Patent Grant
7385462
a. Field of invention
The invention relates to radial power divider/combiners and, in particular, to radial power divider/combiners that are suitable for use in solid-state power-amplifier (SSPA) devices.
b. Background of the invention
Solid State Power Amplifiers (SSPAs) are used in a variety of applications ranging from satellites, radar, and other RF applications requiring high output power. Typical SSPAs can achieve signal output levels of more than 10 watts using solid-state amplifiers such as Monolithic Microwave Integrated Circuits (MMICs), or individual tube amplifiers.
A fundamental problem with conventional SSPA technology is that individual MMICs produce less power and operate at lower efficiency compared to the individual tube devices. At Ka-band, for example, currently available MMIC chips have output power capability that is approximately an order of magnitude less compared to the Traveling Wave Tube Amplifier (TWTA). The efficiency is approximately half.
Although a single MMIC amplifier chip cannot achieve the requisite level of output power without excessive size and power consumption issues, MMIC technology is far more practical than tubes in space and other applications. MMIC technology offers a reduction in supply voltage, potential reduction in cost, improvement in linearity and reliability.
Consequently, efforts have been made to combine the outputs of several individual MMIC amplifiers to achieve the desired total transmitter output, and it has been found that a combination of a large number of MMICs is attractive for applications where these advantages outweigh the lost efficiency. Consequently, existing SSPA designs using MMIC chips typically use a radial splitting and combining architecture in which a signal is divided into a number of individual components. Each individual signal component is amplified by a respective amplifier, and the outputs of the amplifiers are combined into a single output that achieves the desired overall signal amplification.
However, to meet the output power requirements of space telecommunication systems, it is necessary to power combine a large number of individual MMICs in the SSPA, and yet this must be done in a highly efficient manner.
Existing power-combiners such as the in-phase Wilkinson combiner or the 90-degree branch-line hybrid combine a number of binary combiners in a cascaded manner, but this architecture becomes very lossy and cumbersome when the number of combined amplifiers becomes large. For example, to combine eight amplifiers using a conventional, binary microstrip branch-line hybrid at Ka-band (about 26.5 GHz), the combiner microstrip trace tends to be about six inches long and its loss tends to exceed 3 dB. A 3 dB insertion loss infers that half of the RF power output is lost, and this is unacceptable for most applications.
To overcome these loss and size problems, other approaches including the stripline radial combiner, oversized coaxial waveguide combiner, and quasi-optical combiner, have been investigated. The stripline radial combiner, using multi-section impedance transformers and isolation resistors, still suffers excessive loss at Ka-band, mainly because of the extremely thin substrate (<10 mil) required at Ka-band. The coaxial waveguide approach uses oversized coaxial cable, which introduces moding problems and, consequently, is useful only at low frequencies. The quasi-optical combiner uses hard waveguide feed horns at both the input and output to split and combine the power, and these are very large and cumbersome.
United States Patent Application 20050174194 by Wu, You-Sun et al. published Aug. 11, 2005 shows an N-Way Radial Power Divider/Combiner in which an input signal is provided to a transmission antenna that propagates into a divider. Within the divider, the input signal is divided into a plurality N of individual signals by waveguides disposed in a radial configuration around the transmitting antenna such that at least a portion of the input signal radiated by the antenna enters an input end of each waveguide. The individual signals are received by receiving antennas and provided to respective amplifiers. The amplifiers amplify the respective individual signals by a desired amplification factor. The amplified individual signals are provided to a plurality of transmitting antennas within the combiner. Inside the combiner, the amplified individual signals are combined to form an output signal that is received by a receiving antenna in the combiner. Though a ten-way divider/combiner is shown, N is said to be in the range of two to 100. The overall insertion loss of the 10-way power divider-combiner was measured using input signals from 20 to 30 GHz, and at 26.5 GHz, the loss for the combiner alone is 0.71 dB at 26.5 GHz.
It would be desirable to adapt a radial power-combiner architecture similar to the foregoing for a higher frequency bandwidth to power combine a larger number of amplifiers with better efficiency, using a smaller combining circuit that has minimum power loss. This is herein achieved by increasing the number of combining ports using reduced height waveguides in the radial base. The radial base has reduced-height waveguides with rectangular waveguide inputs leading a circular waveguide output, defining properly spaced and properly chosen waveguide steps having incremental height changes. The reflections from the walls of the reduced height waveguides are matched by a matching post coupled to a “Marie” mode transducer. The present invention provides a low-loss, compact radial power divider/combiner for use in high-frequency SSPAs that offers an unparalleled size, weight, and power combination, thereby offering a replacement for tube-based flight and ground amplifiers used in earth-orbiting defense missions and radar applications, as well as satellite secure communications systems requiring large bandwidths (secure satellite uplinks, downlinks, and cross-links), etc.
It is, therefore, an object of the present invention to provide a radial power divider/combiner for dividing/combining large number of amplifier signals within a wide bandwidth using reduced height waveguides inside a radial base.
It is a more specific object to provide a low-loss, compact radial power divider/combiner for use in wideband high-frequency (15% bandwidth in the 30-36 GHz range) Solid State Power Amplifier (SSPA) applications that offers an unparalleled size, weight, and power combination.
It is another object to provide a radial power divider/combiner that facilitates replacement of tube-based flight and ground amplifiers with solid state MMIC-based amplifiers for use in earth-orbiting defense missions and radar applications, as well as satellite secure communications systems requiring large bandwidths (secure satellite uplinks, downlinks, and cross-links), etc.
According to the present invention, the above-described and other objects are accomplished by providing a novel radial power combiner/divider with a higher order of power combining/dividing within a wide high-frequency bandwidth. The radial power combiner/divider generally comprises an axially-oriented mode transducer coupled to a radial base. The unique mode transducer transduces circular TE01 waveguide into rectangular TE10 waveguide, and the unique radial base combines/divides a plurality of ports into/from the single circular TE01 waveguide end of the transducer. The radial base incorporates full-height waveguides at the plurality of ports that are stepped down to reduced-height waveguides using stepped impedance transformers. This presents a stepped-impedance configuration that allows for reduced height waveguides inside the radial base (the height of the waveguides otherwise limiting the order N of combining), and hence a higher order combiner/divider. The reduced-height waveguides in the base converge radially to a matching post at the bottom center of the radial base which matches the reduced height rectangular waveguides into the circular waveguide that feeds the mode transducer. The matching post allows for a better output match at the circular waveguide of the radial base, which in turn with the mode transducer allows for a good output match of the divider/combiner as a whole.
The combiner/divider is herein illustrated in detail in the context of an N=24 combiner for use in Ka-band over the band of 31-36 GHz, with an input match <−20 dB under equal excitation of all input ports, an output match <−24 dB coming out of the mode transducer, and an insertion loss <0.6 dB. Of course, those skilled in the art will understand that certain exemplary specifications described herein in regard to the preferred embodiment are not limiting, and that the invention may be modified for other frequency ranges (other than 30-36 GHz), to power combine a different number of amplifiers (other than N=24), and that standard waveguide notations such as WR sizes and the like are for illustrative purposes only with regard to the illustrated embodiment.
While for the purposes of this description the innovation has been described as a power combiner, it also functions as a power divider.
The present invention is a radial power divider and/or combiner for dividing/combining a increased number N of amplifier signals within a wide bandwidth using compact radial format. The radial power combiner/divider generally comprises an axially-oriented mode transducer coupled to a radial base. The mode transducer transduces circular TE01 waveguide into rectangular TE10 waveguide, and the radial base combines/divides a plurality of ports into/from the single circular TE01 waveguide end of the transducer. The radial base is formed with a plurality of internal waveguides leading from peripheral output ports and converging radially to the center, the internal waveguides incorporating a stepped impedance configuration that allows a reduction in their size and increase in the order N of combining. The base also includes a matching post at the bottom center which matches the reduced height rectangular waveguides into the circular waveguide that feeds the mode transducer.
The invention may be implemented as a power combiner or power divider, or may be combined in a power divider/combiner.
The combiner/divider is herein illustrated in detail in the context of an N=24 combiner for use in Ka-band over the band of 31-36 GHz, with an input match <−20 dB under equal excitation of all input ports, an output match <−24 dB coming out of the mode transducer, and an insertion loss <0.6 dB.
The power divider/combiner 2 generally comprises a radial base 20 with a plurality N of internal waveguides (here N=24) running axially and internal to the base 20 from peripheral ports 22 (spaced evenly around the base 20) and converging to a matching post (obscured) in the center of base 20. For testing purposes, a plurality of matching loads 30 are shown mounted axially around the base 20 to balance the ports 22 not in use, and each load 30 is coupled to a non-use port 22 by machine-screw attachment to the periphery of the base 20. The base 20 has a topside center output port (obscured) for mounting a mode transducer 10. The mode transducer 10 is a three-section transducer with distal ports 16, 18 that convert the TE01 circular waveguide mode at the center output port of base 20 back into standard rectangular TE01 waveguide mode at transducer port 16.
As best seen at exploded illustrations (D) & (E) the sectioned internal waveguides 50 of the first section (D) are evenly spaced and radially converge toward a central cylindrical cavity 52 that is formed with a central cylindrical matching post 54 at the center. The matching post 54 protrudes upward to a plateau even with the inner surface of the first section. The sectioned internal waveguides 50 of the second section (E) likewise converge to the topside center output port which is formed as a central cylindrical aperture 55 that conforms to the cavity 52. In accordance with the present invention, each axial waveguide 50 (along both sections) is formed with a rectangular cross-section that extends uniformly from ports 22 to one or more constricted steps 56 (three successive steps 56A-C being here illustrated), the steps 56 effectively forming a rectangular stepped-impedance configuration with incremental height changes.
In general operation when used as a combiner, rectangular TE01 waveguide signals input to ports 22 form reflections along the walls of the stepped-height waveguides 50 which must be combined properly into a TE01 circular waveguide mode, and this purpose is served by the matching post 54, which provides a circular waveguide output through the topside center output port (aperture 55) into the mode transducer 10 described below. Thus, the radial base 20 has standard rectangular TE10 mode waveguide input and a circular waveguide TE01 mode output at aperture 55.
A cross-section of the mode transducer body 11 is show at
The transducer body 11 of the mode transducer 10 is designed to convert the radial base 20 circular TE01 waveguide output at aperture 55 back to rectangular TE10 waveguide mode. Generally, the transducer body 11 of the mode transducer 10 was designed based on the concept of S. S. Saad, J. B. Davies, and O. J. Davies, “Analysis and Design of a Circular TE01 Mode Transducer,” Microwave, Optics and Acoustics, vol. 1, pp. 58-62, Jan. 1977. Saad et al. therein disclose the concept of a “Marie Mode” transducer for transducing multiple rectangular TE10 modes to circular TE01 mode. Multiple TE01 modes are transitioned into an intermediate mode, which is transitioned into a circular TE01 mode and vice versa. The present transducer employs different symmetry considerations and dimensions.
As seen in
The exact profile, contour and length of each section 110-114 must be precisely tuned in order to make it possible to combine 24 inputs, operating from 31 to 36 GHz. Consequently,
Beginning at the TE01 end,
The cylindrical waveguide section 110 merges into an outwardly-tapered rectangular waveguide section 112 shown in
Finally,
The three above-described sections 110, 112, and 114 are preferably integrally formed in a unitary transducer body 11, which is then attached to ports 16, 18.
It is noteworthy that the above-described transducer 10 can easily be designed to provide two different rectangular waveguide outputs by modification of only section 114, leading to an alternate design for multiple frequency ranges with a common circular waveguide input.
For operation of the power divider/combiner 2 as a divider. In this case a signal generator will provide an input signal to the divider 2 at the input flange 16 of mode transducer 20 via a coaxial cable attached to the flange 16 via a connector, which may be an SMA connector, for example. Once inside the mode transducer 20, the signal propagates down through the transducer body 11 through the a pyramidal section 114 which transitions from rectangular TE01 mode to intermediate rectangular mode, then through tapered rectangular waveguide section 112 which transitions the intermediate rectangular mode to an intermediate cylindrical mode, and finally through the tapered cylindrical waveguide section 110 which transitions the intermediate rectangular mode to a single cylindrical TE10 modes which is propagated into base 20. The matching post 54 provides a circular waveguide output from the transducer 10 into a rectangular mode within each axial waveguide 50 in the base 20. The waveguides 50 maintain a rectangular cross-section to the constricted steps 56 which impart a rectangular stepped-impedance configuration as a result of their incremental height changes. Thus, inside the base 20 the signals are effectively divided to form N output signals (here N=24). One or more of these output signals may then be provided to a signal receiver coupled to ports 22. The signal receiver may be a test device, such as a spectrum analyzer, or a multiple-amplifier module.
By reversing signal direction, the divider/combiner may function as a combiner. In this case, a plurality N of TE01 waveguide signals are input at ports 22 (via coaxial cables or the like) and propagate in through the waveguides 50, which maintain a rectangular cross-section to the constricted steps 56. The steps 56 impart a rectangular stepped-impedance configuration as a result of their incremental height changes. The N signals are combined and transitioned by matching post 54 from rectangular TE01 mode to circular TE10 mode, and the combined signal is output through port 18 into the mode transducer body 11. Inside the transducer body the signal propagates through the tapered cylindrical waveguide section 110, then through tapered rectangular waveguide section 112, and finally through the rectangular waveguide section 114 which transitions the intermediate rectangular mode to a single rectangular TE01 mode which is output through port 16. A prototype N=24 combiner has been constructed for Ka-band and demonstrated over the band of 31-36 GHz with an input match <−20 dB under equal excitation of all ports 22, output match <−24 dB at the port in flange 16 of the mode transducer 10, and an insertion loss <0.6 dB. The functional bandwidth of the combiner/divider 2 exceeds the initial design goal of 31-36 GHz.
Generally, for normal operation all ports 22 will be used. However, for testing purposes only one or two ports 22 will be used, and a plurality of matching loads 30 are shown mounted axially around the base 20 (
In
In
Having now fully set forth the preferred embodiment and certain modifications of the concept underlying the present invention, various other embodiments as well as certain variations and modifications of the embodiments herein shown and described will obviously occur to those skilled in the art upon becoming familiar with said underlying concept. It is to be understood, therefore, that the invention may be practiced otherwise than as specifically set forth herein.
The present application derives priority from U.S. Provisional Application No. 60/663,330 filed 18 Mar. 2005.
The invention described hereunder was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law #96-517 (35 U.S.C. 202) in which the Contractor has elected not to retain title.
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| Number | Date | Country | |
|---|---|---|---|
| 60663330 | Mar 2005 | US |
