Cryogenic Sapphire Oscillator
A highly stabilised frequency sources can be made by linking the frequency of electromagnetic radiation of a microwave field to the physical dimension of a cryogenically cooled mechanical resonator. In this project a cylindrical Sapphire oscillator is used (pictured) in which the microwave field bounces around the cylinders circumference in a “whispering gallery mode” configuration.
The oscillator is cooled to near 4 Kelvin to reduce the frequency standards dependence on temperature through utilising a minimum in the Sapphire’s thermal expansion coefficient. Our goal is to achieve a relative frequency instability of 10-17 during at 1s observation time, a factor of 10 better than any previous demonstration. This expected frequency stability corresponds to 0.1uHz at the optical frequency of 11GHz.
The culmination of 20 years of leading-edge fundamental research, combined with cutting-edge engineering, has led to a disruptive technology that is now revolutionizing a vital Australian defence asset. The sapphire clock offers a 1000-fold improvement in timing precision, which delivers an improved ability for Australian Defence to identify threats to Australia using the Jindalee Over-The-Horizon Radar Network.
The Sapphire Clock Team at one of the JORN sites
Radar works by broadcasting highly complex radio or microwave signals from a transmitter. If these emitted signals intercept an object then part of the signal may be reflected back to a receiver antenna. These reflected signals contain information on the location, speed and size of the object. However, the performance of the radar is critically dependent on the purity and quality of the signals that are broadcast: if one can broadcast noiseless signals then will be possible to detect smaller objects that are further away and which are moving slower. This additional surveillance power is critically important in a defence context by providing additional insight.
To answer this call for better radar signals the Sapphire Clock team commenced working closely with the High-frequency Radar team at DST Group who are responsible for the research behind the JORN project. The Sapphire Clock is the culmination of 20 years of leading-edge fundamental research which has shown world-beating performance in the lab. The clock is so good its performance is the equivalent of only losing or gaining one second every 40 million years. When applied to the JORN radar application it delivers a signal that is more than 1000 times purer than its existing approach. It is important to note that this improvement can still be delivered despite the existing JORN solution making use of the best commercial devices that money can buy.
In addition to the remarkable Sapphire Clock, the team has developed two additional technologies that directly result in purer signals for JORN, and which thus assist Australian defence to be better able to observe threats to Australia. The team has developed ultra-low noise synthesis technology that can take the clock signals and generate the frequencies that are needed by the radar: it can do this while preserving the signal purity of the clock. The team has also developed signal dissemination technology, which enables them to deliver the pure signals to the numerous locations necessary to broadcast the JORN signals effectively. The range of technologies developed by the team provides the revolutionary leap in the performance in this outstanding Australian invention.
In the News
W. A. Al-Ashwal, A. Hilton, A. N. Luiten, J. G. Hartnett, Low Phase Noise Frequency Synthesis for Ultra-Stable X-Band Oscillators, IEEE Microwave and Wireless Components Letters 27, 392-394 (2017).
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