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.
Highlights
The observed difference between the two stability curves (HBAR-based source and laboratory-type synthesizer) is within the measurement errors and can be attributed to different loop settings. These results show that the performance of the high-overtone bulk acoustic wave resonator (HBAR)-based oscillator is well suited for realizing miniature atomic clocks with excellent frequency stabilities
We demonstrated that HBAR oscillators exhibit ultra-low phase noise performances
It is interesting to note that MACs using HBAR-based oscillators could allow the development of time-frequency references combining in a single device excellent phase noise and long-term frequency stability properties, opening potentially the MAC technology to a wider spectrum of applications
Summary
Over the last decade, outstanding progress in microelectromechanical systems (MEMS) technologies and semiconductor lasers, combined to coherent population trapping (CPT) physics,[1,2] has allowed the development of highperformance miniature atomic clocks (MACs) that combine a volume of about 15 cm[3], a total power consumption lower than 150 mW and a fractional frequency stability lower than 10−11 at 1 h and 1 day integration time.[3,4,5] Such frequency references, commercially available,[6] can provide the base for a number of mobile and embedded applications including network synchronization, new-generation mobile telecommunication systems, satellite-based navigation systems on-earth receivers, secure banking data transfer or military and avionic systems. In 2009, Yu et al demonstrated a HBAR-based 3.6 GHz Pierce oscillator with a phase noise of −77 dB rad2/Hz at 1 kHz Fourier frequency, a power consumption of 3.2 mW, a frequency stability of 1.5 × 10−9 at 1 s17 in free-running regime and claimed to be improved at the level of 1 × 10−10 when locked to a cm-scale Rb vapor cell. Note that in this reference, the output signal of the HBAR oscillator was mixed with a RF signal from an external synthesizer, highlighting the difficulty to tune the output frequency to the exact atomic transition frequency.
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