Abstract:
The RF transformer of the present invention couples a transmission line between a magnetic transformer and a balun. The location and function of the transmission line improves frequency response across a wide operational bandwidth by permitting the circuit to be tuned, thereby providing a greater degree of impedance matching.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to broadband impedance matching transformers. More particularly, the invention relates to high impedance broadband transmission line transformers which exhibit low insertion loss, minimum voltage standing-wave ratios and provide a tuning network between balanced and unbalanced circuits. 
     2. Description of the Related Art 
     Transmission lines are used to connect various radio frequency circuit elements including connections from radio frequency (RF) circuits to antenna systems. Typical RF engineering practice dictates that a signal source should have an impedance equal to the impedance of the load. In addition, a load coupled to a transmission line should present an impedance equal to the characteristic impedance of the transmission line. 
     The importance of a matched load is that a transmission line terminated with a load equal to its characteristic impedance will transfer a signal without reflection. In that instance, all power contained in the signal is transferred from the transmission line to the load. Loads with a resistance unequal to the characteristic transmission line impedance produce reflections. 
     Short sections of transmission lines can be used to tune a mismatched load by inserting the section across the conductors as a shunt, or in series with the mismatched line. The length of the transmission line, the type of termination, (open or shorted), and its location determine the effect on the circuit. At very short wavelengths, transmission lines function as circuit tuning elements. 
     One application of a matching network would be employed at the output of an RF signal amplifier. A typical push-pull RF amplifier output stage would require an output transformer with a center tap for carrying equal, direct currents through each half of the primary winding to the transistors. The secondary winding provides a balanced output at a different impedance for conversion to an unbalanced line and for further circuit connection. A matched load is therefore essential to maximize power transfer. 
     A balun (BALanced-UNbalanced) is a passive device which permits a transition between an unbalanced circuit and a balanced circuit and also permits impedance matching if necessary. The balun provides electrical isolation, but passes the transmission line currents. Baluns avoid the high frequency limitations of conventional magnetic transformers since the windings are arranged such that winding capacitance and inductance form a transmission line free of resonances. Baluns can also provide impedance transformations with excellent broadband performance. 
     A prior art network converting a balanced output to an unbalanced output including an intermediate filtering network is disclosed in U.S. Pat. No. 5,495,212. However, the intermediate filtering network revealed does not perform a tuning function for the equivalent circuit; the network provides low-pass filtering. 
     While the prior art has shown impedance matching transmission line transformers using a combination of external devices incorporating intermediate filtering, the conventional devices are overly complex when designed to operate over a wide RF bandwidth. What is needed is a balanced-to-unbalanced transmission line transformer that permits tuning of the overall frequency response characteristics of the circuit. 
     SUMMARY OF THE INVENTION 
     The balanced-to-unbalanced broadband RF transmission line transformer of the present invention couples a twisted-wire transmission line between a center-tapped magnetic transformer and a balun. The location and function of the twisted-wire transmission line improves frequency response across a wide operational bandwidth by permitting the circuit to be tuned; thereby providing a greater degree of matching. The invention significantly improves frequency response over a 50-860 MHz operational bandwidth, while providing a conversion from a balanced to an unbalanced circuit with a high (4:1) impedance ratio. The RF transformer exhibits a low voltage standing wave ratio (VSWR) with a minimal circuit burden. 
     Accordingly, it is an object of the present invention to provide a transmission line transformer that converts balanced inputs which are 180° out of phase with each other to an unbalanced circuit while performing circuit tuning using a compensation transmission line to equalize the response characteristics over a large bandwidth. 
     Other objects and advantages will become apparent to those skilled in the art after reading the detailed description of a presently preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is an electrical schematic of the preferred embodiment of the RF transformer. 
     FIG. 2 is a perspective view of the magnetic transformer. 
     FIG. 3 is a plot of the frequency response of the RF transformer over the operational bandwidth both with and without the compensation transmission line. 
     FIG. 4 is a top view of the entire preferred embodiment. 
     FIG. 5 is an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiment will be described with reference to the drawing figures where like numerals represent like elements throughout. 
     Referring to the electrical schematic of FIG. 1, the preferred embodiment of the RF transformer  17  is shown. The RF transformer  17  includes three discrete sections: 1) a center-tapped magnetic transformer  19 ; 2) a twisted-wire compensation transmission line  23 ; and 3) a balun  25 . 
     The first section of the RF transformer  17  is a center-tapped magnetic transformer  19  with balanced primary input terminals  21  and secondary output nodes a and b. Nodes a and b are coupled to the second section, the twisted-wire compensation transmission line  23 . The compensation transmission line  23  is shunted across nodes a and b and has a calculated variable characteristic impedance Z 0  and an electrical length. The output of the magnetic transformer  19 , (nodes a and b), is also coupled to the third section, the 1:1 balun  25 . The balun  25  converts the balanced output a and b of the magnetic transformer  19  to an unbalanced RF output  27 . 
     The physical construction of the magnetic transformer  19  and the balun  25  determines the characteristic inductance and capacitance of the RF transformer  17  and also determines the overall frequency response. The common mode inductance, or the primary inductance for a magnetic coupled transformer, determines the low frequency response of a transformer. Frequencies above the low frequency limit are coupled through the transformer core  39  and are unaffected by the common mode inductance. The high frequency limit is determined by transformer winding length and parasitic capacitance introduced by the common mode inductance. 
     In the preferred embodiment  17 , the magnetic transformer  19  has a center-tapped primary  29  with five (5) turns and a balanced secondary  31  output having three (3) turns. A wire gauge of 36 AWG (American Wire Gauge) is used to form the primary  29  and secondary  31  around ferrite core  39 . The input  21  is balanced across the primary positive  33  and negative  35  input terminals with the center tap terminal  37  providing a common voltage supply for the balanced input  21 . The input  21  is typically connected to a push-pull amplifier output stage (not shown). 
     The balun  25  is preferably wound with nine (9) turns of 38 AWG on a separate ferrite core  41 . The output  27  of the balun  25  is unbalanced with a positive terminal  43  and a signal common (earthed) terminal  45 . 
     The compensation transmission line  23  is constructed of twisted magnetic 36 AWG wire having a film insulation. As one skilled in this art would appreciate, the insulation may vary in thickness among four groups. A wide variety of characteristic impedances can be accomplished by varying the wire diameter, number of twists per inch, length, insulation film thickness and insulation film type. In the preferred embodiment, the compensation transmission line  23  is constructed of 36 AWG magnet wire, 0.5 inch in length, with sixteen (16) twists per inch. 
     The characteristic impedance, Z 0 , of the compensation transmission line  23  equals the ratio of voltage to current. The characteristic impedance of the preferred embodiment is 41 Ω. This characteristic impedance can also be expressed as the series wire inductance and inter-wire capacitance distributed along the length of the compensation transmission line  23 . These relationships are well known to those skilled in the art of electronics. The result:                Z   0     =       L     C   t                 Equation                   (   1   )                                  
     where Z 0  equals the characteristics impedance, L equals the parallel-wire inductance and C t  equal the total inter-wire capacitance. 
     As shown in FIG. 2, the present invention  17  inside an amplifier preferably locates the compensation transmission line  23  within the ferrite core  39  of the magnetic transformer  19 . The placement of the compensation transmission line  23  within the ferrite core  39  further provides a solid form around which to wrap the compensation transmission line  23  and keep it held in place. This ensures that the physical parameters of the compensation transmission line  23  will be the same for all manufactured units, and that the compensation transmission line  23  will not be inadvertently displaced once the RF transformer  17  leaves the manufacturing plant. 
     A plot of the frequency response of the RF transformer  17  inside an RF amplifier with and without the compensation transmission line  23  is shown in FIG.  3 . For the present invention, it was desired to limit the amplifier return loss to less than −18 dB. The input RF signal is a sinusoid which sweeps over a 0-900 MHz bandwidth. As shown, the frequency response curve  60  for the RF transformer  17  without the compensation transmission line  23  exhibits a rise of over 5 dB at 860 MHz. Accordingly, the return loss at 860 MHz is −13 dB. 
     To equalize the response characteristics, the compensation transmission line  23  is inserted to tune the frequency response. The effect of the compensating transmission line  23  is shown by the frequency response curve  62  of FIG.  3 . The curve  62  shows a noticeable reduction in amplitude at 860 MHz and an overall flatter response across the design bandwidth of the RF amplifier. The use of the compensation transmission line  23  clearly ensures that this return loss is kept below the −18 dB reference line  64 . It should be noted that the response characteristics shown in FIG. 3 is representative of one embodiment tuned for a specific application. 
     Physical realization of the simplicity of the RF transformer  17  likewise is shown in FIG.  4 . The balun  25  is located adjacent to the magnetic transformer  19  upon a single substrate  50 . This provides a compact and efficient utilization of space within a single package. The location of the compensation transmission line  23  is critical since improper placement may significantly degrade RF performance. The location of the compensation transmission line  23  is used to optimize the matching of the RF transformer  17  to an amplifier. Preferably, the compensation transmission line  23  is inserted through the core  39  of the magnetic transformer  19  and wrapped around a portion of the core  39 , as shown in FIGS. 2 and 4. The compensation transmission line  23  could also be located around the periphery of the core  39  of the magnetic transformer  19 . In this case, it would be preferable to include a groove (not shown) such that the compensation transmission line  23  is held securely in place. 
     It should be noted that alternative embodiments of the RF transformer  17  may use compensating transmission lines  23  constructed of coaxial cable. Additionally, physical construction of each transformer  19 ,  25  may include toroids, rods, or symmetric cores of powered iron or ferrite. For example, as shown in FIG. 5, a multi-hole (greater than 2) core  100  may be utilized to combine the transformer core  39  of the magnetic transformer  19  with the ferrite core  41  of the balun  25 . This is particularly desirable for applications which require a compact design, since only a single core  100  is utilized.