron environment of 25xAE8 for approximately twenty-four hours. The center conductor potential (V) can be as high as 840 Volts if the Teflon insulation  24  has a high resistivity, i.e. no leakage. However, if the resistivity should approach somewhere in the range of 10 16  Ohms, then the center conductor potential (V) could be less than 100 Volts. Consequently, the resistivity of the Teflon plays a key role in determining the potential of a floating center conductor  22 . 
     In general, the resistivity of a material is temperature dependent. Orders of magnitude of resistivity could occur over a temperature range of 100° C. For purposes of the present example, assume the expected operation temperature of RF switches to be between −5° C. and 60° C. 
     The resistivity of Teflon covers a wide range of values, i.e. 10 16  Ohm-cm to 10 22  Ohm-cm. Therefore, a test was performed to calculate the resistivity of the Teflon on a coaxial cable/R type switch combination. Resistivity measurements were taken at temperatures of 20° C. and at 60° C. The resistance between the center conductor  22  and ground were used to measure resistance value and derive a value for the resistivity of Teflon. The leakage resistance was determined by measuring the time constant decay of the center conductor potential. The resistivity of Teflon over the entire operation temperature range was determined by extrapolation of the test data. FIG. 3 is a graph  200  having a plot  202  that represents the center conductor potential (V) as a function of temperature in ° C. 
     In operation, an RF switch is used to leave a port open circuited and to switch the coaxial cable  10  into a RF path. At this point in time, it is possible for the center conductor  22  of the coaxial cable  10  to dump any charge it has stored onto a sensitive electronic circuit in the RF path, inducing a transient pulse on that circuit. 
     A test was performed to determine the amplitude of the induced pulse. A diagram of the test setup is shown in FIG. 4. A power supply  30  feeds an RF switch  32 . The switch  32  has a coaxial cable  34  attached at an input port  36 . A 50-Ohm load  38  is connected to an output port  40 . The test included charging the center conductor  35  of the coaxial cable  34  using the power supply  30 , and then switching the cable  34  onto the 50 Ohm load  38 , then measuring the pulse induced across the 50 Ohm load using a scope  42 . FIG. 5 is a graph  300  showing a typical waveform  302 . The rise time of the pulse is approximately 2 ns. Table 1 summarizes the results as follows: 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Applied Voltage 
                 Peak Induced Voltage 
                 Pulse Width 
               
               
                 (V) 
                 (V) 
                 (ns) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 10 
                 4.7 
                 2 
               
               
                 10 
                 3.9 
                 2 
               
               
                 20 
                 11.8 
                 2 
               
               
                 20 
                 11.4 
                 2 
               
               
                 20 
                 11.2 
                 2 
               
               
                   
               
            
           
         
       
     
     The amplitude of the induced pulse was 40%-60% of the voltage bias applied to the center conductor. 
     It was estimated that the highest floating center conductor potential is 820 Volts. Should the RF switch be activated with the center conductor at this potential, a 500 Volt pulse could be injected into a sensitive electronic circuit. A pulse of this amplitude would damage the electronic components. In fact, typical sensitive electronic circuit components should not be subjected to a transient pulse greater than 50 Volts. This implies that, in the present example, the center conductor potential should be kept below 80 Volts. 
     In order to consistently maintain the center conductor potential below 80 Volts, the coaxial cable must have a direct current, (DC) ground. This is accomplished by incorporating the disk  20  of the present invention to the SMA connector  12  in the manner described above. The disk  20  adds a low-level attenuator in series with the coaxial cable  10 . A typical attenuator has relatively low impedance, i.e. &lt;1 MΩ, between the center conductor and ground, and is adequate to bleed-off charge stored on a floating center conductor. The disk  20  of the present invention provides a DC path to ground for the center conductor of each coaxial cable in a spacecraft system. 
     While a particular embodiment of the invention has been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.