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

NASA Tech Briefs Magazine

This article first appeared in the October, 2015 issue of NASA Tech Briefs Magazine.

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