A high-overtone bulk acoustic wave resonator-oscillator-based 4.596 GHz frequency source: Application to a coherent population trapping Cs vapor cell atomic clock

Abstract

This article reports on the design and characterization of a high-overtone bulk acoustic wave resonator (HBAR)-oscillator-based 4.596 GHz frequency source. A 2.298 GHz signal, generated by an oscillator constructed around a thermally controlled two-port aluminum nitride-sapphire HBAR resonator with a Q-factor of 24,000 at 68 °C, is frequency multiplied by 2-4.596 GHz, half of the Cs atom clock frequency. The temperature coefficient of frequency of the HBAR is measured to be -23 ppm/ °C at 2.298 GHz. The measured phase noise of the 4.596 GHz source is -105 dB rad(2)/Hz at 1 kHz offset and -150 dB rad(2)/Hz at 100 kHz offset. The 4.596 GHz output signal is used as a local oscillator in a laboratory-prototype Cs microcell-based coherent population trapping atomic clock. The signal is stabilized onto the atomic transition frequency by tuning finely a voltage-controlled phase shifter implemented in the 2.298 GHz HBAR-oscillator loop, preventing the need for a high-power-consuming direct digital synthesis. The short-term fractional frequency stability of the free-running oscillator is 1.8 × 10(-9) at one second integration time. In locked regime, the latter is improved in a preliminary proof-of-concept experiment at the level of 6.6 × 10(-11) τ(-1/2) up to a few seconds and found to be limited by the signal-to-noise ratio of the detected CPT resonance.

Frequency flicker of 2.3 GHz AlN-sapphire high-overtone bulk acoustic resonators

“…In this region, phase noise performances of the oscillator are about 10 dB better than those reported in Ref. 6. This is mainly due to the use of sustaining amplifiers with lower 1/f noise.…”

“…We claim and checked that phase noise performances in the 20 kHz-1 MHz range reported in Ref. 6 were limited by the phase noise measurement system (Agilent E5052B). For f < 3 Hz, the phase noise spectrum is well-fitted by a f À6 slope, signature of a frequency drift.…”

“…1-3) and making them well-suited for the development of ultra-low phase noise oscillators. [4][5][6] A HBAR, which can be seen as an acoustic analogy to the optical Fabry-Perot interferometer, is obtained by stacking a thin piezoelectric transducer above a thick low-loss acoustic substrate. The piezoelectric film generates acoustic waves that propagate in the whole material stack.…”