Spatial Combining Creates Linear Power

Dec. 16, 2011
This unique approach to combining the outputs of large numbers of GaAs and GaN MMIC amplifiers can provide solid-state power comparable to vacuum-tube devices.

Microwave power at appreciable levels is usually associated with vacuum tube devices, such as traveling wave tubes (TWTs). But with Spatium power-combining technology, CAP Wireless has achieved amplifier power levels once attained only by tubes, adding the contributions of multiple GaAs MMIC amplifiers via a patented quasi-optical power-combining approach. The technique has demonstrated better than 90% power combining efficiency in a number of different solid-state power amplifier (SSPA) designs, from S-band through millimeter-wave frequencies.

Spatium technology has also been used with gallium nitride (GaN) active devices to achieve SSPAs with output levels rivaling TWT amplifiers (TWTAs). This compact and efficient amplification approach has grabbed the attention of the United States Air Force, among other branches of the military, for its capability of delivering high RF/microwave power levels in relatively light-weight packages. And in contrast to a high-power amplifier based on vacuum-tube technology, which can shut down with the failure of a single device, Spatium amplifiers exhibit what is known as a "soft" failure mode: One or more of the solid-state devices within the amplifier can fail, and the amplifier will continue operatingalbeit with reduced gain and output power.

Spatium amplifiers employ quasi-optical combining to add the outputs of individual amplifier devices, such as GaAs or GaN MMIC or discrete power amplifiers, in free space rather than via a stripline or microstrip power combiner. In much the same way that a phased-array antenna combines numerous antenna elements to form the equivalent beam of a much larger structure, a Spatium amplifier combines the power outputs of multiple amplifiers to create the equivalent output power of a much larger amplifier. The bandwidth of a Spatium amplifier is essentially limited only by the performance of its individual amplifiers. Its output power is limited only by the characteristics of the individual amplifiers, the thermal dissipation possible, the number of devices that can be incorporated within a Spatium amplifier structure, and the breakdown voltage of the output coaxial connector. In contrast, the high insertion loss of traditional passive power combiners limits their use when adding the output contributions of large numbers of amplifiers, since the loss for high combining numbers starts to approach the additive amount of power provided by the additional amplifiers.

This idea of spatial power combining is actually not new but was practiced in a basic form with multiple RF/ microwave power tubes and dipole antennas since before World War II. The technology has been enhanced over the years, with the power-combining structure in a Spatium amplifier based on advancements on work originally performed as part of a Small Business Innovative Research (SBIR) grant awarded to CAP by the United States Air Force in 2004. The structure has been extensively analyzed and refined using modeling and simulation techniques, including SOLIDWORKS software from Dassault Systemes SolidWorks Corp. and HFSS software from Ansoft.

The Spatium amplifier is best envisioned as a large coaxial waveguide with amplifier elements radially distributed within the coaxial structure. CAP implemented a coaxial spatial combiner structure because of its numerous advantages over other spatial combining architectures. The coaxial structure enables each of the multiple amplifiers to operate on the same EM field, due to the TEM nature of the coax, resulting in optimum linearity and efficiency. The inherent bandwidth capability is effectively unlimited, the low frequency limited by length and the high frequency by mechanical tolerances. Heat distribution is uniform, and mutual heating is minimized. The sections of the coax are identical and interchangeable. The bandwidth and mechanical implementation enable substantial design reuse, enabling new semiconductor devices and technologies to be rapidly implemented and fielded as they become available.

At the input of a Spatium amplifier, the center conductor from the coaxial input connector is transitioned to a larger center conductor and then to multiple antipodal finline antenna elements (Fig. 1). These broadband antenna elements are arranged radially around the center to gather all the RF/microwave energy from the input connector. They then transition that energy to numerous (typically 16) microstrip transmission lines to feed the individual amplifiers. The latter are also mounted around the center of the cylindrical amplifier structure, each in a resonance-free, hermetically sealable, ceramic package incorporating an appropriate heat spreader. Input signals are simultaneously amplified by the amount of gain in the individual amplifiers. The outputs of the amplifiers are then launched onto microstrip line. These in turn couple to antipodal finline antenna elements and back to a coaxial waveguide transmission structure. Here, the multiple EM fields generated by the outputs of the amplifiers are coherently combined and transitioned back to the center conductor of a coaxial connector at the output of the Spatium amplifier.

Of course, high combining efficiency in a Spatium amplifier assumes uniform amplitude and phase responses from the individual amplifiers, and tightly controlled amplitude and phase balance for the feed lines to the amplifiers and the output lines from the amplifiers that ultimately combine for the final output signals. The power combining structure of a Spatium amplifier can sum the outputs of 16 MMIC amplifiers with total combining and insertion loss of less than 0.5 dB over more than a decade of bandwidth, for typical combining efficiency of better than 90% through millimeter-wave frequencies. The power-combining losses in a Spatium amplifier do not increase with the number of elements, as in a conventional microstrip or stripline power combiner, nor is there a requirement that the number of elements be binary. Hence, low combining losses and high efficiency can be expected in Spatium amplifiers even when as many as 18 or 23 amplifiers are being combined to achieve the final output power.

The transition at the input of a Spatium amplifier takes the form of a 16-way power divider; on the output, the transition has the form of a 16-way power combiner. Both transitions increase in length inversely proportional to the lowest frequency of operation. The first generation of Spatium power amplifiers, designed for use from 2 to 20 GHz, measured 2.81 x 3.00 x 9.90 in. (Fig. 2). By raising the lower-frequency cutoff point to 4.5 GHz, the second-generation design, which covered 4.5 to 20 GHz, measured 2.40 x 2.40 x 5.00 in. The length of a third-generation Spatium amplifier, designed for Ka-band applications, was reduced further. The diameter of the amplifiers is largely dictated by the MMIC amplifiers, their associated bias and packaging requirements, and thermal considerations. These Spatium building-block amplifiers are then used in larger designs with control circuitry to form advanced units such as the firm's line of instrumentation amplifiers (Fig. 3).

As an example of one of these instrumentation amplifiers based on Spatium technology, model RM022020 delivers 20-W typical saturated output power from 2 to 20 GHz. It provides typically 40-dB gain, with a 10-dB gain adjustment range. Measuring 17.0 x 17.1 x 5.25 in. and weighing 35.3 lbs, it suppresses spurious levels to typically -85 dBc. The broadband amplifier is unconditionally stable. It is also suitable for electronic-warfare (EW) and electronic-countermeasures (ECM) applications (Fig. 4).

The Spatium technology is also used in a variety of CAP Wireless' more narrowband standard amplifiers, including model KS2699-OEM with 40-W minimum and 55 W typical saturated output power from 14 to 17 GHz, and 50-W typical output power at 1-dB compression across that frequency range (Fig. 5). It delivers 20-dB minimum and 23-dB typical gain across its bandwidth, with gain flatness of typically 1 dB across frequency and temperatures from -40 to +70C. The power-added efficiency (PAE) is typically 20%, with typical noise figure of 10 dB and spurious levels controlled to -60 dBc. Model KS2699-OEM draws 35 A current from a +7-VDC supply. It measures 5.7 x 2.4 x 2.4 in. and is supplied with female SMA input and output connectors.

Model KK0198-OEM provides 40 W typical saturated output power from 29.5 to 31.0 GHz, with typical output power of 30 W at 1-dB compression. It boasts 15- dB minimum gain and 18-dB typical gain, with typical gain flatness of 1 dB across frequency and with temperature (-40 to +85C). Based on the combined outputs of multiple GaAs MMICs, model KK0198- OEM achieves PAE of 20% while drawing 32 A from a +5.5-VDC supply. It is supplied with female 2.92-mm input and output connectors.

In addition to its many GaAs-based designs, CAP Wireless has also applied the Spatium power-combining approach to gallium-nitride (GaN) active devices to achieve saturated outputpower levels as high as 300 W at 6 GHz. For example, model CN5199-OEM is a solidstate Spatium power amplifier with 300 W typical saturated output power from 2 to 6 GHz, and 150 W output power at 1-dB compression across that 4-GHz bandwidth. It features 16-dB minimum gain and 19-dB typical gain, with 2 dB typical gain flatness from 2 to 6 GHz. The GaN amplifier achieves typical power-added efficiency (PAE) of 38%. As with the other Spatium power amplifiers, it draws high current from a low-voltage supply (because of the multiple internal solid-state amplifiers), typically drawing 28 A current from a +28-VDC supply. The 2-to-6-GHz amplifier, which employs conduction cooling, measures 9.0 x 2.4 x 2.4 in. and is supplied with Type N female connectors. It is specified for operating temperatures from -40 to +85C.

Although the Spatium power-combining approach is quite efficient in terms of combining the individual contributions of high-frequency GaAs and GaN MMIC amplifiers, the high power density of Spatium amplifiers, especially with 16 or more MMIC stages, presents a challenge in thermal design. Most of the Spatium amplifiers are designed for use with conduction cooling. In order to ensure the long-term reliability of the individual MMIC stages, each of the 16 sections or wedges of a Spatium amplifier must be maintained below +85C in order to prevent overheating.

The firm has worked with outside companies such as Quantum Focus Instruments Corp. in performing thermal infrared (IR) scans of prototype amplifier wedges. Prototype wedges were fabricated with packaged MMICs mounted on different heat-sink materials and thermal IR scans performed to better understand the thermal load on each wedge when mounted within the final Spatium amplifier design. These measurements also shed light on the accuracy of the thermal simulations provided by thermal material suppliers.

A great deal of thought was put into developing a test fixture that would accurately represent the thermal conditions each amplifier wedge would see in a full Spatium amplifier. For example, in a full amplifier, heat would only be removed from one side of the wedge in contact with a thermal clamp, so the test fixture must also account for this type of heat flow.

To push Spatium amplifier technology to higher power levelsmaking it a practical replacement for vacuum-tube amplifiers in such applications as electronic warfare, commercial satellite communications (satcom) systems, and military airborne data linksCAP Wireless has explored the use of enhanced heat removal systems. These include liquid cooling, as well as proprietary composite thermal management materials. For now, these standard instrumentation and component amplifiers are examples of the great promise of this innovative amplifier technology.

About the Author

Jack Browne | Technical Contributor

Jack Browne, Technical Contributor, has worked in technical publishing for over 30 years. He managed the content and production of three technical journals while at the American Institute of Physics, including Medical Physics and the Journal of Vacuum Science & Technology. He has been a Publisher and Editor for Penton Media, started the firm’s Wireless Symposium & Exhibition trade show in 1993, and currently serves as Technical Contributor for that company's Microwaves & RF magazine. Browne, who holds a BS in Mathematics from City College of New York and BA degrees in English and Philosophy from Fordham University, is a member of the IEEE.

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