Micro atomic frequency standards employing an integrated FBAR-VCO oscillating on the 87RB clock frequency without a phase locked loop

We applied a 3.5-GHz-band thin film bulk acoustic resonator (FBAR) to a voltage controlled oscillator (VCO), which is designed as a local oscillator of miniaturized atomic frequency standards (AFSs). This frequency reference instead of a bulky quartz resonator may enable an AFS with a chip component size. By using the 3.5-GHz-band FBAR, the clock frequency at the
⁸⁷
Rb transition (fclock/2=3.42 GHz) was directly generated without a phase locked loop (PLL), reducing the cost, size, and power consumption of the RF control unit. Also, MEMS based Rb gas cells were developed, where the cylindrical cavity was 3 mm in both diameter and length. We observed a coherent population trapping (CPT) resonance. By incorporating the FBAR-VCO into the feedback loop of the CPT system, a sustained clock operation was achieved with a frequency instability of 2.1×10
⁻¹¹
at 1 s.