A review of sapphire whispering gallery-mode oscillators including technical progress and future potential of the technology

We present the results of recent progress in the development of low phase noise signal generators based on sapphire "whispering gallery"-mode resonators, and look at the future potential of these oscillators to meet the ever-increasing demands for precision signal generation and processing. Beyond improving phase noise performance, future oscillators will be required to work over more demanding temperature ranges, with lower vibration sensitivity and greater reliability. We discuss the performance of sapphire oscillators, particularly those operating near room temperature, and present recent results of vibration sensitivity less than 10^-10 per g, phase noise of -146 dBc/Hz at 1 kHz offset and -170 dBc/Hz at 10 kHz offset from a 10.24 GHz carrier, and extended temperature operation.