Abstract:
The invention is a balun transformer that converts a single-ended (or unbalanced) signal to a differential (or balanced) signal. The balun is a printed metal pattern on a circuit board in conjunction with several low cost chip capacitors and a low cost chip inductor. The balun transformer is a modified Marchand balun that is implemented using printed transmission lines. The balun has a plurality of coupled transmission lines to improve tolerances to variations in PC board fabrication. To make the balun compact, it is electrically lengthened through the use of capacitive loading, which reduces the required physical size. Additionally, the capacitors increase the bandwidth due to the resonant interaction between the short inductive balun and the capacitors that are placed in series with the input and the output.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 09/892,755, filed Jun. 28, 2001 now U.S. Pat. No. 6,819,199, entitled “Balun Transformer with Means for Reducing a Physical Dimension Thereof,” which claims benefit of U.S. Provisional Application No. 60/262,629, filed Jan. 22, 2001, each of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to balanced to unbalanced transformers (baluns) and more particularly to an improved Marchand balun adapted for use on printed circuit boards. 
     2. Background Art 
     In radio frequency integrated circuits, it is often desirable for the input and output connections to be differential. An example of a differential connection is two wires having an equal impedance to a common ground conductor, their respective signals 180 degrees out of phase. A transmission line having these characteristics is known as a balanced line, as opposed to an unbalanced line. The advantages of a balanced radio frequency signal input over an unbalanced input include higher dynamic range, higher bandwidth, and lower pick-up and generation of interference. 
     Unfortunately, many radio frequency components, such as coaxial cable, are unbalanced. An adapter is required to convert the unbalanced signal into a balanced one without loss or distortion and while maintaining the proper matching impedance to terminate the transmission line. A passive device that achieves this function is known as a balun, and can be constructed in various ways. Many existing passive baluns either are too large, too expensive, too complex or have an insufficient bandwidth to be effectively implemented in a printed circuit board RF application. 
     What is needed is a passive balun that exhibits low signal distortion, high bandwidth, low loss, and has a good impedance match. Additionally, the passive balun should be small, simple to manufacture, and tolerant of variations. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention is a modified Marchand balun implemented using a plurality of first coupled printed metal traces electromagnetically coupled to a plurality of second coupled printed metal traces. The second coupled metal traces are also coupled to ground. Capacitors are coupled between the balun input and the first coupled metal traces, between ground and the first metal traces and between the second coupled printed metal traces and each of a pair of balun outputs. 
     The balun outputs a balanced signal, one output having an equal amplitude and opposite phase relative to a second output. Some of the metal traces comprising the balun are configured as transmission lines. These transmission lines are either coplanar waveguide transmission lines or microstrip transmission lines. A ground is located at the periphery or beneath the balun. Some embodiments place a ground at the periphery and below the balun. An impedance matching network is coupled to the input of the balun and an impedance matching network is coupled to the balanced output. Both the input and output impedance matching networks can comprise lumped or distributed element components. 
     A capacitor and spiral inductor can be coupled to the balun input. A direct current or voltage source and low frequency digital control signals can be applied to the balun input without electrically loading the balun at a desired input signal frequency. The applied direct current or voltage source can power various active circuits such as a low noise block via a coaxial cable. 
     The value of each lumped element, each distributed element and the width and spacing of the printed metal traces is incrementally varied in a simulator to determine the specific values that result in the desired balun circuit characteristics. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The present invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the leftmost digit(s) of a reference number identifies the drawing in which the reference number first appears. 
         FIG. 1A  illustrates a satellite receiver and set top box embodying the present invention. 
         FIG. 1B  illustrates a classic prior art Marchand balun. 
         FIG. 2A  illustrates a cross section of a coplanar waveguide. 
         FIG. 2B  illustrates a cross section of a coplanar waveguide with ground. 
         FIG. 3  illustrates a coplanar waveguide implantation of the classic prior art Marchand balun. 
         FIG. 4A  illustrates a circuit diagram for a 3-Finger balun according to the present invention. 
         FIG. 4B  illustrates a circuit diagram for a 4-Finger balun according to the present invention. 
         FIG. 5A  illustrates a balun according to the present invention. 
         FIG. 5B  illustrates an embodiment of a balun after calculating element values. 
         FIG. 6A  illustrates cross section of a two layer printed circuit board without ground plane under the balun traces. 
         FIG. 6B  illustrates cross section of a two layer printed circuit board with ground plane under the balun traces. 
         FIG. 6C  illustrates a cross section of four layer printed circuit board without ground plane under the balun traces. 
         FIG. 6D  illustrates a cross section of four layer printed circuit board with ground plane under the balun traces. 
         FIG. 6E  illustrates a cross section of six layer printed circuit board without ground plane under the balun traces. 
         FIG. 6F  illustrates a cross section of six layer printed circuit board with ground plane under the balun traces. 
         FIG. 7  illustrates an embodiment of the present invention on a printed circuit board. 
         FIG. 8  illustrates an optimized inductor that can be used to transmit DC power or low frequency digital control signals to the center conductor of a balun according to an embodiment of the present invention. 
         FIG. 9  illustrates the present invention using three transmission lines and a lumped element matching network. 
         FIG. 10  illustrates an alternate embodiment of the 3-Finger balun with a ground below the balun according to the present invention. 
         FIG. 11  illustrates an inductor for providing DC power or low frequency digital control signals at the balun input in an embodiment of the present invention. 
         FIG. 12  illustrates steps of a method for initially designing a balun according to the present invention. 
         FIG. 13  illustrates details of the method step of selecting initial design parameters. 
         FIG. 14  illustrates details of the method step of simulating balun performance. 
         FIG. 15  illustrates details of the method step of incrementally varying balun parameters. 
         FIG. 16 . illustrates steps of a method for final balun design according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     I. Example Environment 
     Referring to  FIG. 1A , an example system implementing this invention is illustrated. A data signal  122  from a satellite  124  is received at a dish antenna  126  and routed to a low noise block  120 . Typically, low noise block  120  comprises a low noise amplifier, a mixer, an oscillator and an IF amplifier. Low noise block  120  amplifies and converts data signal  122  to a desired frequency range from data signal  122  downlink frequency. In one embodiment the desired frequency range is 950–2150 MHz. Data signal  122  is passed over a coaxial cable  132  from low noise block  120  to a television set-top box  134 . Set-top box  134  comprises a tuner circuit  138  that converts data signal  122  into a signal suitable for reception by a television  136 . An example commercial embodiment of tuner  138  is the Broadcom BCM3440A0. 
     It is preferable that tuner  138  have a differential radio frequency input in order to achieve the low second order nonlinear distortion that is critical to a direct conversion receiver architecture. A balun  130  is placed between coaxial cable  132  and tuner  138  to convert the unbalanced radio frequency signal carried in coaxial cable  132  to a balanced radio frequency signal  112  and  114  at the input of tuner  138 . 
     An embodiment of the invention is a balun implemented as a printed metal pattern on a circuit board containing the tuner chip and its ancillary components. In addition to meeting the required electrical specifications, the printed balun is tolerant of parameter variations during printed circuit board manufacturing. These parameter variations include metal line width, spacing between metal lines, printed circuit board thickness, dielectric constant of the board, and proximity to other metal objects. Tolerance of printed circuit board manufacturing variations enables the printed balun design to be easily incorporated into a standard printed circuit board assembly process. In a preferred embodiment the balun output lines should be located close together to interface properly with an integrated circuit package. 
     II. Marchand Balun 
     One suitable structure is the Marchand balun  100 , shown in  FIG. 1B . This classic balun implementation uses two quarter-wavelength (λ4) sections of coaxial cable inside another coaxial shield. One section includes electromagnetically coupled lines  104  and  108 , and the other section includes electromagnetically coupled lines  102  and  106 . The electromagnetic coupling between coaxial line  102  and  106  and between  104  and  108  results in a signal at balun output  112  that is equal in amplitude and opposite in phase to a signal at balun output  114  relative to an input signal at balun input  110 .  FIG. 1B  includes an exemplary impedance value of 75Ωat outputs  112  and  114 . 
     A coaxial cable can be flattened and adapted into printable form by cross sectioning the coaxial structure and flattening the conductors into coplanar waveguides. Referring to  FIG. 2A , a coplanar waveguide  202  comprises a signal trace  206  flanked on both sides by a ground  208 . Signal trace  206  and ground  208  are laid on a substrate  212 . Referring to  FIG. 2B , a coplanar waveguide with ground  204  comprises the elements of waveguide  202  and an additional ground  209  under metal trace  206 . In coplanar waveguide  204 , ground  208  can be connected with ground  209  by vias  220  through substrate  212 . In  FIGS. 2A and 2B , the reference labels “s” and “w” represent conductor spacing width: the reference label “h” represents dielectric height of the substrate  212 ; and the reference label “ε r ” represents the dielectric constant of the substrate  212 . Balun  100  is modified for printed circuit board use by transforming coaxial cable into the coplanar waveguide  202  as illustrated in  FIG. 3 . Vias  220  (e.g., see  FIG. 2B ), also known as plated through holes, provide electrical connection between different layers in multi layer printed circuit boards. 
     Referring to  FIG. 3 , the coplanar balun  300  consists of balun input  110  coupled to input coplanar waveguide  308 . First coplanar waveguide  304  and second coplanar waveguide  306  are coupled to balun output  114  and balun output  112 , respectively. Coplanar waveguides  304  and  306  are coupled to a ground  302 . The electromagnetic coupling between coplanar waveguides  304 ,  306 ,  308  result in a signal at output  112  that is equal in amplitude and opposite in phase to a signal from balun output  114  relative to an input signal input to balun input  110 . 
     Previous printed circuit implementations of the Marchand balun have had disadvantages relative to the present invention. For example, in  Compact and Broad - Band Three Dimensional MMIC Balun , IEEE Transactions on Microwave Theory and Techniques, Vol. 47, No.1 January, 1999, which is incorporated herein by reference in its entirety, a printed circuit balun implementation is presented. The described balun requires the input and output lines to be placed in different layers of the circuit board and thus the balun&#39;s proper operation depends critically on dielectric thickness, which is not desirable for printed circuit board implementation. Due to the use of full quarter-wavelength lines, the described balun transmission lines must be meandered across the printed circuit board to achieve a compact size. The meandered transmission lines disturb the described balun&#39;s operation, which is corrected by adding an additional transmission line that, undesirably, necessitates spreading the output lines far apart. 
     Another Marchand balun is discussed in  Design and Performance of GaAs MMIC CPW Baluns Using Overlaid and Spiral Couplers,  1997 IEEE MTT-S Digest, which is incorporated herein by reference in its entirety. The balun discussed is a conventional two-wire Marchand using a coplanar layout. To achieve the required coupling between the transmission lines is difficult with only two wires. To compensate, a complicated overlay scheme is used which is not compatible with a printed circuit board fabrication process. No size reduction schemes are used, so the balun is undesirably large for use on a printed circuit board. 
     A third example of the conventional Marchand balun is discussed in  A New Compact Wideband Balun,  1993 IEEE MTT-S Digest, which is incorporated herein by reference in its entirety. The balun discussed is made of multiple coupled microstrip lines, so the balun depends critically on dielectric parameters and distance between layers. Multiple lines are used to relax the conductor spacing, but the layout results in the output ports exiting on opposite sides of the balun. 
     III. The Invention 
     The preferred embodiment of the present invention is now described with reference to the figures where like reference numbers illustrate like elements. Furthermore, the left digit of each reference number corresponds to the figure in which the reference number is first used. While specific methods and configurations are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the art will recognize that other configurations and procedures may be used without departing from the spirit and scope of the invention. 
     Referring to  FIG. 4A , a 3-Finger balun transformer, according to the present invention; is presented as an electrical circuit schematic. Balun  400  consists of transmission lines  404 ,  406 ,  408 ,  410 ,  412 , and  414 . Transmission line  404  is coupled to transmission line  406 . Input capacitor  402  is coupled to transmission line  404  and balun input  110 . A loading capacitor  418  is coupled to transmission line  406  and a ground  416 . Transmission lines  408 ,  410 ,  412 , and  414  are coupled to ground  416 . Output capacitor  422  is coupled to transmission lines  408 ,  412  and to positive balun output  112 . Output capacitor  420  is coupled to transmission lines  410 ,  414  and to negative balun output  114 . Transmission lines  404 ,  408 , and  412  are electromagnetically coupled. Transmission lines  406 ,  410 , and  414  are electromagnetically coupled. The electromagnetic coupling between transmission lines  404 ,  406 ,  408 ,  410 ,  412 , and  414  result in a signal at output  112 , in response to a signal applied to balun input  110 , that is equal in amplitude and opposite in phase to a signal at output  114 . 
       FIG. 4B  is a 4-Finger balun transformer. Balun  401  consists of transmission lines  404 ,  405 ,  406 ,  407 ,  408 ,  410 ,  412 , and  414 . Transmission line  404  is coupled to transmission lines  405  and  406 . Input capacitor  402  is coupled to transmission lines  404 ,  405  and balun input  110 . Transmission line  405  is coupled to transmission  407 . A loading capacitor  418  is coupled to transmission lines  406 ,  407  and to a ground  416 . Transmission lines  408 ,  410 ,  412 , and  414  are coupled to ground  416 . Output capacitor  422  is coupled to transmission lines  408 ,  412  and to positive balun output  112 . Output capacitor  420  is coupled to transmission lines  410 ,  414  and to negative balun output  114 . Transmission lines  404 ,  405 ,  408 , and  412  are electromagnetically coupled. Transmission lines  406 ,  407 ,  410 , and  414  are electromagnetically coupled. The electromagnetic coupling between transmission lines  404 ,  405 ,  406 ,  407 ,  408 ,  410 ,  412 , and  414  result in a signal at output  112 , in response to a signal applied to balun input  110 , that is equal in amplitude and opposite in phase to a signal at output  114 . 
     Transmission lines are formed from metal traces. Metal traces provide electrical and electromagnetic coupling. Transmission lines can be configured to function as coplanar waveguide transmission lines or microstrip transmission lines. 
       FIG. 5A , illustrates printed balun  500 , which is a printed metal trace implementation of a balun according to the present invention. Printed balun  500  has two metal traces  508 ,  516  and two connecting traces  501 ,  514  electrically connected and laid out in a rectangular pattern. Metal traces  508  and  516  function as transmission lines. Four additional metal traces  520 ,  524 ,  534 , and  540  are laid parallel to metal traces  508  and  516 . Trace  520  and  524  are coupled to a ground  512  and connector  538 . Metal traces  534  and  540  are coupled to ground  512  and connector  536 . Metal traces  520 ,  524 ,  534 , and  540  function as transmission lines. Input capacitor  504  is coupled to metal trace  508  and to input inductor  502 . Input inductor  502  is coupled to balun input  110 . A loading capacitor  530  is coupled between metal trace  508  and ground  512 . Balun positive output  112  is coupled to thin metal trace  510 . Thick metal trace  544  is coupled to thin metal trace  510  and output capacitor  532 . Output capacitor  532  is coupled to metal trace  534 . Balun negative output  114  is coupled to thin metal trace  506 . Thick metal trace  509  is coupled to thin metal trace  506  and output capacitor  526 . Capacitor  526  is coupled to metal trace  520 . Electromagnetic coupling between transmission lines  508 ,  516 ,  520 ,  524 ,  534 , and  540  result in a signal at output  112 , in response to a signal applied to balun input  110 , that is equal in amplitude and opposite in phase to a signal at output  114 . 
     Inductor  502  and capacitor  504  add an input impedance matching network to balun  500 . The addition of thick metal traces  544 ,  509  and thin metal traces  506 ,  510  add an output impedance matching network to balun  500 . In the embodiment illustrated in  FIG. 5A  elements  506 ,  509 ,  510 , and  544  are distributed elements. Metal trace width, trace spacing, trace proximity to ground and trace thicknesses are varied to achieve the capacitance and inductance values necessary to impedance match with the output circuit. In alternate embodiments elements  506 ,  509 ,  510 , and  544  can be lumped element components. 
     The extent of ground  512  is indicated on  FIGS. 5 ,  8 ,  9 , and  11  by diagonal lines. For the embodiment illustrated in  FIGS. 5A and 5B , ground  512  is at the periphery of balun  500 . Ground  512  can be placed on different layers of the printed circuit board and coupled by vias  220  to the desired layer. In other embodiments ground  512  is placed under the balun. 
     The present invention, exemplified by balun  500 , modifies the classic Marchand balun illustrated in  FIGS. 1 and 2  by adding input capacitor  504 , loading capacitor  530 , output capacitor  532  and output capacitor  526 . Coupling these elements to the balun transmission lines provides multiple improvements as discussed below. 
     The capacitance of elements  504 ,  530 ,  526 ,  532  is calculated so the balun operates at its most efficient internal impedance. Ground  512  is removed from beneath the balun metal traces and placed at the periphery for simplicity of fabrication. Four coupled transmission lines are employed rather than two in order to achieve the desired internal balun impedance without requiring the trace spacing to be the manufacturing minimum of 5-mils. A high, even mode/odd mode, impedance ratio increases the balun&#39;s impedance matching bandwidth and lowers insertion loss across a wider band of input frequencies. 
     The added capacitance of elements  504 ,  530 ,  526 , and  532  electrically lengthens the balun by reducing transmission line wave velocity, thereby enabling the physical length of the transmission lines to be reduced to less than the classic balun&#39;s λ/4 length without affecting the unbalanced to balanced signal transformation. Reducing the required transmission line length means the physical size of the balun can be reduced. Reduced size makes it easier for a circuit designer to implement the balun on a crowded printed circuit board. 
     A further improvement to the classic Marchand balun is the addition of impedance matching networks at the balun input and outputs. The input matching network consists of inductor  502  and capacitor  504  series coupled at balun input  110 . The output matching network consists of thick metal trace  509  coupled to thin metal trace  506 , which is coupled to balun output  114 . Thick metal trace  544  is coupled in series with thin metal trace  510 , which is coupled to the balun output  112 . These networks are designed to match the input impedance of the balun to the impedance of the input circuitry and the output impedance of the balun to the impedance of the output circuit. The matched input and output impedances provide improved signal transfer between the input and output circuits while the internal balun impedance is unaffected and optimized for high bandwidth, low loss and other factors. 
     The output impedance value is selected to match the circuitry coupled to the balun output. This impedance is typically 50 or 75 ohms; but is variable to match other circuit values. 
     In the present invention the input and output matching networks can be constructed from lumped elements or distributed elements. A person skilled in the art will understand the advantages and disadvantages of both types of elements. 
     In a preferred embodiment, essential features of the balun illustrated in  FIG. 5A , such as the transmission lines, lumped and distributed components, are placed on the top layer of a printed circuit board. This simplifies balun construction and makes it more tolerant to manufacturing changes in printed circuit board layer width, dielectric constant, and printed trace dimensions. A lower metal layer can function to connect metal traces. Alternatively surface mount zero-ohm chip resistors can be used. 
     Referring to  FIG. 5B , an embodiment of a balun  500  is presented as balum  550  with calculated element values and metal trace dimensions. Balun  550  shows common reference numbers with the balum  500  that were already discussed with reference to  FIG. 5A . Balun  550  has the following electrical characteristics: 
     These performance specifications are for example only and are not meant to be limiting. Other performance specifications will be apparent to persons skilled in the art based on the disclosure provided herein. 
     The physical arrangement of metal traces in relation to each other and to electrical ground, determines whether the traces perform as coplanar waveguides or as microstrip transmission lines. Either type of transmission line can be used in the present invention to achieve the performance benefits discussed above. 
     The use of coplanar waveguide makes a ground under the balun traces optional, since the necessary configuration can be set on a single layer. Some embodiments use a ground under the balun metal traces to achieve better isolation and shielding from external noise sources. 
     Referring to  FIGS. 6A–6F , different arrangements of balun metal traces  206 , ground  208  and ground  209  are illustrated. Vias  220  is used to connect ground  208 ,  209  together across printed circuit board (PCB) layers.  FIG. 6A  illustrates a two-layer PCB  602  with balun traces  206 , ground  208  and vias  220  connecting ground  208  to ground  209 .  FIG. 6B  illustrates a two-layer PCB  604  with ground  209  under balun traces  206 .  FIG. 6C  illustrates a four-layer PCB  606  without ground  209  under balun traces  206 .  FIG. 6D  illustrates a four-layer PCB  608  with ground  209  under balun traces  206 .  FIG. 6E  illustrates a six-layer PCB  610  without ground  209  under balun traces  206 .  FIG. 6F  illustrates a six-layer PCB  612  with ground  209  under balun traces  206 . Balun  500  can be embodied in any of the printed board cross sections described in  FIGS. 6A–6F  as well as additional cross section arrangements that one of skill in the art would recognize based on the teachings herein. 
     One embodiment of balun  500  uses four coupled coplanar waveguides to strengthen the electromagnetic coupling, making the balun tolerant to variation in printed circuit board materials, dimensions and customer layout. Inexpensive lumped components can be used for tuning and impedance matching to a capacitive balun input. The geometry of the output lines functions as a distributed tuning network to provide another degree of freedom in matching the balun to a particular integrated circuit. Other embodiments use three couples transmission lines to achieve the desired performance. 
       FIG. 7  illustrates printed circuit  700 , which is a printed circuit embodiment of the invention. Printed circuit  700  is balun  500  coupled to a radio frequency signal input  702  at balun input  110 . Capacitor  706  is a one picofarad capacitor coupled approximately 425 mils from balun input  110 . Balun output  112  and balun output  114  are coupled to a tuner  704 . In one commercial embodiment, tuner  704  is the Broadcom BCM3440A0. 
       FIG. 8  illustrates an apparatus  800  for transferring direct current power and low frequency digital control signals to low noise block  120  (see  FIG. 1A ) adapted for use with balun  500  (see  FIG. 5A ) the embodiment of  FIG. 8  is presented with exemplary dimension values for the balun  800  including widths equal to 7 mils, 21 mils, 28 mils, 35 mils, and 63 mils, and spacing between element equal to 21 mils, 75 mils and 425 mils. Direct current power is defined as power supplied from a current source as direct current or from a voltage source as direct voltage. In addition to direct current power, low frequency digital control signals can be supplied to low noise block  120 . A direct current power and low frequency digital control signal source  802  is coupled to spiral inductor  810 . Direct current power and low frequency digital control signals can be supplied from source  802  together or either signal separately. Spiral inductor  810  is connected to balun radio frequency input  702 , approximately 425 mils from balun input  110 . Radio frequency input  702  is connected to coaxial cable  132 . (see  FIG. 1A ) Coaxial cable  132  is connected to low noise block  120 . A capacitor  804  is also coupled to ground  512  and to radio frequency input  702  approximately 425 mils from balun input  110 . Capacitor  804  and inherent capacitance from the connection of spiral inductor  810  reduce undesirable cross over interference at balun input  110 . Ground  512  is provided from vias  220 . Individual vias are shown as solid dots but, for clarity, each is not labeled. 
     Spiral inductor  810  is designed to provide a very high impedance at the signal frequency of interest at balun input  110 . Connection of spiral inductor  810  to balun input  110  has negligible loading effect on balun  500  and radio frequency input  702 . The high impedance of spiral inductor  810  does not affect the transmission of direct current power or low frequency digital control signals. Spiral inductor  810  coupled to balun  500  is an embodiment of the present invention to provide direct current power and low frequency digital control signals along coaxial cable  132  to power low noise block  120 . This embodiment does not deleteriously affect the radio frequency signal at balun input  110  or the electrical characteristics of the balun. 
       FIG. 9  illustrates balun  900 , which is an embodiment of the present invention that uses lumped capacitive and inductive components and three coupled transmission lines. This embodiment illustrates the design flexibility of the present invention. It shows metal trace dimensions and the number of coupled transmission lines can be varied to achieve a desired result. Balun  900  consists of a first input transmission line  902  electrically coupled to a second input transmission line  904  and laid out in a rectangular pattern. A first output transmission line  906  is coupled to a second output transmission line  908  and to ground  512 . Input transmission lines  902  and  904  are coupled to an input capacitor  910 . An input inductor  912  is coupled to input capacitor  910  and to balun input  110 . First output transmission line  906  is coupled to an output capacitor  914 . Output capacitor  914  is coupled to an output inductor  928 . Output inductor  928  is coupled to a positive balun output  918 . Second output transmission line  908  is coupled to an output capacitor  916 . Output capacitor  916  is coupled to an output inductor  930 . Output inductor  930  is coupled to a negative balun output  920 . Electromagnetic coupling between transmission lines  904  and  906  and electromagnetic coupling between  902  and  908  result in a signal at balun output  918  in response to a signal applied to balun input  110 , which is equal in amplitude and opposite in phase to a signal at output  920 . 
     A loading capacitor  922  is coupled to transmission lines  902  and  904  and to ground  512 . Loading capacitor  922  is equivalent to capacitor  530 . (see  FIG. 5A ) Capacitor  922  can be fabricated as a distributed or a lumped element capacitor. A tuning capacitor  926  is coupled across the outputs of capacitor  914  and capacitor  916 . Capacitor  926  provides a differential capacitance on balun  900  to allow finer tuning of the internal balun impedance and thereby reduce input return loss. Ground  512  is provided from vias  220 . Individual vias are shown as solid dots but, for clarity, each is not labeled. 
       FIG. 10  illustrates balun  1000  which is and embodiment of the present invention the embodiment of  FIG. 10  is presented with exemplary dimension valurs for the balun  1000  including widths equal to 0.007″, 0.011″, 0.016″, 0.020″, 0.030″, 0.050″, 0.480″, and 0.525″, spacing between elements equal to 0.020″ and 0.250″, and diameters equal to 0.010″. Balun  1000  has balun input  110  coupled to input capacitor  912 . Capacitator  912  is connected to conductor  910 . An input transmission line  1006  is coupled to inductor  910  ant to loading capacitor  922 . Capacitor  922  is coupled between transmission line  1006  and ground. Transmission lines  1002 ,  1004 , and  1010  are electrically coupled to output capacitor  914  and output capacitor  916 . Output inductor  930  is connected to balun negative output  920 . Output inductor  928  is connected to output capacitor  914  and balun positive output  918 . Tuning capacitor  926  is connected between the output side of capacitors  914  and  916 . Transmission line  1006  is electromagnetically coupled to transmission lines  1002 ,  1004 , and  1010  that results in a signal at output  918 , in response to a signal applied to balun input  110 , that is equal in amplitude and opposite in phase to a signal at output  920 . 
     Ground is provided from vias  220 . Individual vias are shown as solid dots but, for clarity, each is not labeled. Also for clarity, diagonal lines are not used to show the location of ground. Elements containing vias  220  are coupled to ground. In the embodiment shown in  FIG. 10  ground is located on layer two under everything except balun. There is also a ground on layer four located beneath everything. 
       FIG. 10  also illustrates an embodiment of a device used to provide direct current and voltage power or low frequency digital control signals to low noise block  120  (see  FIG.1A ) Direct current power and low frequency digital control signal source  802  is coupled to meandered trace  1025 . Trace  1025  is coupled to balun  1000  between input  110  and input capacitor  912 . Meandered trace  1025  provides a high impedance to data signal  122  to minimize undesired electrical loading of balun  1000  and low noise  120 .  FIG. 10  shows meandered trace  1025  as having exemplary dimensions of 1.016″ long and 0.007″ wide. 
       FIG. 11  illustrates an alternate embodiment of a spiral inductor used to transfer direct current power and low frequency digital control signals to coaxial cable  132 . (see  FIG. 1A ) The embodiment of  FIG. 11  is presented with exemplary dimension values for the balun  1100  including widths equal to 0.007″, 0.021″, 0.060″, and 0.238″ and a diameter equal to 0.010″. Spiral inductor  1100  has direct current power and low frequency digital control signal source  802 . A connection  1120  couples spiral  1100  to balun input  110 . Ground  512  is provided from vias  220 . Individual vias are shown as solid dots but, for clarity, each is not labeled. Ground  512  is also located under the spiral elements. For clarity the ground under spiral inductor  1100  is not illustrated with diagonal lines. The top layer ground  512  is shown with diagonal lines. 
     The high impedance exhibited by inductor  1100  does not effect the operation of balun  500  (see  FIG. 5A ) or coaxial cable  132  at signal frequencies of 950 to 2150 MHZ. Direct current power and low frequency digital control signals are unaffected by the high impedance. The direct current power and low frequency digital control signals are placed on the center connector of coaxial cable  132  and applied to low noise block  120 . Inductor  1100  is an embodiment of inductor  800  modified to function with ground under the metal traces. Spiral inductor  1100  can be used in place of meandered trace  1025  for coupling direct current power and low frequency digital control signal source  802  to balun input  110 . 
       FIG. 12  illustrates a method  1200  for initial design of a balun according to the present invention. In step  1210 , a design for the balun is selected. In step  1230 , the performance of the balun is simulated. In step  1230 , the performance of the balun is compared with the design goal performance. If the simulated performance is equal or better than design goal performance (YES), the initial design is complete, step  1250 . If simulated performance is less than design goal performance (NO), the existing parameters are varied, step  1240 . Then step  1220  is performed again to simulate the balun performance. Steps  1220 ,  1230  and  1240  continue until the initial balun design is complete in step  1250 . 
     Step  1210  is shown in further detail in  FIG. 13 . In step  1310 , an initial length, width and spacing of the metal traces are selected. These values are the balun designer&#39;s best estimates of the components and configuration necessary to achieve the balun design goals. In step  1320 , initial physical size constraints and an initial metal trace layout are selected. These constraints account for any size or configuration constraints placed on the balun design in its intended use. In step  1330 , an initial ground plane configuration is selected. In step  1340 , an initial value is selected for individual capacitors  404 ,  410 ,  426 , and  432 . 
     Step  1220  is shown in further detail in  FIG. 14 . In step  1405 , the balun parameters are encoded in a simulator. In step  1410 , the balun simulation is driven with a characteristic input impedance. In an embodiment the input impedance is 75 ohms. In step  1420 , the balun is loaded with a substantially balanced differential load. This output load is a simplification of the load expected in the actual circuit embodiment. An ideal value is selected to allow the balun simulation to more easily converge on a solution. The substantially balanced differential load does not reflect the floating load anticipated in an actual circuit embodiment of the balun. In step  1430 , the balun passband insertion loss, input return loss, bandwidth and differential signal balance are calculated by the simulator. 
     Step  1240  is shown in further detail in  FIG. 15 . In step  1520 , the value of a capacitor is varied incrementally in a manner to result in balun performance closer to the design goal. In one embodiment, an example of the specific capacitors varied in this step are the input capacitor  404 , the loading capacitor  410 , output capacitor  426 , and output capacitor  432 . In step  1540 , the printed metal trace lengths are varied incrementally in a manner to result in balun performance closer to the design goal. In step  1560 , the printed trace widths are varied incrementally in a manner to result in balun performance closer to the design goal. In step  1580 , the printed metal trace spacing is varied incrementally in a manner to result in balun performance closer to the design goal. 
       FIG. 16  illustrates the method  1600  of final balun design. Step  1250  occurs after the balun initial design is complete. In step  1620 , the actual load impedance is encoded in the balun simulator. In step  1640 , an impedance matching network is coupled to the balun input. In step  1650 , an impedance matching network is coupled to each side of the balun differential output. In step  1660 , the balun performance is simulated. In step  1670 , the balun simulated performance is compared with design goal performance. If simulated is equal or greater than design goal performance, (YES), step  1690  is performed. In step  1690 , balun design is completed. If simulated performance is less than a design goal, (NO), step  1680  is performed. In step  1680 , the value of an element in the input and output matching networks is varied incrementally in a manner to result in balun performance closer to the design goal. Then step  1660  is performed. Steps  1660 ,  1670 , and  1680  are performed in sequence until simulated balun performance is equal to or better than design goal performance. 
     Conclusion 
     Example embodiments of the methods, systems, and components of the present invention have been described herein. As noted elsewhere, these example embodiments have been described for illustrative purposes only, and are not limiting. Other embodiments are possible and are covered by the invention. Such embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.