This invention relates to high power microwave and millimeter wave RF sources, and, more particularly, to a high power RF amplifier structure that combines the RF power output of a large number of individual semiconductor millimeter wave RF amplifiers to achieve higher power levels within a physically compact package.
The microwave and millimeter wave (MMW) frequency range has long been the range of choice for various electronic devices, such as radar, satellite up-link and down-link transmitters, LMDS ground station transmitters and smart munitions. To achieve RF at frequencies in that range, as example, at 35 GHz, millimeter microwave integrated circuit (xe2x80x9cMMICxe2x80x9d) devices have been developed to produce and/or amplify RF signals with reasonable levels of efficiency. Many of those MMIC devices employ high electron mobility transistors (xe2x80x9cHEMTxe2x80x9d) as the active element providing amplification. One example of such a high efficiency MMIC source is described in an article by Ingram et. al. (a co-inventor) appearing in the IEEE Transactions on Microwave Theory and Techniques, Vol. 45, No. 12, December 1997 at pages 2424-2430.
Although the MMIC amplifier described in the foregoing article achieved a benchmark in power level in the achievement of a six watt RF output, due to the nature of the semiconductor device and the high frequency, the high power achieved by a single such MMIC amplifier device is much less than the power levels which are achieved at the lower microwave frequencies at which the familiar magnetron or klystron devices are used. Although the RF power is relatively high for a semiconductor device at the 35 GHz frequency, that power is less than customarily desired for the typical radar and/or up link and down link communications channels. It may be said that the more power available, the better. To achieve greater RF power levels at millimeter wave frequencies, it has been necessary to combine the RF outputs of multiple numbers of MMIC amplifier devices so that the total output power from the combination is much larger than that available solely from a single MMIC amplifier.
The familiar binary combiner has typically been used for that RF combining function in those plural MMIC power amplifier combinations. In implementation of the binary combiner, RF inputted from separate sources to a pair of waveguide arms are combined by use of a Magic-T junction, and the combined output is then introduced by a third port of the Magic-T junction to another arm. In turn, the RF in that third arm is then combined by another Magic-T junction with the RF output of another like waveguide arm that introduces the combined power from a different pair of arms. The combining structure must be symmetrical. That is, each arm to a MMIC power amplifier must be of the same length as the corresponding arm associated with any other MMIC power amplifier so that the RF from separate amplifiers is equal in intensity and phase when combined at a Magic-T junction. The foregoing inverted pyramiding structure, at least theoretically, may be built up ad-infinitum to produce very high power levels.
In practice, power loss is inherent in the binary combiner structure due to the waveguide media, such as the air environment within the waveguide, resistivity of the waveguide walls and imperfection of the construction. Some portion of the RF energy heats the air and the waveguide, and is essentially lost as heat, reducing the energy that is output. That power loss serves as one limit to the size of the binary combiner and the combination of multiple power amplifiers. As the number of combining elements is increased, the power losses in the arms carrying the higher power levels become excessive, and the combining efficiency drops substantially as the number of stages in the binary combiner increases beyond eight.
A contributing factor to such loss of RF is the physical size of the assembly. Each MMIC amplifier assembly, though small, is of a finite size. To combine the outputs of multiple power amplifiers using the binary combiner technique, the power amplifiers must be arranged, as example, in a single row so as to satisfy the described requirement for symmetry in the binary waveguide combining arrangement. The number of individual power amplifiers may be represented as 2n, where n is equal to a whole number greater than 1. Increasing the number of amplifiers from 4 to 8 spreads the row of amplifiers over double the width than before, and, hence, requires an increase in length of the intermediate waveguide arms of the binary combiner that join the amplifiers together. Because the RF must then propagate over greater path lengths, the energy lost due to heating of air in the waveguide and dissipation on the waveguide walls, increases. Hence, the overall electrical efficiency becomes lower.
The physical size of the binary combiner becomes excessively large as the number of included power amplifier elements increase beyond eight. Eventually, the combiner loss increases exponentially beyond between eight to sixteen elements, and additional combining does not produce a net higher power. As an advantage, the present invention allows the RF of a greater number of microwave semiconductors to be combined without incurring such exponentially increasing losses. Considered separately, a large physical size is not typically desirable, since size could pose a problem in applications in which limited space is available, such as in aircraft installations. As a further advantage, the present invention provides both a high power RF amplifier and a more compact physical structure than is available with the existing high power amplifier designs that employ the binary combining structure.
A secondary effect of increased heating is that the MMIC amplifiers, which are sensitive to temperature, are adversely affected by temperature increases. Being a semiconductor material, the lower the temperature of operation, the greater is the power gain achieved. Hence, the amplifier structure typically includes cooling apparatus both active and/or passive types to conduct heat away from the amplifier. Thus, not only do loses increase when the transmission path lengths increase, but the operating efficiency of the individual amplifiers falls off, unless more active cooling can be provided. Even if greater cooling capacity is employed to maintain the amplifier efficiency, the energy expended to provide that cooling instead reduces overall system efficiency.
Accordingly, a principal object of the invention is to provide a new high power RF amplifier this is capable of providing very high power levels at millimeter and microwave frequencies with reasonable efficiency.
Another object of the invention is to provide an efficient means to combine the RF outputs of semiconductor devices to achieve ultra-high power levels in the millimeter/microwave frequency range.
A still further object of the invention is to provide an RF power combining structure whose efficiency and power level surpasses that available in designs that use binary combiners and affords a more compact physical size.
In accordance with the foregoing objects and advantages, a new waveguide power combining structure is defined by a compact power module that contains numerous (16 will be used as an example to illustrate the concept) MMIC amplifiers and novel manifold structures. The input manifold structure provides an RF feed that evenly distributes inputted RF in equal amplitude and phase to each MMIC amplifier. Each MMIC amplifier amplifies the RF power and the RF output power from each amplifier is combined in the output manifold structure of the power module from which the combined higher power level RF is output.
The individual MMIC power amplifiers are organized in four separate rows and those rows of amplifiers are stacked in layers, one over the other. The input waveguide manifold that distributes the RF to be amplified amongst all the power amplifiers and the output waveguide manifold in which the individual outputs are combined into a single higher power output, are identical to one another, defined waveguides arranged in a pattern of H""s, referred to herein as xe2x80x9ccrazy-H""sxe2x80x9d. In such configuration the input (or output) is located at the center stem of a large English letter xe2x80x9cHxe2x80x9d shaped waveguide distribution network and the outer arms of the xe2x80x9cHxe2x80x9d each feed into the center of the connecting stem of a smaller size xe2x80x9cHxe2x80x9d sub-distribution network. The outer end of each of the outer arms of the small size H distribution network is coupled to the RF input of a respective one of the amplifiers in respect of the input manifold (or to the output of such amplifier in respect of the output manifold) by means of an E-plane transition.
To split a signal into equal parts for distribution to the inputs of various amplifiers each location at which a division is to occur in the input manifold (and each location at which a combination is to occur in the output manifold) includes a Magic-T, as is familiar from the prior binary combiner system. Thus a Magic-T is included at the center stem of each H, both large and small size, and at each juncture between the center stem of the H and the respective arms of the larger size H (and at the output port or input port as the case may be).
The level of combined RF power produced by the foregoing power combining structure, and, hence, the number of MMIC amplifiers incorporated within the structure to amplify RF, is limited by the ability of the structure (and associated cooling devices) to dissipate the heat generated therein by the portion of the RF energy that is consumed in the lossy material. Once that power limit is reached in a given power module design, increased power levels are achieved by combining the RF output of each power module using another arrangement.
As an additional and important aspect to the invention, a plurality of the foregoing power modules are distributed evenly about the periphery of a radial combiner and are respectively coupled in parallel to the inputs of the radial combiner. The output of the radial combiner combines the RF outputs of each power module to produce a power level that is many times greater than the power level output of a single power module. The radial combiner is capable of dissipating greater amounts of heat, since the latter combiner does not contain semiconductor elements. As assembled the combination of power modules and radial combiner provides a symmetrical and compact package.
As a still further aspect to the invention, multiple power module and radial combiner combinations may be assembled into power sources of even greater RF power by stacking the combinations and incorporating a binary waveguide in the vertical direction between the stacked radial combiners.
The foregoing and additional objects and advantages of the invention together with the structure characteristic thereof, which was only briefly summarized in the foregoing passages, will become more apparent to those skilled in the art upon reading the detailed description of a preferred embodiment of the invention, which follows in this specification, taken together with the illustrations thereof presented in the accompanying drawings.