Frequency stability and phase noise characteristics of digital OCXOs using dual-mode excitation

A high-stability oven-controlled crystal oscillator (OCXO) (frequency stability; +/- 10 ppb from -30 to 70/spl deg/C) has been developed using a dual-mode SC-cut quartz crystal oscillator. The oscillator's frequency stability includes a frequency-temperature hysteresis over the range of operational temperatures, and repeatability of frequencies after power is switched off then back on. In this OCXO a conventional oven-control system is used for coarse compensation and a digital correction system is used for fine compensation. The combination of these forms of compensation greatly improves the C-mode frequency stability and expands the operational temperature range with a very small additional requirement for electric power. Experimental results indicate that the frequency-temperature stability of the OCXO was 20 times better, and its operational temperature range 30 degrees wider, than the conventional OCXOs. We also measured the phase noise characteristics of this OCXO. The experimental results indicate that very low phase-noise (-157 dBc/Hz @ 10 kHz) can be obtained at the floor region. This noise level corresponds well with a calculation based on our dual-mode oscillator noise estimation.