Abstract:
A balun is a device for coupling together balanced and unbalanced electrical signals. An ultra-wide bandwidth balun can operate in a frequency band of more than 1.5 GHz to 26.5 GHz. The balun can be based upon a resistively loaded choke structure. The loading can be in the form of resistive cards or vanes. The vanes may be aligned with the electric field between the choke and an outer ground to prevent effective short circuits at points where the choke is half wavelength multiples in length. The resistive loading may also suppress higher order modes within the choke structure. The wideband balun can be very small to satisfy the tight space constraints of many modern communication applications. The balun may be fabricated using standard printed circuit board manufacturing techniques which may dramatically reduce production costs.

Description:
PRIORITY CLAIM TO PROVISIONAL APPLICATION 
   This application claims priority to provisional patent application entitled, “Ultra Wide Bandwidth Balun” filed on Jan. 24, 2006 and assigned U.S. Application Ser. No. 60/761,347. The entire contents of the provisional patent application mentioned above are hereby incorporated by reference. 

   TECHNICAL FIELD 
   The invention is generally directed to signal transmission systems requiring baluns for coupling balanced and unbalanced transmission lines. The invention relates more specifically to baluns in radio frequency (RF) applications where systems operate at extreme bandwidths and at RF or millimeter frequencies. The invention also relates to baluns with an integrated RF power splitting capability. 
   BACKGROUND OF THE INVENTION 
   A balun is a device designed to couple together balanced and unbalanced electrical signals. A balun can be considered a simple form of transmission line transformer. The most basic baluns use an actual transformer, with the unbalanced connection made to one winding, and the balanced to another. Other types of baluns use transmission lines of specific lengths, with no obvious transformer component. These are usually designed for narrow radio-frequency (RF) ranges where the lengths involved are some odd multiple of a quarter wavelength of the intended operating RF frequency. A common application of such a balun is in making a coaxial cable connection to a balanced antenna. 
   A balanced line or balanced signal pair is an RF transmission line that usually includes two conductors in the presence of a ground. The RF transmission line relies on balanced impedances to minimize interference. The RF signals on each line are typically the inverse of one another and each conductor is equally exposed to any external electromagnetic fields that may induce unwanted noise. The balanced line may be operated so that when the impedances of the two conductors at all transverse planes are equal in magnitude and opposite in polarity with respect to ground, the electrical currents in the two conductors are equal in magnitude and opposite in direction. These symetries can allow balanced lines to reduce the amount of noise per distance, which can enable longer cable runs. This is because electromagnetic interference will generally affect both signals the same way. Similarities between the two signals are automatically removed at the end of the transmission path when one signal is subtracted from the other. Balanced lines often also have electromagnetic shielding to reduce the amount of noise that may be introduced. 
   In contrast, an unbalanced line is a transmission line whose conductors have unequal impedances with respect to an electrical ground. Generally, in an unbalanced transmission line, one of the conductors is grounded. 
   Traditional narrow-band sleeve baluns generally use a quarter wavelength conductive cylinder. A coaxial (coax) cable is placed inside the conductive cylinder. At one end, the shielding braid of the coaxial cable is wired to the conductive cylinder while at the other end no connection is made between the cable and the conductive cylinder. The balanced end of the resulting balun is at the open end of the conductive cylinder, opposite from the end wired to the coax braid. At this point the coax cable separates into two conductors. One conductor is the center conductor separated from the braid, and the second conductor is the braid shielding of the cable or a connection to the braid. The quarter wavelength structure acts as a transformer converting the zero impedance at the end shorted to the braid to infinite impedance at the open end. This forces any current introduced by the balanced connection, such as a dipole antenna, to flow into the unbalanced coax connection as the infinite impedance of the cylinder prevents any currents from flowing on the outside of the coax cable. The conductive cylinder can be considered a choke structure. This type of balun is narrow-band or band-limited because the balun only functions well at odd multiples of quarter wavelengths. The baluns function particularly poorly at resonant frequencies (half wavelength multiples) where they may act as a short circuit. 
   In light of the bandwidth limitations of traditional narrow-band balun designs, there is a need for a balun system that operates over a very wide bandwidth and at millimeter RF frequencies. There is also a need in the art for a balun system that splits power splitter at the balanced end in order to support multiple balanced loads, such as multiple antenna elements. These wide bandwidth and power splitting qualities of a balun system are highly desirable in applications such as broadband, multiple-antenna communication systems. 
   SUMMARY OF THE INVENTION 
   The inventive broadband balun can comprise a loaded choke structure. The loading can be in the form of resistive cards or vanes. The vanes may be aligned with an electric field between the choke and an outer ground. The significance of this balun design is that it can support an ultra-wide RF bandwidth of more than 1.5 GHz to 26.5 GHz. Such an ultra wide band balun may be useful in many kinds of electronic systems for coupling balanced and unbalanced transmission lines over an extremely wide band of RF operating frequencies. A feed network of a wide band antenna is one exemplary application of this electronic component. For example, spread-spectrum techniques requiring a wide frequency bandwidth are becoming more common in communication systems. 
   Compared to traditional multi-octave baluns that are based on quarter wavelength transmission lines and are generally only capable of a ten-to-one bandwidth ratio, the inventive ultra wide band balun may operate at an eighteen-to-one bandwidth ratio. The design can utilize a lossy balun approach. When the impedance of a load attached to the balun has considerable reactance, this lossy balun design may be advantageous resulting in a system that is lossy by design. Such a system may be considered lossy because it expends a portion of the RF energy supplied to or through it. The lost energy is usually converted to heat, radiated, or dissipated in some way. 
   The invention may also provide resistive loading of its choke structure to prevent effective short circuits at points where the choke is a half wavelength multiple. The resistive loading may also suppress higher order modes within the choke structure. The resistive loading can be achieved with resistive cards, also referred to as vanes. The resistive loading may also be accomplished using a discrete resistor or an array of discrete resistors. 
   The inventive balun can be very small, on the order of 30 millimeters, to satisfy the tight space constraints of many modern communication applications. While the resistive vanes and the power splitting capability are two significant features of the technology, an additional feature of the invention is that it may be embodied using standard printed circuit board (PCB) manufacturing techniques. PCB manufacturing can be highly scalable and may dramatically reduce production costs. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a single input, single output wide-band balun using a stripline structure according to an exemplary embodiment of the invention. 
       FIG. 2  illustrates a cross-sectional view of the balanced end of a resistively loaded choke balun implemented in stripline according to one exemplary embodiment of the invention. 
       FIG. 3  illustrates a cross-sectional view of the unbalanced end of a resistively loaded choke balun implemented in stripline according to one exemplary embodiment of the invention. 
       FIG. 4  illustrates a cross-sectional view of the balanced output of a stripline balun with non-radial vanes according to one exemplary embodiment of the invention. 
       FIG. 5  illustrates balanced propagation within the cross-section of the balanced transmission line of the balun according to one exemplary embodiment of the invention. 
       FIG. 6  illustrates unbalanced propagation within the cross-section of the balanced transmission line of the balun according to one exemplary embodiment of the invention. 
       FIG. 7  illustrates a coaxial cable and a sleeve choke according to one exemplary embodiment of the invention. 
       FIG. 8  illustrates how the resistive vanes can also be embodied as a set of resistors. 
       FIG. 9  illustrates a perspective view of a single input, dual output stripline balun featuring a power split according to one exemplary embodiment of the invention. 
       FIG. 10  illustrates a perspective view of a single input, dual output stripline balun featuring a power split according to one exemplary embodiment of the invention. 
       FIG. 11  illustrates a system of single input, dual output power splitting baluns arranged in a linear fashion to make up an RF power distribution system according to one exemplary embodiment of the invention. 
       FIG. 12  illustrates a close up the unbalanced to balanced junction of a single input, single output stripline balun according to one exemplary embodiment of the invention. 
       FIG. 13  is a plot of the insertion loss for a single input, single output balun loaded with resistive cards according to one exemplary embodiment of the invention. 
       FIG. 14  is a logical flow diagram representing a method for coupling wideband RF signals between a balanced transmission line and an unbalanced transmission line according to one exemplary embodiment of the invention. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   The inventive balun system can support an ultra wide bandwidth spanning over an eighteen-to-one bandwidth ratio. Additionally, a power splitter arrangement can be incorporated into the balun system allowing the balun system to be used in a one input, one output arrangement or a one input, two output arrangement. 
   The inventive balun system may provide solutions for two challenges in the design of baluns with extreme bandwidth operation. First, a problem with wideband choke baluns is that a choke that is near a quarter wavelength at the lowest operating frequency will be near a half wavelength for a frequency higher in the band. Such a choke will perform well at the quarter wavelength but very poorly at the half wavelength and is thus band limited. Second, at higher RF frequencies the resistive cards dampen out higher order modes in the choke to further extend the useful frequency range. 
   One exemplary embodiment of the inventive balun system uses stripline technology. Such a design may result in a compact component for electronic systems such as antenna feed networks. The design may also improve reliability and yield high repeatability for quality manufacturing at a reasonable cost while achieving superior bandwidth performance. 
   Like most electromagnetic systems, the inventive balun system can be used reciprocally. The balun system can work equally well converting a balanced signal to an unbalanced signal as it can converting an unbalanced signal to a balanced signal. Also, a dual output balun system can function as a signal combiner just as it can function as a power splitter. 
   Turning now to the drawings, in which like reference numerals refer to like elements,  FIG. 1  illustrates a single input, single output wide-band balun system using a stripline structure according to an exemplary embodiment of the invention. An unbalanced single line input  101  to the balun  100  is formed from a stripline  170  surrounded by a loaded choke structure  120 . A typical width of the stripline trace is approximately 0.050 inches. The loaded choke structure  120  is illustrated as a rectangular metal structure enclosing a dielectric material  125 . The choke structure  120  can be characterized as “loaded” because it has resistive cards  110  or other resistive elements installed within the choke structure  120 . The resistive cards  110 , also called vanes, may each be oriented to extend outward from the outside wall of the choke structure  120  towards or to one or more inside surfaces of a grounded outer housing  190 . The resistive cards  110  can be positioned such that they interact with the radio frequency electric field around the choke structure  120 . 
   The resistive cards  110  are illustrated as a first vane  110  extending from the top of the choke structure  120 , a second vane  110  extending from a side of the choke structure  120 , and a third vane  110  extending from the other side of the choke structure  120 . A fourth vane can be positioned on the bottom broad surface of the choke structure  120  which is not visible in  FIG. 1 . 
   The unbalanced input  101  transitions to a balanced output  102  with the input stripline  170  extending into one of the output striplines  160  at balanced output  102 . The bottom output stripline  160  in the balanced section  140  is an extension of the narrower stripline  170  in the unbalanced section  130  of the balun  100 . Similarly, the top stripline  150  at the balanced output  102  is an extension of the choke structure  120 . Specifically, stripline  150  is an extension of the top metal wall of the choke structure  120 . 
   The signals of the two striplines  150 ,  160  at output  102  are one-hundred-eighty degrees out of phase with each other. The grounded outer housing  190  of the balun  100  can be a metallized box that serves as the outer conductor, or ground of the choke  120  around the unbalanced line  170  in section  130  of the balun  100 . The grounded outer housing  190  also serves as a shielding for the balance lines  150 ,  160  in section  140  of the balun  100 . A transition takes place at a line  135  in the midpoint of the balun  100 . This transition separates the unbalanced section  130  and balanced section  140  of the balun  100 . 
   The resistive cards, or vanes  110  may be made from a thin dielectric film such as Mylar coated with a resistive film. Such a resistive film may have a continuous resistance, for example 100 ohms per square inch. The vanes  110  may also comprise a discrete resistor, an array of discrete resistors, or a bulk resistive material. Other card types may be used as well as other structures and other resistive values all without departing from the scope of the invention. 
   Referring now to  FIG. 2 , the figure illustrates a cross-sectional view of the balanced end  102  of a resistively loaded choke balun implemented in stripline according to one exemplary embodiment of the invention. The balanced end  102  is shown with the upper stripline  150  and the lower stripline  160 . The choke structure  120  can support the resistive vanes  110  that extend outward from the choke structure  120  to an outer ground  190 . The resistive vanes  110  can be arranged radially. That is, the resistive vanes  110  can be in line with the center point of the choke structure  120  and normal to the outer surfaces of the choke structure  120 . The dielectric circuit board  180  can support the striplines  150  and  160 . The upper stripline  150  and the lower stripline  160  are spaced apart in a parallel fashion by the dielectric circuit board  180 . The choke dielectric material  125  can fill the area within the choke body  120 . The choke dielectric material  125  may be circuit board dielectric, some other dielectric, or air. 
   Referring now to  FIG. 3 , the figure illustrates a cross-sectional view of the unbalanced end of a resistively loaded choke balun implemented in stripline according to one exemplary embodiment of the invention. The unbalanced end  101  is a single stripline  170  supported by printed circuit board  180  or other dielectric material  180 . The choke structure  120  can support the resistive vanes  110  that extend outward from the choke structure  120  to an outer ground  190 . The resistive vanes  110  can be arranged radially. That is, the resistive vanes  110  can be in line with the center point of the choke structure  120  and normal to the outer surfaces of the choke structure  120 . The dielectric circuit board  180  can support the stripline  170 . The choke dielectric material  125  can fill the area within the choke body  120 . The choke dielectric material  125  may be circuit board dielectric, some other dielectric, or air. 
   Referring now to  FIG. 4 , the figure illustrates a cross-sectional view of the balanced output of a stripline balun with non-radial vanes according to one exemplary embodiment of the invention. The balanced end  102  is shown with the upper stripline  150  and the lower stripline  160 . The choke structure  120  can support the resistive vanes  110  that extend outward from the choke structure  120  and normal to the surfaces of the choke structure  120 . The resistive vanes  110  can extend out to an outer ground  190 . The resistive vanes  110  can be arranged non-radially. That is, the resistive vanes  110  do not have to be positioned in line with the center point of the choke structure  120 . The dielectric circuit board  180  can support the striplines  150  and  160 . The upper stripline  150  and the lower stripline  160  are spaced apart in a parallel fashion by the dielectric circuit board  180 . The choke dielectric material  125  can fill the area within the choke structure  120 . The choke dielectric material  125  may be circuit board dielectric, some other dielectric, or air. Resistive vanes  110  extend outwardly from the choke structure  120 . In this exemplary embodiment the resistive vanes may not extend off of the top and bottom of the choke structure. This exemplary embodiment demonstrates that the vanes can be placed as needed outside of the choke structure  120  to support ease of manufacturing and to reduce the unwanted modes of the electric fields within the balun. To be most effective, the resistive cards or vanes  110  may extend radially outward from the choke structure towards or to the grounded outer housing  190 . However, as we see here, the vanes need not be exactly radial to function. For example, the vanes  110  may lie in a line substantially parallel to the electric fields. 
   Referring now to  FIG. 5 , the figure illustrates balanced propagation within the cross-section of the balanced transmission line of the balun according to one exemplary embodiment of the invention. The top conductive trace  150  and the bottom conductive trace  160  are at opposite potentials. The top conductive trace  150  is positive and the bottom conductive trace  160  is negative. Thus, there is an electric field  530  between the two conductors. This represents a balanced or odd mode of propagation. Such a mode is the desired mode for a balanced transmission line. 
   Referring now to  FIG. 6 , the figure illustrates unbalanced propagation within the cross-section of the balanced transmission line of the balun according to one exemplary embodiment of the invention. Here, the top conductor  150  is positive, the bottom conductor  160  is also positive, no voltage potential exists between the two outputs, and the potential  640  is referenced to an outside ground not shown. This represents an even mode or unbalanced mode of propagation where. Such a mode is the undesired mode for a balanced transmission line. 
   Referring now to  FIG. 7 , the figure illustrates a coaxial cable and a sleeve choke according to one exemplary embodiment of the invention. Coaxial cable  730  comprises center conductor  740  and coaxial exterior shielding or braid  750 . Sleeve choke structure  760  is a cylindrical conductor that can be placed coaxially around the coaxial cable  730  such that they share a common center line. Resistive vanes  110  can extend outwardly within the choke structure  760  from the braid  750  of the coaxial cable  730 . The vanes  570  may extend outward from coaxial braid  750  to the cylindrical choke structure  760 . A balanced output from the sleeve balun  700  is shown at  720 A where center conductor of the coaxial cable  740  becomes one of the balanced conductors and the braid  750  of coaxial cable  730  becomes the other balanced conductor. There is also a power split where the balance output is split between one balanced pair  520 A and a second balanced pair  520 B. 
   Typically, the impedance at each of the two balanced outputs  720 A,  720 B may be twice that of the impedance of the input  710 . In this example, the output impedance at each output is 100 ohms and the input impedance is 50 ohms. While a two-way power split is illustrated, the power split may also be an N-way power split without departing from the spirit or scope of the invention. 
   Referring now to  FIG. 8 , the figure illustrates how the resistive vanes  110 A can also be embodied as a set of discrete resistors  110 B. As discussed with reference to  FIG. 1 , the resistive vanes may also be embodied as a single discrete resistor, a resistive film, bulk resistive material, or any other mechanism for providing a resistive loading to the choke structure of the balun. 
   Referring now to  FIG. 9  and  FIG. 10  together, both figures illustrate perspective views of a single input, dual output stripline balun  900  featuring a power split according to one exemplary embodiment of the invention. The unbalanced input  901  is a single transmission line. The transmission line enters the rectangular choke structure  910 . The rectangular choke structure  910  is similar to the choke structure  120  of  FIG. 1 . Resistive vanes ( 110 , not illustrated) can extend beyond the outer surface of the choke structure  910  to an external ground conductor. The resistive vanes  110  may be substantially normal to the outer surfaces of the choke structure  910  and may be arranged radially as discussed with relation to  FIG. 2 , or non-radially as discussed with relation to  FIG. 4 . The choke structure  910  is in the unbalanced section of the balun  900 . The unbalanced section of balun  900  may be substantially identical to the unbalanced portion  130  of a non-power-splitting stripline balun  100 , such as those discussed in relation to  FIGS. 1 ,  2 ,  3 , and  4 . 
   At the splitter location  950 , the balanced end of the choke structure  910  can split out to service two balanced outputs  902 ,  903 . A first balanced output  902  can be is fed by the balanced transmission line made up of an upper trace  964  and a low trace  968 . A second balanced output  903  can be fed by the balanced transmission line made up an upper trace  960  and a lower trance  962 . In the exemplary embodiment illustrated in  FIG. 9 , the two upper traces  964 ,  960  can split off of the upper portion of the choke structure  910 , while the lower traces  968 ,  962  can split off of the single transmission line (not illustrated) within the choke structure  910 . Such a splitting can provide for the two balanced outputs  902 ,  903  being in phase with one another. In another exemplary embodiment the upper traces  964 , 960  can split off of the center transmission line of the choke structure  910  while the lower traces  962 ,  968  can split off of the lower portion of the choke structure  910 . In this second example, the splitting can provide for the two balanced outputs  902 ,  903  being in phase with one another but in opposite phase from the first example. In other exemplary embodiments, the balanced outputs  902 ,  903  can be out of phase from one another by one extending from the upper portion of the choke structure  910  and the other extending from the lower portion of the choke structure  910 . Such an arrangement may require more printed circuit layers on the balanced end of the dual output balun  900 . 
   The balanced end  902 , 903  of the balun system may be constructed of three dielectric layers,  1010 ,  1011 , and  1012 . The upper conductors  962 ,  964  of the balanced outputs  902 ,  903  can lie on the metallization layer  1020  positioned between the top dielectric layer  1010  and the second dielectric layer  1011 . The lower conductors  962 ,  968  of the balanced outputs  902 ,  903  can lie on the metallization layer  1021  positioned between the second dielectric layer  1011  and the third dielectric layer  1012 . 
   While a two-way power split is illustrated, the power split may also be an N-way power split without departing from the spirit or scope of the invention. 
   Referring now to  FIG. 11 , the figure illustrates a system  1100  of single input, dual output power splitting baluns  900  arranged in a linear fashion to make up an RF power distribution system according to one exemplary embodiment of the invention. The distribution system  1100  shows a plurality of single input, dual output baluns  900 . The baluns  900  are arranged in a linear fashion and connected by a rigid support structure. Multiple linear arrays  1100  may be arranged to form a two dimensional plane of balanced outputs. 
   Referring now to  FIG. 12 , the figure illustrates a close up the unbalanced to balanced junction of a single input, single output stripline balun according to one exemplary embodiment of the invention. Near the point where the unbalanced input trace  170  (not visible in  FIG. 12 ) and one surface of the choke structure  120  extend to become the conductors  150 ,  160  of the balanced transmission line, a transition in the width of the trances may serve to match the impedance between the single unbalanced conductor and the balanced transmission line. 
   Referring now to  FIG. 13 , the figure is a plot of the insertion loss for a single input, single output balun loaded with resistive cards according to one exemplary embodiment of the invention. The plot shows frequency in gigahertz (GHz) on the horizontal axis and power in decibels (dB) on the vertical axis. The top trace  1310  of the plot is the desired output signal at the balanced output port  102 . This is the odd field between the output conductors. It is this odd, balanced, or transverse electromagnetic (TEM) mode that is the desired output. The bottom trace  1320  is the undesired output signal obtained by shorting out the two output conductors and measuring the voltage to the grounded outer housing. Electric fields exist between the pair and the outer ground surfaces. This is the undesired output signal of the unbalanced or the even mode. 
   Referring now to  FIG. 14 , the figure shows a logical flow diagram representing a method for coupling wideband RF signals between a balanced transmission line and an unbalanced transmission line according to one exemplary embodiment of the invention. Certain steps in the processes or process flow described in all of the logic flow diagrams referred to below must naturally precede others for the invention to function as described. However, the invention is not limited to the order of the steps described if such order or sequence does not alter the functionality of the present invention. That is, it is recognized that some steps may be performed before, after, or in parallel other steps without departing from the scope and spirit of the present invention. 
   Step  1410  involves propagating an RF signal over an unbalanced transmission line  170 . The source of the RF signal can be a signal detector, an antenna, a mixer, an oscillator, another transmission line, a connection to another transmission line, or any other component, device, or system that can be used to feed an RF signal into a transmission line. 
   In Step  1420 , an RF signal is coupled from the unbalanced transmission line  170  into a choke balun  100 . The unbalanced transmission line is the same as the transmission line  170  discussed in relation to Step  1410 . 
   In Step  1430 , nulls in the RF signal at resonant frequencies of the choke balun  100  are substantially reduced by proving a resistive load  110  within the choke structure  120  of the balun. These undesirable resonances take place at half wavelength multiples of the length of the choke structure. The resistive loading  110  may be provided by resistive cards, vanes, resistive films, a single resistor, an array of resistors, a bulk resistive material, or any other mechanisms for resistively loading the choke structure of the balun. This RF loading can be optimized by modeling software such as High Frequency Structure Simulator (HFSS) or by empirical testing. 
   In Step  1440 , the RF signal is coupled from the choke balun  100  into a balanced transmission line  102 . Finally, in Step  1450 , the RF signal is propagated along the balanced transmission line  102  mentioned with respect to Step  1440 . This balanced transmission line  102  may feed into some balanced load. The load can be a transmitter, antenna, laser, amplifier, another transmission line, a coupling into another transmission line, or any other component, device, or system that an RF signal can be fed into. 
   Alternative embodiments of the wide band balun system will become apparent to one of ordinary skill in the art to which the present invention pertains without departing from its spirit and scope. Thus, although this invention has been described in exemplary form with a certain degree of particularity, it should be understood that the present disclosure has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts or steps may be resorted to without departing from the spirit or scope of the invention. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description.