Improved frequency stability in low power consuming oscillators and clocks: CSAC, MEMS and dual-mode crystal oscillators

Nowadays, there are growing demands to decrease power consumption, to reduce start-up or warm-up time, to reduce dimensions and volume of oscillators and clocks; many applications today are looking for further improvement in long-term and/or short-term frequency stability of existing clocks as well. First part of my talk will review progress during last two decades in different clock categories with respect to: long-term and short-term frequency stability, power consumption, operating temperature range, dimensions, costs, etc. Miniaturization of quantum based atomic clocks is one of the most interesting technological progresses during last two decades; worldwide activities in this field will be emphasized. Major challenges during development of the chip-scale atomic clocks (CSACs) will be discussed. Next, the progress and great improvements in frequency stability of MEMS based oscillators will be presented. Then, the talk will concentrate to dual-mode oscillators, which enable significant improvement in frequency vs. temperature stability of the low power clocks. Mostly used temperature compensated cuts will be compared; and advantages of stress compensated cut will be emphasized. Several implementation examples of the resonator self-temperature-sensing based on simultaneous excitation of two modes in the resonator will be presented; methods of compensation for frequency vs. temperature dependencies utilizing digital circuit implementations will be illustrated. Finally, the new combined clock system with improved frequency vs. temperature stability comparing to CSAC operating in low power mode will be presented. It utilizes self-temperature-sensing of the resonator instead of the internal TCXO of the CSAC. The goal has been further reduction of the frequency vs. temperature dependence; typically, by two orders (to ±0.01ppm), while maintaining very low overall power consumption of the system.