Radar remote sensing plays an important role in deriving unique measurements that address fundamental questions in the National Aeronautic and Space Administration (NASA) Earth Science Enterprise (ESE) strategic plan. Synthetic aperture radar (SAR) can provide measurement key to the water cycle (e.g. soil moisture and water level), global ecosystem (biomass estimation, land cover change), and ocean circulation and ice mass (ice motion). L-band radar provides the ability to make these measurements under a variety of topographic and land cover conditions, day or night, with wide coverage at fine resolution and with minimal temporal decorrelation.

Large, lightweight, high power, electronically-steerable L-band T/R modules are required to enable the next-generation NASA Earth Science SAR missions. One of the most critical parts of the T/R modules is the power amplifier (PA) in the last stage, since the last stage determines the overall efficiency of the T/R module.

Switching-mode amplifiers are promising candidates for UHF transmitters that require high efficiency, but only limited bandwidth and linearity. Recently Kee et al. reported a new class of switching power amplifiers, the Class-E/F amplifier [1]. Figure 1 shows the schematic of an E/Fodd,2 amplifier. This amplifier has zero-voltage switching like a Class-E amplifier, while terminating the odd harmonics and the 2nd harmonic like a Class-F-1 amplifier. The amplifier uses a pair of transistors, driven as switches 180º out of phase. A second-harmonic trap helps shape the current waveform. Figure 2 shows a photograph of the actual amplifier.

 
High-Efficiency Transmitters for Space Radar
 

The PA operates at 1100 MHz with an output power of 60 W, a gain of 12 dB and power-added efficiency (PAE) of 65%. Compact, low-loss and impedance-transforming microstrip baluns are designed for the push-pull configuration. The amplifier is realized using silicon laterally diffused MOSFET (LDMOS) devices. The amplifier is built on a Duroid substrate with a dielectric constant of 2.2. Our amplifier has an output power five times larger than the other recently reported switching amplifiers, with an area five times smaller [2].

References:

W. N. Edelstein, C. Andricos, A. Moussessian, F. Wang, D. B. Rutledge, "High-Efficiency L-band Transmit/Receive Module for Synthetic Aperture Radar," 2003 IEEE Radar Conf. Proceedings, pp. 238-243, May, 2003.

Scott Kee, Ichiro Aoki, Ali Hajimiri, and David Rutledge, The Class - E/F Family of ZVS Switching Amplifiers , IEEE Microwave Theory and Techniques, Vol. 51, No. 6, June 2003.


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