Next-generation synthetic aperture radar (SAR) remote sensing platforms utilize new concepts such as the SweepSAR techniques that provide increased swath size, high resolution, rapid global coverage, and subcentimeter interferometry and polarimetry. An L-band SweepSAR mission would use multiple transmit/receive (T/R) channels and digital beamforming to achieve simultaneously high resolution and large swath. One of the key challenges in implementing the SweepSAR concept is the development of space-qualified efficient transmit/receive modules (TRMs) that provide the amplitude and phase stability necessary for repeat pass interferometry.

The transmit/receive module (TRM) mechanical design shows both the (a) radiator and (b) cavity sides. The four major TRM components (front-end module, HPA, energy storage system, and TRM chassis) are highlighted.
This work presents the design and measured results of a fully integrated, switched power, two-stage, GaN HEMT (high electron mobility transistor) HPA (high-power amplifier) achieving 60% power-added efficiency at over 120 W output power. This HPA is an enabling technology for L-band SweepSAR interferometric instruments that enable frequent repeat intervals and high-resolution imagery. The amplifier exhibits over 34 dB of power gain at 51 dBm of output power across an 80-MHz bandwidth. In addition, due to harmonic tuning, the harmonic output of the TRM is better than –50 dBc, therefore eliminating the need for lossy and large highpower filters. The HPA is divided into two stages: an 8-W driver stage and 120-W output stage.

(a) Picture of fabricated integrated transmit/receive module with the GaN high-power amplifier module highlighted; (b) fabricated two-stage GaN HPA module highlighting the DC and RF boards and the GaN HEMT devices.
Typically, the HPA is most sensitive to thermal drifts due to the nonlinear large signal operation; therefore, a key focus on the TRM design for the proposed L-band SweepSAR mission was to create a thermally stable HPA, optimizing both the electrical performance and packaging design. Electrically, the HPA was stabilized by operating the device at or near saturation by creating a compressed two-stage design, and temperature-compensated gate bias circuitry. Mechanically, HPA packaging was optimized to provide an efficient path to the thermal radiator, and optimized thermal radiator area to maintain temperature.

The amplifier is designed for pulsed operation, with a high-speed DC drain switch operating at the pulsed-repetition interval, and settles within 200 ns. In addition to the electrical design, a thermally optimized package was designed that allows for direct thermal radiation to maintain low-junction temperatures for the GaN parts, maximizing long-term reliability. Lastly, real radar waveforms are characterized, and analysis of amplitude and phase stability demonstrate ultra-stable operation over temperature using integrated bias compensation circuitry, allowing less than 0.2 dB amplitude variation and 2° phase variation over a 70 °C range.

This work was done by Tushar Thrivikraman, Stephen J. Horst, Douglas L. Price, James P. Hoffman, and Louise A. Veilleux of Caltech for NASA’s Jet Propulsion Laboratory. NPO-49388

Topics:
Data acquisition and handling Electrical systems Electronics & Computers Energy storage systems Remote sensing
This Brief includes a Technical Support Package (TSP).
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Compact, Two-Stage, 120-W GaN High-Power Amplifier for SweepSAR Radar Systems

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NASA Tech Briefs Magazine

This article first appeared in the October, 2015 issue of NASA Tech Briefs Magazine (Vol. 39 No. 10).

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Overview

The document presents a Technical Support Package detailing the development of a Compact, Two-Stage, 120-W GaN (Gallium Nitride) High-Power Amplifier (HPA) specifically designed for SweepSAR (Synthetic Aperture Radar) Radar Systems. This work is conducted by researchers at NASA's Jet Propulsion Laboratory (JPL) and is part of the NASA Earth Radar Mission task.

The introduction outlines the necessity for high-peak power HPAs that maintain high efficiency for advanced radar systems. The document emphasizes the use of GaN HEMT (High Electron Mobility Transistor) technology, which is characterized by its wide bandgap material, high breakdown voltage, and high power density compared to traditional materials like GaAs (Gallium Arsenide). The design incorporates AlGaN/GaN layers on a SiC (Silicon Carbide) substrate, which provides low thermal resistance and enhances performance.

The document is structured into several sections, including electrical and mechanical design, measured results, and performance analysis. The HPA is noted for achieving over 120 W output power with a power-added efficiency (PAE) of 60%. It features a high gain of 34 dB, which improves overall transmit efficiency and reduces the need for high-power driver stages. The design is thermally stable, utilizing effective packaging, bias temperature compensation, and saturated operation to ensure consistent performance across varying temperatures.

Measured results demonstrate the amplifier's capability to handle realistic radar signals with minimal distortion, thereby reducing calibration requirements. The document also discusses the implications of the HPA's performance in ambient conditions and over temperature variations, highlighting its reliability and efficiency.

In summary, this Technical Support Package showcases the advancements in GaN technology for high-power applications in radar systems, emphasizing the compact design, high efficiency, and thermal stability of the two-stage amplifier. The research is supported by NASA and aims to contribute to the broader field of aerospace technology, with potential applications beyond radar systems. The document serves as a resource for understanding the technical aspects and benefits of this innovative amplifier design.