Patent Publication Number: US-2004041659-A1

Title: Compact broadband divider/combiner

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     [0001] This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 60/388,092 entitled “Compact Broadband Divider/Combiner”, filed Jun. 12, 2002, which is herein incorporated by reference in its entirety. 
    
    
     
       BACKGROUND  
       [0002] This invention relates to dividing and combining signals. More particularly, this invention relates to an apparatus capable of dividing a signal into numerous signals and to combining numerous signals into a single signal, and which does so without the need for significant tuning and while substantially maintaining amplitude and phase balance.  
       [0003] Generally, there are three types of radio-frequency (RF) or microwave power dividers: lumped element equivalent bandpass dividers, radial waveguide dividers, and corporate dividers. Lumped element equivalent bandpass dividers are usually realized as a cascade of stepped impedance radial transmission lines, and are usually not capable of a large transmission bandwidth in a small diameter structure. Although the thickness of a lumped element bandpass divider design is relatively small, the diameter and the number of sections required can be relatively large.  
       [0004] Radial waveguide dividers have cavities that are large relative to a wavelength of a signal at their operating frequency. While able to provide very high power transmission, cavity type dividers do not usually provide a large bandwidth, being capable of usually only octave bandwidth performance. With radial waveguide cavity type dividers, the divider diameter tends to increase with larger bandwidth, for a given frequency. For example, for a 3 GHz signal, a 50-way divider having an 8″ cavity provides a bandwidth of less than 10% and a voltage standing wave ratio (“VSWR”) of about 1.5:1.  
       [0005] Corporate dividers usually include a cascade of two-way dividers, and are generally lossy and physically large because, to obtain a relatively large bandwidth, corporate dividers require a lot of sections. Corporate dividers employ a cascade of sections to split one signal into an even number of output signals. However, such a cascaded divider is very lossy due to the insertion loss associated with each divider section, and losses associated with junction discontinuities where one section is connected to another. Furthermore, since a corporate divider accommodates only an even number of sections, and the number of divider levels increases as the number of sections increases (at a power of two for each additional section), the corporate divider does not allow an arbitrary or an odd number of levels. For these reasons, corporate dividers are rendered impractical for high-power, signal-amplifier applications.  
       SUMMARY OF THE INVENTION  
       [0006] Therefore, a relatively small and simple divider having broad bandwidth capability is needed. In some embodiments, a divider not suffering from the higher order waveguide molding such as that occurring in radial waveguide dividers is needed.  
       [0007] In one embodiment, a radio frequency (“RF”) power divider/combiner includes a circular, symmetric interdigital filter. The power divider/combiner includes a circular top housing having a first center and a first circumference, and a circular bottom housing having a second center and a second circumference, a plurality of ports at circumferential positions, and a longitudinal axis extending from the first center to the second center. The circular top housing includes a first top side and a first bottom side, and a first plurality of finite thickness radial walls extending from the first bottom side. The first plurality of radial walls are concentric about the first center. The circular bottom housing, having a second top side and a second bottom side, includes a second plurality of finite thickness radial walls extending from the second top side, the second plurality of radial walls being concentric about the second center. The circular top housing and the circular bottom housing are joined to form a substantially closed structure, and the first plurality of radial walls and the second plurality of radial walls are substantially and concentrically interleaved forming an interdigitated radial chamber extending from the longitudinal axis to approximately the circular symmetric interdigital filter first and second circumferences.  
       [0008] The circular bottom housing also includes a bottom conductor having a first longitudinal axis which is substantially parallel to the longitudinal axis, extending from the second top side of the circular bottom housing, and being substantially parallel to the longitudinal axis. The bottom conductor is substantially enclosed by a top conductor extending from the first top side of the circular top housing, the top conductor having a second longitudinal axis which is substantially parallel to the longitudinal axis.  
       [0009] The present invention also provides a method of transmitting an input signal. The method includes dividing the input signal into a plurality of in-phase, and substantially equal-amplitude signals, and transmitting the plurality of in-phase, and substantially equal-amplitude signals. The present invention further provides a method of providing the same signals to a phased array antenna. Furthermore, the present invention also provides a method of transmitting a combined signal. The method includes receiving a plurality of in-phase and equal amplitude input signals, combining the plurality of in-phase and equal amplitude input signals, and transmitting the combined signals at a common port.  
       [0010] Other features and advantages of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011] In the drawings:  
     [0012]FIG. 1 is a top view of a compact broadband divider/combiner according to the present invention;  
     [0013]FIG. 2 is an isometric sectional view of a compact broadband divider/combiner taken along line A-A of FIG. 1;  
     [0014]FIG. 3 is a simplified sectional view of a compact broadband divider/combiner taken along line A-A of FIG. 1;  
     [0015]FIG. 4A is a first equivalent circuit of a compact broadband divider/combiner;  
     [0016]FIG. 4B is a second equivalent circuit of a compact broadband divider/combiner including a plurality of series capacitors;  
     [0017] FIG,  4 C is a third equivalent circuit of a compact broadband divider/combiner including an input series capacitor;  
     [0018]FIG. 4D is a fourth equivalent circuit of a compact broadband divider/combiner including an output series capacitor;  
     [0019]FIG. 4E is a wedge equivalent circuit of a compact broadband divider/combiner;  
     [0020]FIG. 5 is a first basic sectional view of a compact broadband divider/combiner, showing a first plurality of impedances;  
     [0021]FIG. 6 is a second basic sectional view of a compact broadband divider/combiner, showing a second plurality of impedances; and  
     [0022]FIG. 7 is a plot of a power split of the compact broadband divider/combiner. 
    
    
     DETAILED DESCRIPTION  
     [0023] Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.  
     [0024] To obtain multi-octave bandwidth performance with a compact structure, a circular symmetric interdigital filter is used because the theoretical performance of the interdigital filter is known a priori. A compact broadband divider/combiner  100  (hereinafter “divider”), according to one embodiment of the present invention is shown in FIG. 1. As would be apparent to those of ordinary skill in the art, the “divider”  100  can operate in two modes: one mode where it divides one input signal into many output signals and another mode where it combines many input signals into one output signal. However, rather than referring to a “divider/combiner,” for simplicity the term “divider” is used throughout this discussion. Of course, it should be understood that the invention may also be used as a divider only, or as a combiner only, if desired. The divider  100  includes a center conductor  105 , a top housing extension  110 , and a first center  115 . The divider  100  also includes an interdigital filter, as shown in FIG. 2.  
     [0025] Referring to FIG. 2, the divider  100  may include a circular top housing  120  having a first circumference  125 , and a circular bottom housing  130  having a second center  135  and a second circumference  140 . The circular top housing  120  is mounted to the circular bottom housing  130  with a plurality of fasteners  142 , thereby creating an overall inside height  145 . The fasteners  142  may be, for example, screws, bolts, or the like, or adhesives or tapes or combinations thereof. A longitudinal axis  148  extends from the first center  115  to the second center  135 . The circular top housing  120 , having a first top side  150  and a first bottom side  155 , may include a first plurality of finite thickness radial walls  160  extending from the first bottom side  155 , the first radial walls  160  being concentric about the first center  115 . The circular bottom housing  130 , having a second top side  165  and a second bottom side  170 , may include a second plurality of finite thickness radial walls  175  extending from the second top side  165 , the second radial walls  175  being concentric about the second center  135 , thereby creating a plurality of spacings  180  between the radial walls  160  and  175 . The circular top housing  120  and the circular bottom housing  130  fit together and provide a substantially closed structure. The radial walls  160  and the radial walls  175  are substantially and concentrically interleaved, thereby forming an interdigitated chamber radially extending from the longitudinal axis  148  to approximately the circular symmetric interdigital filter first and second circumferences  125  and  140 .  
     [0026] The circular bottom housing  130  also includes a center conductor  105  having a first longitudinal axis, extending from the second top side  165  of the circular bottom housing  130 , and being substantially parallel to the longitudinal axis  148 . The center conductor  105  is substantially enclosed by an outer conductor or a top housing extension  110  extending from the first top side  150  of the circular top housing  120 , and the top housing extension  110  has a second longitudinal axis which is substantially parallel to the longitudinal axis  148 .  
     [0027] Referring to FIG. 3, each of the radial walls  160  and  175  has a height  205 . A plurality of output ports  210  are spaced along the radial walls  160  and  175 . The spacing  180  between the walls determines the unit element (“LJE”) impedance of an equivalent circuit (shown in FIG. 4A). Specifically, the height  205  essentially determines an electrical length of a transmission line whose impedance is determined by a spacing  180  between the first wall  160  and the second wall  175 . In one example, the overall inside height  145  is essentially a quarter wavelength at the center of the divider operating frequency range. According to one embodiment, the divider  100  may be based on an N =11 chebychev stub/UE filter having a bandwidth of 6.6:1, with a voltage standing-wave ratio (“VSWR”) level of 1.05:1, and the number of output ports  210  set at 36. The first two UEs of the equivalent circuit, illustrated in FIG. 4A, may be realized in a two-section feed line  215 , having two diameters between the inside diameter of the top housing extension  110  and the diameter of the center conductor  105 . In addition, each gap  212  between the walls corresponds to a UE in the equivalent circuit in FIG. 4A. The last spacing  180  realizes a shunt inductor  220  in the equivalent circuit in FIG. 4A. The plurality of output ports may attach directly to the last wall at 10° intervals around the outer periphery of the divider for  36  outputs. The center conductor  105  may also have a different number of sections and thus different diameters to represent a different number of UEs, as shown in FIGS. 5 and 6, if desired.  
     [0028]FIG. 4A shows one example of an equivalent circuit  300  of the entire divider  100 . The equivalent circuit  300  includes a cascade of UEs  305  with a shunt inductor  310  at an output port  210  (FIG. 3), a source impedance  315 , and a load impedance  320 . A source resistance (labeled R s ) may be preferably about 50Ω, and a load resistance (labeled R L ) is, therefore 50Ω/N where N is the number of output ports  210 . The impedance of each UE  305  decreases as the UEs get closer to the shunt inductor  310 . For example, with R s  set at 50Ω and N set at 36 (which results in R L /N being 50/NΩ and the phase angle φ being 90° at the center frequency), the impedance (expressed as z n ) for each UE shown is as follows: z i  is 46.8Ω, z 2  is 41.4Ω, z 3  is 33.9Ω, z 4  is 25.4Ω, z 5  is 17.6Ω, z 6  is 11.5Ω, z 7  is 7.2Ω, z 8  is 4.4Ω, z 9  is 2.7Ω, z 10  is 1.7Ω, and z 11  is 5.5Ω. Additional examples of equivalent circuits illustrating the entire divider circuit are shown in FIGS. 4B, 4C and  4 D. These circuits include a plurality of series capacitors  325 . Each capacitor  325  is distributed and is a quarter wavelength at the design center frequency, where n is the number of sections. FIG. 4E shows a wedge equivalent circuit  400  where a 360°/N wedge of a divider is taken. The wedge equivalent circuit  400  includes a source impedance  405  (R s ), a load impedance  410  (R L ), and a plurality of UE impedances  415 . The source impedance  405  is normally 50NΩ, the load impedance R L =R s /N and is, therefore, 50Ω, and the plurality of UE impedances  415  are N*z i Ω, where i ranges from 1 to N+1.  
     [0029] Referring to FIG. 3, the divider  100  receives a signal at the input port, which is at the center  115  of the divider  100 . The signal energy travels in and meanders through the concentric rings. The gap  212  between the wall  160  and the second top side  165  acts as a transmission line. The signal energy goes in and travels through the gap  212 , making a right angle bend and travels into the next gap or transmission line. The diameter of the cascade of transmission lines get larger as the signal energy moves through the chamber to the outer diameter of the concentric circle, while the impedance level drops with the lowest impedance being at the load  320 .  
     [0030] When the device is operating as a divider, each of the N output ports may be preferably terminated with an impedance of 50Ω. Since the output ports are in parallel, the equivalent circuit in FIG. 4A has a load of 50Ω/N where N is the number of output ports. A signal at an input port  225  travels through the feed line  215  in a transverse electromagnetic (“TEM”) mode and meanders radially outward through the radial chamber towards the plurality of output ports  210 . Because of circular symmetry, all output signals have equal phase and amplitude characteristics. The output port  210  (in the last concentric ring at 10° intervals) is filled with a pressed-in spring finger which accepts a center pin of an output connector. According to one embodiment, the connectors may be subminiature RF connectors or SMA connectors that meet predefined interface mating dimensions. Preferably, the dimensions meet the requirements of the MIL-C-39012 specification. In one example, a Type N connector pin is soldered to the center conductor  105  at the top housing extension  110 , and the center conductor  105  is press fit into the circular bottom housing  130 . The plurality of gaps  212  from an open end of the concentric rings are adjusted to compensate for the 180° bend between adjacent UEs. As would be apparent to one of ordinary skill in the art, almost any arbitrary number of output ports can be used. The output port intervals can be adjusted to the number of ports chosen. For example, if 24 output ports are desired, the interval will be 360°/24=15°. Furthermore, adjusting the thickness of the radial walls  160  and  175  affects the response of the divider  100 . For example, the radial wall  175  thickness affects the spacing  180  and the overall diameter of the divider  100 . Increasing the radial wall thickness requires an increase of spacing  180  and, therefore, an increase in the diameter of the divider  100 .  
     [0031] In the combiner mode, N in-phase and equal amplitude signals are fed into the N ports, the signals are then combined in-phase at the common port with no loss of power except for reflection and dissipation losses. Reflection losses are the same as those at the common port when used as a divider. Additional combining losses are related to errors in amplitude and phase of the N signals. FIG. 7 shows a power split response plot using the embodiment described above. The measured result illustrates the broadband capability of the invention with a return loss that is approximately less than 15 dB over a 6.5:1 band.  
     [0032] Embodiments of the invention are capable of multi-octave bandwidth performance in a compact structure. In the embodiment shown, the physical structure is simple and requires at most one solder joint, requires little or no tuning, and the number of ports ranges from 2 to more than 100. Furthermore, because of the circular symmetry design, both the amplitude and the phase balance are theoretically perfect.  
     [0033] Due to the circular symmetry design of the invention, phase and amplitude balance is not affected by the discontinuities in the structure. However, discontinuities can affect the VSWR of the system, and the amplitude and the phase are, in most instances, affected by any eccentricity between the circular top housing  120  and the circular bottom housing  130 . For example, the response plot shown in FIG. 7 shows, instead of a flat response from an ideal divider/combiner, a series of ripples  350 . Eccentricity may also cause some output ports  210  to receive more power at a particular frequency than the other output ports  210 . However, amplitude and phase imbalance may be minimized by monitoring the output power balance and adjusting the relative position of the circular top housing  120  relative to the circular bottom housing  130 .  
     [0034] As can be seen from the above, embodiments of the invention provide a divider/combiner with broad bandwidth capability. The invention may be used to divide a signal into plurality of output signals and to combine a plurality of input signals to a single output signal. The combiner can be used in many applications, such as telecommunications applications, and with phased array antennas.