Phase noise analysis of a tuned-input/tuned-output oscillator based on a GaN HEMT device

This paper reports on an experimental analysis of phase noise in a tuned-input/tuned-output oscillator based on a bare-die GaN HEMT device. To investigate phase noise dependency on resonator coupling factor (β), the circuit is designed with flexibility to modify the resonant tank, in terms of unloaded quality factor (Q₀) and impedance level. The reflection coefficient (Γ_amp) from the reflection amplifier can also be varied. With exception for very high values of Γ_amp, the circuit is robust to variations in β. It is more important to choose a bias point where the flicker noise is low and the power reasonably high. A minimum phase noise of -150dBc/Hz @ 1MHz off-set from a 1GHz oscillation frequency and a power normalized figure of merit (FOM) of 186 are reached. An interesting analytical result is also presented; power normalized figure of merit, commonly used for benchmark of oscillators is bound by Q₀ and can be related to the theoretical noise floor limit.