Abstract:
A multi-stage power divider particularly adapted for use in microwave circuits consists of a plurality of transmission lines and resistances uniquely arranged to achieve a wide range of power division and to give the power divider broad bandwidth and high isolation. The power divider is particularly easy to design and manufacture in stripline and microstrip constructions. The divider provides coupling in the range of 3 dB to 20 dB with high isolation and in a single-layer construction.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to power dividers and, more particularly, to a multi-stage power divider for microwave circuits. 
     There are many applications in which it is desirable to divide a signal into a plurality of signals. In antenna systems, for example, it is often desirable to supply a portion of an input signal to each of a plurality of individual antenna units. Signal division may also be used in electronic circuitry to drive plural solid-state amplifiers with the same signal, in cable transmission systems to divide an original signal among a number of output cables, and in numerous other applications. 
     Coupled line dividers are often used in microwave applications for supplying power from an input port to a pair of output ports. Coupled line dividers, however, are not fully satisfactory because they require precise line gaps and spacings to achieve the desired power division, and often require line widths and gap spacings that are either too wide or too narrow. 
     Various other devices for accomplishing power division are known in the art, as shown by U.S. Pat. Nos. 2,148,098; 2,244,756; 3,091,743; 3,516,025; 3,904,990; and 4,556,856. 
     Such power dividers are not capable of providing a wide range of power division and a sufficiently broad bandwidth and isolation of their output ports for many microwave applications. In addition, such dividers are also costly to manufacture for microwave applications. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a multistage power divider which consists of a plurality of radio frequency pathways and resistors that are uniquely connected to achieve power division and to give the device broad bandwidth and high isolation. The power divider of the invention is particularly designed for use in microwave circuits, and permits a simple, single layer stripline, or microstrip, construction to provide effective broad bandwidth power division and coupling in the range of 3 dB to 20 dB with high isolation. 
     In the power divider of this invention, a plurality of passive circuit elements are arranged to define a plurality of radio frequency pathways between a power input and a plurality of power outputs, and to divide incoming radio frequency power among the plurality of outputs in a preselected ratio. The passive circuit elements are connected to define a plurality of power-dividing junctions that are located in sequence in at least one radio frequency pathway between the power input and the power output to further divide the radio frequency power in at least one radio frequency pathway, and to connect the radio frequency power further divided from that one pathway with the radio frequency power in another pathway at a power-combining junction in the other pathway, and to provide electrical resistance between the junctions for the further divided power and the adjacent radio frequency pathways. 
     A two-stage power divider of the invention comprises: a power input, a first power output for a first power pathway, and a second power output for a second power pathway; a first input transmission line coupled to the power input; a first power-dividing stage coupled to the input transmission line and including second and third transmission lines; a second power-dividing stage coupled to the second transmission line and including fourth and fifth transmission lines; a sixth transmission line connected to the fifth transmission line for combining the power from the second power-dividing stage and the third transmission line at a power-combining junction; and a seventh and eighth output transmission line connecting, respectively, the forth transmission line to the first power output and the power-combining junction of the fifth and sixth transmission lines to the second power output. 
     In preferred embodiments, each of the eight transmission lines comprises a quarter-wavelength transformer uniquely connected to achieve the power division; and the circuit further includes first and second resistors coupled across the outputs of the first and second power-dividing stages, respectively, to provide the circuit with broad bandwidth and high isolation. 
     With the present invention, power division is dependent only upon the line impedances. Accordingly, the need for close control of gap spacings and line widths, as required in coupled line dividers, is eliminated. The multi-stage design also maintains more practically realizable line impedances than in conventional broad band dividers, and allows for greater power division ratios than can be implemented in a single, resistive, power divider. The power divider circuit of the invention can readily be made with stripline or microstrip construction in a single-layer package. 
     Further features and advantages of the invention will be set forth hereinafter in conjunction wit the detailed description of a presently preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically illustrates a two-stage, resistive power divider according to the invention; and 
     FIG. 2 illustrates the two-stage, resistive, microwave power divider of FIG. 1 implemented as a stripline on a printed circuit board. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     To assist in understanding the invention, FIG. 1 schematically illustrates the invention in a two-stage, resistive, power divider network. The twostage power divider 10 divides the power at power input 22 between two power outputs 23 and 24. The power divider 10 comprises a plurality of passive circuit elements, preferably eight quarter-wavelength transmission line transformers 11-18, and two resistors 19 and 21. 
     More particularly, transmission line 11 comprises a first impedance-matching transformer connected between the power input 22 and a first power-dividing stage. The first power-dividing stage comprises second and third quarter-wavelength transformers 12 and 13 connected at junction 26, and resistive element 19. The power from the second quarter-wavelength transformer 12 is then further divided in a second power-dividing stage in the first power pathway. The second power-dividing stage comprises fourth and fifth quarter-wavelength transformers 14 and 15 connected at junction 27, and resistive element 21. In the second power pathway, a sixth quarter-wavelength transformer 16 is connected to the three-fourths-wavelength transformer 13, and the fifth quarter-wavelength transformer 15 is connected to the sixth quarter-wavelength transformer 16 to define a power-recombining junction 28 in the second pathway. Quarter-wavelength transformer 17 is connected between the quarter-wavelength transformer 14 and the first power output 23, and an eighth quarter-wavelength transformer 18 is connected to the power-combining junction 28 and the second power output 24. The seventh and eighth quarter-wavelength transformers 17 and 18 comprise impedance-matching transformers between the power divider and the circuitry to which it is connected. 
     The two resistive elements 19 and 21 are connected across the first and second power-dividing stages and the outputs of the second and third transmission lines 12 and 13, and the fourth and fifth transmission lines 14 and 15, respectively. 
     The resistance of resistive elements 19 and 21 contribute to broad bandwidth and high isolation. Impedance and resistance values for the elements of circuit 10 are determined by the desired power division, the external characteristic impedance, and the maximum allowed impedance within the power divider. Power division in the circuit is dependent only upon the line impedances; and thus, the need for close control of gap widths, as required in conventional microwave power dividers, is eliminated. The multi-stage design also maintains more practically realizable line impedances than conventional broad band dividers and allows for greater power division ratios than can be implemented in a single, resistive, power divider. The circuit can thus provide reliable power division in the range of 3 dB to 20 dB. 
     The design equations for the two-stage, resistive power divider of FIG. 1 are set forth below. In the equations, the following definitions apply: 
     Pd/Pc is the ratio of power at output port 24 divided by the power at power output 23; 
     Z 0  is the characteristic impedance to which the circuit is matched; 
     Z m  is the maximum allowable impedance to be used in the circuit; 
     K 2  is the ratio of the power in the fourth quarter-wavelength transformer 14 divided by the power in the fifth quarter-wavelength transformer 15, the power division occurring in the second stage of the power divider; 
     Z 01  -Z 08  are the line impedances of the transmission line transformers 11-18, respectively; and 
     R 1  and R 2  are the resistances of resistive elements 19 and 21, respectively. ##EQU1## The equations have been prepared, for ease of calculation, on the basis that the power at power output 24 will be greater than the power at power output 23, and that in the second stage of power division K 2  will be a fraction. 
     FIG. 2 illustrates how simply the power divider 10 of FIG. 1 can be implemented in stripline and microstrip construction. The transmission lines 31-38 correspond, respectively, to the transmission lines 11-18 of FIG. 1. Transmission lines 31-38 can comprise strips of conductive material, preferably copper or gold, on the surface of an electrically non-conductive substrate 30. The substrate 30, in conjunction with an adjacent ground plane 30a, forms a power divider 10 of this invention. Transmission lines 32, 34 and 37 form one pathway for radio frequency power from the power input port 42 to the first power output port 43, and transmission lines 33, 36 and 38 form a second radio frequency power pathway from the input power port 42 to the second output power port 44. Transmission line 35 connects the first and second pathways, and power from the first pathway is combined with power from the second pathway at junction 52. Resistors 39 and 41 are connected from the power-dividing junction 51 to the second pathway, and from the power-combining junction 52 to the first pathway, respectively. Resistors 39 and 41 are thus connected across the first power-dividing stage formed by transmission lines 32 and 33 and the second power-dividing stage formed by transmission lines 34 and 35. 
     Thus, a multi-stage radio frequency power divider can be formed by providing a non-conductive substrate 30 with a plurality of electrically conductive strip portions 31-38 carried by the substrate. The plurality of conductive strip portions 31-38, in conjunction with an adjacent ground plane, can form a power input port 42, an impedance-matching power input pathway 31 leading to a first power-dividing junction 50, a first radio frequency pathway 32, 34, 37 leading to a first radio frequency power output port 43, and a second radio frequency pathway 33, 36, 38 leading to a second radio frequency power output port 44. The conductive strip portions forming the first pathway 32, 34, 37 lead first to a second power-dividing junction 51, and then from the second power-dividing stage 51 to the first power output port 43. The conductive strip portions 33, 36 forming the second pathway 33, 36, 38 lead to a power-combining junction 52 and the second output port 44. The second power-dividing junction 51 and the power-combining junction 52 are connected by a conductive strip portion 35, and electrical resistance elements 39, 41 are connected from the second power-dividing junction 51 to the second pathway portions 33, 36, and from the power-combining junction 52 to the first pathway portions 34, 37. 
     The power divider 30 can be conveniently manufactured by conventional printed circuit board and electronic manufacturing techniques without the need for great precision. The dimensions of the conductive strips 31-38 can be determined from the impedances determined from the design equation above by those skilled in stripline and microstrip design techniques. 
     While what has been described constitutes a presently preferred embodiment, the invention can take various other forms. For example, it should be understood that an input signal appearing at input port 22 can be further divided by multi-staging or the connection of two-stage power dividers to the power outputs 23 and 24. Accordingly, it should be understood that the invention should be limited only insofar as is required by the scope of the following claims.