Speaker
Description
A high average power, pulsed RF source operating at 2.85 GHz is described for use in multipactor research. Four state of the art GaN HEMTs from Cree/Wolfspeed capable of 700 W in long pulse mode (~ 100 μs) each are combined for a total maximum power output of 2.8 kW with a rated output of 2 kW total. A low phase noise, free running 55 MHz modulation capable VCO is used as the RF source, buffered and amplified by a LNA. Generation of a 100 μs drive pulse is accomplished via a TTL switch with a rise time of 25 ns and a typical switching time of 35 ns driven by a pulse/delay generator. The pulse is further amplified by a microwave amplifier providing the majority of gain in the system, 45 dB, and the 6 W required for the final stage. A CGHV31500F GaN HEMT amplifier is used to amplify the signal to the required 50.5 dBm (~112 W) with split feeding of the final output stage consisting of four additional parallel connected GaN HEMTs. Each of the four has 12.5 dB of gain, providing 500 W for a combined output of 2 kW. An in-line attenuator provides the operator output power control in the range of approximately 42 to 64 dBm (16 to 2,800 W). A custom control PCB was designed to control, bias, and sequence the various parts of the source. Custom power splitters and directional couplers capable of high power were designed for the system. The amplifier enables multi-carrier mode amplification as well as modulated output amplitudes. A ring resonator is used to study the multipactor phenomenon in a WR284 waveguide section, increasing the effective power to approximately 40 kW, with a high impedance section where the phenomenon is observed within the tapered impedance transformer.
