Current Balanced Balun Network With Selectable Port Impedances

1. Technical Field of the Invention

The present invention relates to balun networks, and more particularly, relates to balun networks having selectable port impedances.

2. Description of the Related Art

Radio designers typically chooses components and feed lines having standard impedances such as 300 Ohms, 75 Ohms, or 50 Ohms, for example. Such standard impedances match the impedances of existing standard components. These include balanced-unbalanced networks or balun networks. Known balun networks have fixed impedance ratios between the balanced and unbalanced inputs and outputs. For example, most baluns such as split sheath and bazooka baluns have impedance ratios of 1:1. Thus, the input impedance is the same as the output impedance. Other baluns such as a half-wave balun has an impedance ratio of 4:1, wherein the balanced port has an impedance four times the impedance of the unbalanced port.

Radio frequency functions have been implemented in integrated circuit chips. Many integrated circuit chips have balanced connections while other components on a printed circuit board require unbalanced connections. Implementing baluns with fixed impedance ratios for coupling to standard impedances of radio frequency components places an impedance requirement constraint on the design of an integrated circuit.

FIG. 1 illustrates a balun network according to a first embodiment;

FIG. 2 illustrates a balun network according to a second embodiment;

FIG. 3 illustrates a balun according to a third embodiment; and

FIG. 4 illustrates a radio having a printed circuit board sharing a balun network and components.

A balun network having a selectively variable impedance ratio between a balanced port 110 and an unbalanced port 120 is illustrated in FIG. 1 according to a first embodiment. The balun network has a conductor section of at least two contiguous portions 130 and 140. An entire length 1 of the line made up of the first and second portions 130 and 140 has a length an odd integral multiple of about one-half a wavelength of a nominal frequency of interest. The balanced port 110 connects between a first end 103 and a second end 105 of the conductor section, e.g., the first and second conductor portions 130 and 140. The unbalanced port 120 connects between the first end 103 and a ground plane or ground 107 of the unbalanced port 120.

The first conductor portion 130 has an impedance Z.sub.1, and the second portion 140 has an impedance Z.sub.2 different than the impedance Z.sub.1 of the first portion. By providing different portions of different impedances, a half-wave balun network is provided having ports with selectively varying impedances. For example, if the impedance Z.sub.2, of the first portion 130 is 45 Ohms and the impedance Z.sub.2 of the second portion 140 is 60 Ohms, then the balanced port 110 has an impedance of 272.333 Ohms and the unbalanced port 120 has an impedance of 50 Ohms. As this impedance difference between the conductor portions 130 and 140 changes, the ratio of impedances can be increased or lowered between the balanced and unbalanced ports 110 and 120. It has been found that the balun network of the present invention can achieve selectable impedance ratios of up to about 9:1 or down to roughly 2:1 and values in between.

In a previous balun with a single conductor section of a constant impedance, however, for example, the balun network has an expected impedance ratio between the balanced port and unbalanced port of 4:1. Thus, the balanced port 110 would have an impedance of 200 Ohms and the unbalanced port 120 would have an impedance of 50 Ohms when the conductor portion had a constant impedance across its half wave length.

The balun network of the present invention is balanced for at least current. As the impedances of portions of the half-wave section are changed, the voltage has been found to be unbalanced at each of the terminals of the balanced port when measured with respect to the ground plane or ground 107.

FIG. 2 illustrates a tapered conductor section 150 having a varying impedance Z(1) which varies according to a position at any given point along the length of the conductor. Because more than two different impedance conductor portions can be provided to adjust the ratios between the ports, the tapered shaped conductor section 150 of the embodiment of FIG. 2 illustrates a number of conductor portions approaching infinity. The number of different impedance portions can thus vary from two up to infinity. The tapered conductor section 150 has likewise been shown by simulation to provide a variable impedance at the balanced and unbalanced ports dependent upon the impedances chosen along the lengths of the conductor.

The balanced port 110 connects between a first end 103 and second end 105 of the tapered conductor section 150, and the unbalanced port 120 connects between the first end 103 and a ground plane or ground 107 of the unbalanced port.

FIG. 3 illustrates an exemplary construction of the balun network on a printed circuit board 210. The printed circuit board 210 has a metalized underside 220 on the same plane of a surface of a dielectric substrate of the board 230. Elongated metallic strips are placed on an upper-side of the dielectric board 230. The elongated metallic strips form a first conductor portion 330 of a first impedance and a second conductor portion 340 of a second impedance different than the first impedance. This is achieved by varying the width of the elongated metallic strip on the dielectric board 230 of the printed circuit board. The first conductor portion 330 and the second portion 340 of FIG. 3 has a U-shaped configuration which loops backwardly at its electrical midpoint and is symmetric along a center line between the two conductor portions.

A balanced port 310 is provided between a first end 303 and second end 305 of the first conductor portion 330 and the second conductor portion 340. An unbalanced port 320 is provided between the first end 303 of the conductor portion and the ground plane or ground 307 of the unbalanced port 320. In the printed circuit embodiment having a metalized under-surface 220 of the embodiment of FIG. 3, the ground plane of the unbalanced port 320 is the same as the metal layer 220.

FIG. 4 illustrates a portable radio 410 with a cutaway view of a printed circuit board 420 therein. The printed circuit board 420 has a balun network 430 plated thereon for connection to an integrated circuit chip 440. The printed circuit board 420 also has another balun network 450 coupled between an antenna 460 and an amplifier 470 of radio transceiver circuitry. By placing the balun network having a selectable impedance ratio according to the present invention on a printed circuit board, the selectable impedance ratio between the balanced and unbalanced ports 103,105 of the balun network provides for greater flexibility when choosing and designing components, such as the integrated circuit chip 440, the radio transceiver circuitry, and the antenna 460. Existing components of fixed impedances are of lesser importance with the variable impedance balun of the present invention.

Although the invention has been described and illustrated in the above description and drawings, it is understood that this description is by example only and that numerous changes and modifications can be made by those skilled in the art without departing from the true spirit and scope of the invention. The present invention is applicable to devices needing a balun requiring current balance. These include radios for cellular, paging, satellite and land mobile products. More flexible provision of integrated circuit chips in these and other devices is also achieved. Besides provision on a printed circuit board in a strip line or microstrip configuration, the balun network can also be implemented using plated dielectric blocks having holes or other conductive structures formed therein with ground planes selectively plated thereon.