A 2 KW, 2.85 GHZ Multipactor RF Source Utilizing Depletion Mode GaN HEMTs

A pulsed RF source for multipactor research capable of providing 2 kW at 2.85 GHz to a multipactor test cell is described utilizing state-of-the-art GaN HEMTs from Cree/Wolfspeed and integrated GaN amplifier from Qorvo/Tri-Quint. A Mini-Circuits ZX95-2920CA+ VCO is used as the signal generator with low phase noise and modulation capable with a bandwidth of 55 MHz. An inline attenuator provides the operator power control in the range of approximately 42 to 64 dBm (16 to 2800 W). A high-speed RF switch creates a 100-microsecond pulse with a rise time of 25 ns and a typical switching time of 35 ns. A single Tri-Quint TGA2585-SM provides the majority of the gain in the system, 32 dB, and the necessary power for the final output stage (6 W). Four CGHV31500F amplifiers operating in parallel comprise the output stage providing the bulk of the power needed, 500 W each for a total of 2 kW with 12.5 dB gain each, with a single unit providing the necessary input power before splitting to the four finals (50.5 dBm, 112 W). A custom PCB was designed to properly bias the GaN stages and prevent device failure due to improper bias sequencing. Custom power splitters and combiners had to be used due to the high-power levels being considered with directional couplers on the output to monitor output (forward) power and reflected (reverse) power during operation. This will provide valuable insight into characteristics of the plasma cloud generated during the multipactor event.