A Phase-Locked GaN Oscillator Using a Mechanically Tunable Cavity Resonator

Abstract

The demand for wireless data transfer is growing rapidly, and local oscillator phase noise has been identified as a performance limiting factor in radio communication systems. Two ways to reduce phase noise are to use a high power amplifier- and a high quality factor (Q) resonator in the oscillator design, such as a gallium nitride high electron mobility transistor (GaN HEMT) amplifier and a metallic cavity resonator. A free-running cavity oscillator is however a poor choice for use as a local oscillator, since they exhibit poor thermal frequency stability. This thesis presents the design and characterization of a phase-locked GaN HEMT cavity oscillator, utilizing mechanical tuning of the cavity as well as electronic tuning using a varactor.

The prototype oscillator works, but performed worse than expected with a phase noise of −103.7 dB at 100 kHz offset, measured at the best operating point. The oscillation frequency of the oscillator was tunable from 11.51 GHz to 12.29 GHz. Furthermore, the phase locked loop (PLL) was not able to lock the phase of the oscillator to a reference due to stability issues. Theoretical evidence that low phase noise can be achieved using this method is however presented, and reasons for the poor performance of the prototype are discussed.