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A Study of Interpulse Instability in Gallium Nitride Power Amplifiers in Multifunction Radars

The incorporation of gallium nitride (GaN) power amplifiers (PAs) into future high-power aperture radar systems is certain; however, the introduction of this technology into multifunction radar systems will present new challenges to radar engineers. This paper describes a broad investigation into amplitude and phase transients produced by GaN PAs when they are excited with multifunction radar waveforms. These transients are the result of self-heating electrothermal memory effects and are manifested as interpulse instabilities that can negatively impact the coherent processing of multiple pulses. A behavioral model based on a Foster network topology has been developed to replicate the measured amplitude and phase transients accurately. This model has been used to develop a digital predistortion technique that successfully mitigates the impact of the transients. The moving target indicator improvement factor and the root mean square pulse-to-pulse stability are used as a metric to assess the impact of transients on radar system performance and to test the effectiveness of a novel digital predistortion concept.