Low Phase Noise InGaP/GaAs HBT MMIC VCO With Nonuniform Transmission Line Resonator for a Chip-Scale Atomic Beam Clock

This letter presents a voltage-controlled oscillator (VCO) monolithic microwave integrated circuit (MMIC) designed in an InGaP/GaAs heterojunction bipolar transistor (HBT) process with a synthesized on-chip nonuniform transmission line (NTL) resonator for application in a new type of chip-scale atomic clock (CSAC). Near the half-hyperfine frequency of ¹³³Cs (4.596...GHz), the NTL oscillator provides a peak RF output power up to 14 dBm, modulating a laser diode used to interrogate the atoms inside the clock. The measured dc-to-RF conversion efficiency is 25.5%(39.2 mW), and the phase noise is −81.8 dBc/Hz at a 10 kHz offset. A second VCO is designed using a uniform on-chip transmission line resonator for comparison, showing the NTL VCO provides a 1.8 dB measured improvement in phase noise at the same bias. An additional bias point is reported, only consuming 9.8 mW, while degrading the phase noise to −77.7 dBc/Hz at 10 kHz. The NTL resonator is synthesized to maximize the unloaded quality factor, achieving a final simulated value of 41 and 48 with and without the varactors, respectively.