Abstract:
A passive, highly efficient, low noise adapter device includes a balun and noise reduction circuitry uniquely configured to converting an unbalanced signal line on a 50 ohm signal line to a balanced signal on a 100 ohm transmission line or vice versa. The device facilitates the use of commercially available and accepted test equipment for accurate transmission measurements on balanced twisted pairs of cables and connectors. A typical utilization includes an adapter device connected between the 100 ohm twisted pair cable and suitable test equipment such as a network analyzer and/or a signal generator for determining losses in the telephone wire and connectors.

Description:
This application is a continuation-in-part of application Ser. No. 09/192,724, filed Nov. 16, 1998, issued Aug. 8, 2000 as U.S. Pat. No. 6,100,772. Said application/patent are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to transmission systems for video and data signals. More specifically the invention relates to a low noise adapter device which is suitable for coupling testing equipment to a balanced two wire unshielded twisted pair for testing same. 
     Cable television (CATV) is transmitted over 75 ohm coaxial cable. Channels 2-60 are transmitted on the cable at frequencies ranging from 5 to 500 MHz. Many homes and commercial buildings have previously installed telephone cables consisting of unshielded twisted pairs. The standard unshielded twisted pairs (UTP) of telephone cables have a characteristic impedance of 100 ohms. It is often convenient to utilize this cable for transmission of video signals. Perhaps more significantly, UTP cable may also be utilized for transmission of computer data signals. Currently such cable and applications, such as local area networks, transmit computer data in the 20 to 30 MHz range. In the foreseeable future such transmission speeds on such cable may exceed 100 MHz. 
     There are difficulties associated with using the unshielded twisted pair cable for the above, particularly the transmission of CATV signals. Any attempt to utilize the UTP cable with unbalanced CATV signals or other unbalanced signals will result in unacceptable radiation and attenuation of the signals. Techniques such as shown in U.S. Pat. Nos. 5,633,614 and 5,495,212 also assigned to the owner of the instant invention, provide generally acceptable matching of the CATV coaxial cable with the UTP cable. 
     It is often useful or necessary to measure the balance and other characteristics of UTP cable particularly with reference to the potential usage of such cable for carrying CATB or data signals. Appropriate test equipment for performing such measurements, including network analyzers and signal generators, generally have a characteristic impedance of 50 ohms. Thus, attempts to make any quality measurements of the cable or other associated equipment such as connectors is impossible without suitably matching the test equipment characteristic impedance with that the UTP cable. Heretofore baluns and adapters suitable for such coupling had a restricted frequency range, for example up to 100 MHz. An adapter device is needed which offers high performance low noise coupling up to and beyond 350 MHz. 
     SUMMARY OF THE INVENTION 
     A passive, highly efficient, low noise adapter device includes a balun and noise reduction circuitry uniquely configured to converting an unbalanced signal line on a 50 ohm signal line to a balanced signal on a 100 ohm transmission line or vice versa. The device facilitates the use of commercially available and accepted test equipment for accurate transmission measurements on balanced twisted pairs of cables and connectors. A typical utilization includes an adapter device connected between the 100 ohm twisted pair cable and suitable test equipment such as a network analyzer and/or a signal generator for determining losses in the telephone wire and connectors. 
     A feature and advantage of preferred embodiments of the invention is that a much higher band width is provided by the device compared to conventional adapters. 
     A feature and advantage of preferred embodiments is that the device provides excellent noise reduction characteristics over the entire band width of the device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an electrical schematic diagram of the circuitry of an embodiment of the invention. 
     FIG. 2 is an end elevational view of a device embodying the invention. 
     FIG. 3 is a front elevational view of the device of FIG.  2 . 
     FIG. 4 is a block diagram of an application of the invention. 
     FIG. 5 is a block diagram of another application of the invention. 
     FIG. 6 is an exploded view of a device embodying the invention. 
     FIG. 7 is a side elevational view of a circuit board including principal components of an embodiment of the invention. 
     FIG. 8 is a top plan view of a circuit board suitable for the embodiment of FIG.  7 . 
     FIG. 9 is a bottom plan view of the circuit board of FIG.  8 . 
     FIG. 10A is a perspective view of the balun (T 1 ) showing the windings. 
     FIG. 10B is a schematic figure of the balun (T 1 ). 
     FIG. 11A is a perspective view of the common mode choke (T 2 ). 
     FIG. 11B is a schematic view of the common mode choke (T 2 ). 
     FIG. 12A is a perspective view of the signal splitter (T 3 ). 
     FIG. 12B is a schematic view of the signal splitter (T 3 ). 
     FIG. 13 is a block diagram of an application showing use of two of the inventions for measuring insertion loss of the invention. 
     FIG. 13A is a typical frequency response using two of the devices when configured as depicted in FIG.  13 . 
     FIG. 14 is a block diagram of an application showing use of the invention for measuring return loss of the invention where the input signal is provided by an unbalanced 50 ohm impedance source and the load is measured across a balanced 100 ohm nominal resistance. 
     FIG. 14A is a typical frequency response of the device when configured as depicted in FIG.  14 . 
     FIG. 15 is a block diagram of an application showing use of the invention for measuring return loss of the invention where the input signal is provided by a balanced 100 ohm impedance source and the load is measured across a 50 ohm nominal resistance. 
     FIG. 15A is a typical frequency response of the device when configured as depicted in FIG.  15 . 
     FIG. 16 is a block diagram of an application showing use of the invention for measuring return loss of the invention where the input signal is provided by an unbalanced 50 ohm impedance source and the load is measured across a network of three resistors, each of 50 ohms nominal resistance. 
     FIG. 16A is a typical frequency response of the device when configured as depicted in FIG.  16 . 
     FIG. 17 is a schematic of a further embodiment in accordance with the invention. 
     FIG. 18 is a schematic of a further embodiment in accordance with the invention. 
     FIG. 19 is a schematic of a further embodiment in accordance with the invention. 
     FIG. 20 is a schematic of a further embodiment in accordance with the invention. 
     FIG. 21 is a schematic of a further embodiment in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The use of input and output as used herein as a matter of convention for differentiating the ports on the device and does indicate or require that the signals must be transmitted in a particular direction through the device. As illustrated in the figures and as discussed below signals are transmitted in either direction through the device. 
     Referring to FIGS. 1,  2 , and  3 , the adapter device, enumerated  20 , is shown in schematic form and as a component part. The adapter device generally comprises a housing  22  through which extend a first unbalanced input port  24 , a second unbalanced input port  26 , and a balanced output port  28 . The balanced output port has two signal connectors  32 ,  33  and also has a grounded connector  34 . The balanced output port signal connectors  32 ,  33  directly receive stripped wire of the unshielded twisted pair. The unbalanced input ports have coaxial type connectors  38  which may be SMA connectors or other suitable coaxial connectors. 
     Positioned in the housing, is a circuit board  42  as indicated by the dotted line in FIG. 3 which generally holds the components as indicated in the schematic of FIG.  1 . The input port is connected to a balun  44  by a signal conductor  50  which extends to a first end  56  of a first winding  58 . A second end  62  of the first winding connects to or comprises a first balun output lead  64 . A second balun winding  70  has a first end  72  connecting to the first balun output lead and a second end  74  connecting to the circuit ground. 
     A third winding  82  has a first end  84  connecting to circuit ground  78  and a second end  86  connecting to a second balun output lead  90 . A capacitor  94  extends across the first balun output lead  64  and the second balun output lead  90  and provides some suppression of high frequency parasitics. 
     A longitudinal common mode choke  96  has a pair of input leads  97 ,  98  and a pair of output leads  101 ,  102 . The longitudinal choke  96  has a first winding  104 , a second winding  105 , a third winding  106 , and fourth winding  107 . The output leads  101 ,  102 , are connected to first and second conductors  110 ,  112  of output port  28 . A signal splitter  116  extends across the first and second conductors of the output port and has a center tap  118  between a first winding  120  and a second winding  121 . 
     The center tap  118  of the splitter is connected to a resistor  125  which connects to the signal conductor  127  of the second unbalanced input port. The resistor, which minimizes parasitics, is 12.4 ohms in the preferred embodiment and is suitably in the range of 2 to 50 ohms. Details of the construction of the balun, choke, and splitter are discussed in detail below. 
     Referring to FIG. 4 one suitable application of the adapter device is displayed. A specimen  130  of unshielded twisted pair cable is connected to the balanced output ports  28 A,  28 B of two adapter devices  20 A,  20 B. A signal generator  134  is connected to the first unbalanced input port  24  of the adapter device  20 A and a network analyzer is connected to the first unbalanced input port  24  of the adapter device  20 B. The output of the signal generator  136  and the input  138  of the network analyzer  137  both has a first characteristic impedance of substantially 50 ohms. Cable  139  connecting the signal generator to the adapter device and the cable  140  connecting the network analyzer to the adapter device  20 B each are 50 ohm coaxial cable. The specimen  130  would be an unshielded twisted pair cable having a second characteristic impedance of 100 ohms. The balun matching the first characteristic impedance. Thus, a signal of a specific frequency or a signal swept over a broad range of frequencies is detected and analyzed by the network analyzer  131  and appropriate return losses, insertion losses, and other suitable measurements can be determined by a convention measurement techniques. 
     Referring to FIG. 5, an alternative configuration is shown in which an adapter device  20  has a signal generator  141  connected to the second unbalanced input and a network analyzer  142  connected to the first unbalanced input. An unshielded twisted pair of cable  146  is connected to the output port  28  and has a 100 ohm resistor  148  connected at the termination  150  of said cable. Said configuration offers an alternate means of determining specific loss characteristics of the unshielded twisted pair of cable  146 . 
     Referring to FIGS. 10A and 10B, details of the construction of the balun T 1  is disclosed. Further specifications and construction procedures are disclosed in Table 1 below. The core is suitably a high permeability core in the range of 7500 or greater. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Impedance Matching Transformer (T1): 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Wire used: 
               
               
                   
                 Three strands for forming a first, second, and third winding of 
               
               
                   
                 Enameled wire of a diameter 0.0035″ or greater. For purposes of 
               
               
                   
                 this description three colors are used as an example. 
               
               
                   
                 Twisted strands of each of the three wires. Then interleave twisted 
               
               
                   
                 bundles all colors. 
               
               
                   
                 Winding Instructions: 
               
               
                   
                 Wind 2 or more turns of the twisted wire on a toroid core structure 
               
               
                   
                 with a permeability of greater than 7500. Thread out one color of 
               
               
                   
                 wire leaving the other two colors twisted. Continue winding the 
               
               
                   
                 other two twisted wires for 3 or more turns. 
               
               
                   
                   
               
             
          
         
       
     
     Referring to FIGS. 11A and 11B the longitudinal choke  96  is disclosed and has a first section  156  and a second section  158 . The first section has a pair of stacked, cores with one being a relatively low permeability in a preferred embodiment of about 35 and the second stacked core has a permeability of about 750. Generally the permeability should vary by a magnitude of 10 or more. 
     The larger core in the second section  158  has a permeability in a preferred embodiment of 5,000. 
     The first, second, and third winding configured to provide an input impedance of substantially 50 ohms and an output impedance of substantially 100 ohms. 
     Further details are shown in table 2 below. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Choke (T2): 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Wire used: 
               
               
                   
                 Two strands of 36 AWG Teflon wire. 
               
               
                   
                 Take 1 strand of each wire and twist. 
               
               
                   
                 Winding instructions: 
               
               
                   
                 Stack one powder iron toroid core with a permeability of about 35 
               
               
                   
                 and diameter not to exceed .250″ and one ferrite core of similar 
               
               
                   
                 diameter and a permeability of about 750 together and wind 8 or 
               
               
                   
                 more turns of the twisted wire. Take one core with a permeability 
               
               
                   
                 of greater than 4000 and a diameter of no more than .200″ and 
               
               
                   
                 wind 8 more turns with the finish of the first set, leaving minimum 
               
               
                   
                 distance between the two sets (Exhibit B). 
               
               
                   
                   
               
             
          
         
       
     
     Each strand of Teflon® wire having a conductor portion and an insulation portion. The conductor portions and insulation portions configured so that the pair of windings has a characteristic impedance substantially matching the second characteristic impedance. 
     Referring to FIGS. 12A and 12B details of the configuration of the splitter  96  are disclosed. The splitter consists of a pair of cores stacked one with a low permeability 35 and the other with a relatively high permeability of 5,000. Appropriately the low permeability core is less than or equal to 35. Appropriately the high permeability core is greater than 4,000. Alternatively, the two cores appropriate would have a different in magnitude of 20 or more. Further details of construction are disclosed in the table below. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Splitter (T3): 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Wire used: 
               
               
                   
                 Two strands of 36 AWG Teflon insulated wire. 
               
               
                   
                 Bifilar wound 
               
               
                   
                 Winding instructions: 
               
               
                   
                 Stack one powder iron toroid core with a permeability of about 35 
               
               
                   
                 and one ferrite core of similar diameter and a permeability of about 
               
               
                   
                 750 and bifilar wind 7 or more turns of the Teflon wire. 
               
               
                   
                   
               
             
          
         
       
     
     Referring to FIG. 6 details of the assembly of the housing is shown. In the ideal embodiment the housing  22  will be formed of brass although other comparable metals or other conductive materials may be used. The housing contains and encloses the circuit board  42  onto which or in proximity with are located the coaxial connectors  38  and the unshielded twisted pair connectors  32 ,  33 . 
     Referring to FIGS. 7,  8 , and  9 , the positioning of the various components on the circuit board is disclosed in detail. The first input port  24  is positioned adjacent the balun (T 1 )  44  which is positioned adjacent the longitudinal common mode choke (T 2 )  96 . Next is the splitter (T 3 ) adjacent to the resistor  125 . FIG. 8 discloses the component side of the board and FIG. 9 discloses the solder side of the board. With regard to the component side, juncture  150  is for the T 1  single color finish and a second single color wire start. Juncture  152  is for T 1  second wire finish. Juncture  153  is for T 2  Teflon wire start. Juncture  154  is for T 1  third wire start. Juncture  155  is for T 1  third wire finish/and T 1  second wire start. Juncture  156  is for T 2  Teflon wire start. Juncture  160  is for T 2  Teflon wire finish. Juncture  161  is for T 2  Teflon wire finish. Juncture  162  is for T 3  Teflon wire start and another Teflon wire finish. Juncture  163  is for the T 3  Teflon wire finish. Juncture  164  is for the T 3  Teflon wire start. The element number  170  is a 0.5 picofarads 50 volt capacitor, for example a surface mount multiplayer ceramic chip 0603 size capacitor. Said capacitor may appropriately range between 0.1 and 5 picofarads. The output signal conductor  171  are disposed on the circuit board and positioned to maintain the second characteristic impedance. 
     FIGS. 13,  13 A,  14 ,  14 A,  15 ,  15 A,  16 , and  16 A, depict block diagrams and typical frequency response charts for the analysis and testing of device  20  and are intended to show typical performance of said device  20 . Detailed descriptions are as follows: 
     FIG. 13 depicts a test configuration utilizing two devices ( 20 A and  20 B) and for the purposes of measuring insertion loss. Signal generator  134  is electrically coupled to the output side of device  20 A by means of first shielded coaxial cable  139 . The input side of device  20 A is electrically coupled to the input side of device  20 B by means of a suitably short segment of balanced, twisted pair telephone wire  131 . The output of device  20 B is electrically coupled to network analyzer  131  by means of second shielded coaxial cable  139 . The second unbalanced input ports on both devices  20 A and  20 B remain unused in this configuration. Typical frequency response noted using said configuration is depicted in FIG.  3 A. It will be appreciated that between the frequencies of 1 MHz and 500 MHz, the total signal loss for this configuration, is 2.4 decibels (dB), or, for a single device  20 , 1.2 dB. 
     FIG. 14 depicts a test configuration for the purposes of measuring return loss over a 100 ohm resistor  148  load when excited by a 50 ohm unbalanced signal source. Signal generator  134  is electrically coupled to the output side of device  20  by means of shielded coaxial cable  139 . The input side of device  20 . 1  is electrically coupled to the parallel combination of resistor  148  and network analyzer  131  by means of a suitably short segment of balanced, twisted pair telephone wire  29 . Typical frequency response noted using said configuration is depicted in FIG.  4 A. It will be appreciated that between the frequencies of 3 MHz and 350 MHz, the return loss of device  20  is a minimum of 20 dB. 
     FIG. 15 depicts a test configuration for the purposes of measuring return loss over a 50 ohm resistor  148 . 1  load when excited by a 100 ohm balanced signal source. Signal generator  134  is electrically coupled to the input side of device  20  by means of a suitably short segment of balanced, twisted pair telephone wire  129 . The output side of device  20  is electrically coupled to the parallel combination of resistor  148 . 1  and network analyzer  131  by means of suitably short segments of shielded coaxial cable  139 . Typical frequency response noted using said configuration is depicted in FIG.  15 A. It will be appreciated that between the frequencies of 3 MHz and 350 MHz, the return loss of device  20  is a minimum of 20 dB. 
     FIG. 16 depicts a test configuration for the purposes of measuring common mode return loss over a load consisting of a network of three 50 ohm resistors  33 A,  33 B, and  33 C, when excited by a 50 ohm unbalanced signal source. Signal generator  134  is electrically coupled to the output side of device  20  by means of shielded coaxial cable  139 . The input side of device  20  is electrically coupled as follows: First port  26  is electrically coupled to first end of resistor  33 A. Port  24  is electrically coupled to first end of resistor  33 B. The first end of resistor  33 C. The second ends of resistors  33 A,  33 B, and  33 C are electrically coupled together and furthermore, electrically coupled to network analyzer  131 . The remaining connection of network analyzer  131  is electrically coupled to ground. All connections on the output side of device  20  are by means of suitably short segments of balanced, twisted pair telephone wire  29 . Connector  21  on device  20  is unused in this configuration. Typical frequency response noted using said configuration is depicted in FIG.  6 A. It will be appreciated that between the frequencies of 1 MHz and 350 MHz, the return loss of device  20  is a minimum of 20 dB. 
     Still within the scope of the invention component values may be adjusted to provide different operating ranges, for example, a lower range of operation, such as 100 MHz to 10 MHz, which is used in some applications. 
     Referring to FIG. 17, a further embodiment of the invention is illustrated. This embodiment is similar to the first embodiment and has a balun  210 , a capacitor  215  across the balun output leads, a common mode choke  222 , and a common mode shunt or splitter  230 . The resistor in series with balanced port  236  has been eliminated. Capacitive coupling between the 100 ohm balanced output  248  may be increased by utilizing a thinner wire insulation with a higher dielectric constant. The materials and configuration are otherwise suitably what is disclosed in the first embodiment. 
     Referring to FIG. 18 this embodiment is designed to provide excellent operating characteristics up to 650 MHz. This embodiment has five principle components: an impedance matching transformer or balun  262 , a common-mode choke  268 , a common-mode shunt  274 , and two capacitors  280 ,  282 . 
     The transformer  262  is used to match the unbalanced 50 ohm source impedance (i.e. a network analyzer) to a balanced 100 ohm impedance (i.e. networking cables, connectors). One small toroidal core with a permeability of 10,000 is wound in a similar configuration as the first embodiment using a multi-stranded, twisted wire. 
     The common mode choke  268  blocks any unwanted common mode signals from the balanced side of the circuit. A combination of a core with low permeability powdered iron with a permeability of 35 and high permeability cores, such as ferrite at 1800 and 5000, are stacked together and wound with a twisted wire to keep a 100 ohm characteristic impedance at the balanced pair. 
     The splitter or common mode shunt  274  is used to direct any common mode signal from the balanced pair to the C 1  port to allow measurement of balance on the 100 ohm pair. A combination of low permeability iron core with a permeability of 35 and a core of high permeability, 5,000, are stacked together and wound with twisted wire that creates a high capacitive coupling between the balanced pairs. This higher capacitive coupling created by the twisted wire enhances the high frequency performance of the device. 
     The first capacitor  280  is placed across the unbalanced input of the T 1  impedance matching transformer  262 . This is suitably a small capacitance (under 10 PF) which enhances the high frequency performance of the device. The second capacitor  282  is placed across the balanced output of the impedance matching transformer  262 . this is also a small capacitance, suitably under 10 pF, which enhances the high frequency performance of the device. 
     Referring to FIG. 19, this embodiment is suitably used to match the 50 ohm impedance of a network analyzer to 130 ohm cables and connectors. This embodiment is similar to the functionality of the first embodiment with the exception of a different impedance match and the elimination of the first embodiment common mode shunt (T 3 ) since balance testing capability is not needed. This embodiment comprises principally three components: an impedance matching transformer  310 , a common mode choke  318 , and a capacitor  324 . 
     This transformer of balun  310  is used to match the unbalanced 50 ohm source impedance (network analyzer) to a balanced 130 ohm impedance (cables, connectors). One small toroidal core with a permeability of 10,000 is wound in the same configuration as the first embodiment using a multi-stranded, twisted wire. 
     This component is used to block any unwanted common mode signals from the balanced side of the circuit. A combination of low permeability (powdered iron 35 perm) and higher permeability cores (i.i. ferrite with permeabilities of 1800 and 5000) are stacked together and wound with twisted Teflon® wire to keep a 130 ohm characteristic impedance in the balanced pair. 
     The capacitor  324  is placed across the unbalanced input of the T 1  impedance matching transformer  310 . This is a small capacitance (under 10 pF) which enhances the high frequency performance of the balun. 
     The second capacitor  282  is placed across the balanced output of the T 1  impedance matching transformer  262 . This is also a small capacitance (under 10 pH) which enhances the high frequency performance of the balun. 
     Referring to FIG. 20 a further embodiment is designed for the same functionality as the first embodiment, but a lower frequency range of 100 kHz to 100 MHz. To accommodate the lower frequency range, larger and higher permeability (10,000) ferrite cores configured as toroids are appropriately utilized. This embodiment is comprised principally of an impedance matching transformer  360 , a common mode choke  368 , a common mode shunt  376 , and a capacitor  382 . 
     The transformer  360  is used to match the unbalanced 50 ohm source impedance (network analyzer) to a balanced 100 ohm impedance (networking cables, connectors). A toroid with a permeability of 10,000 is wound as in the same twisted wire as the first embodiment of 040-0055 but in a different winding configuration as shown. 
     The common mode choke  368  is used to block any unwanted common mode signal from the balanced side of the circuit. Two cores with permeabilities of 10,000 are suitably stacked together and wound similar to the first embodiment to keep characteristic impedance of 100 ohms between the balanced output. 
     The common mode shunt  376  is used to direct any common mode signal to the C 1  port to allow measurement of balance between the 100 ohm pair  382 . A core with a permeability of 10,000 is wound with wire as on the first embodiment. Tefzel® wire is suitably used to create capacitive coupling between the balanced pairs  382  to improve high frequency performance. 
     The capacitor  382  is placed directly across the 100 ohm side of T 1  to improve the product&#39;s high frequency performance. 
     Referring to FIG. 21, this embodiment is used to match the 50 ohm impedance of a network analyzer to 100 ohm telecommunication circuits and devices. This embodiment suitably does not have a common mode shunt since balance testing capability is not needed. This embodiment is principally comprised of impedance matching transformer  408 , common mode choke  414 , and two capacitors  420 ,  424 . 
     The transformer  408  is used to match the unbalanced 50 ohm source impedance (network analyzer) to a balanced 100 ohm impedance. Suitably larger and high permeability (10,000) toroidal cores are implemented to obtain low frequency response. This component is wound as an isolation transformer to keep any DC current from flowing into the input of the analyzer. 
     The common mode choke  414  is used to block any unwanted common mode signals from the balanced side of the circuit. A large, high permeability (10,000) toroidal core is wound with twisted wire to keep consistent capacitive coupling between the 100 ohm balanced pair. 
     The first capacitor  420  is placed across the unbalanced input of the impedance matching transformer  408 . This suitably has a capacitance of less than 30 pF which enhances the high frequency performance of the balun. 
     The second capacitor  424  is placed across the balanced output of the T 1  impedance matching transformer. This suitably has a capacitance of less than 15 pF which enhances the high frequency performance of the balun. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and it is therefore desired that the present embodiment be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than to the foregoing description to indicate the scope of the invention.