RF HBT oscillators with low-phase noise and high-power performance utilizing (λ/4 ± δ) open-stubs resonator

This paper presents a new type of transmission-line resonator and its application to RF (microwave and millimeterwave) heterojunction bipolar transistor (HBT) oscillators. The resonator is a parallel combination of two open stubs having length of λ/4 ± δ(δ ≪ λ), where λ is a wavelength at a resonant frequency. The most important feature of this resonator is that the coupling coefficient (β^C) can be controlled by changing δ while maintaining unloaded Q-factor (Q^u) constant. Choosing a small value of δ allows us to reduce β^C or equivalently to increase loaded Q-factor (Q^L). Since coupling elements such as capacitors or electromagnetic gaps are not needed, β^C and Q^L can be precisely controlled based on mature lithography technology. This feature of the resonator proves useful in reducing phase noise and also in enhancing output power of microwave oscillators. The proposed resonator is applied to 18-GHz and 38-GHz HBT oscillators, leading to the phase noise of -96-dBc/Hz at 100-kHz offset with 10.3-dBm output power (18-GHz oscillator) and - 104-dBc/Hz at 1-MHz offset with 11.9 dBm (38-GHz oscillator). These performances are comparable to or better than state-of-the-art values for GaAs- or InP-based planar-circuit fundamental-frequency oscillators at the same frequency bands.