Aging of dual mode resonator for microcomputer compensated crystal oscillator

A microcomputer compensated crystal oscillator (MCXO) utilizes the dual c-mode excitation (fundamental mode and 3rd overtone (OT)) of an SC-cut resonator for self-temperature sensing and compensation. The long-term stability of the MCXO depends primarily on the aging of the dual mode resonator. When two modes age differently in time, the aging MCXO's output frequency curve would shift with a tilt over its operating temperature range. In this paper, we report the aging of the dual modes of the 20 MHz 3^rd OT SC-cut MCXO resonators. The resonators were measured over the -55degC ~ +85degC temperature range with the self-temperature sensing technique utilized for the MCXO. The measured aging of the two modes ranges from 0.083 ppb to 0.386 ppb per day. The fundamental mode shows more aging than the OT. Observing the different aging of the two modes is not surprising since the mode shape effect and stress relief would be different between the two modes. The different aging rates exacerbate the long-term aging in the MCXO by increasing the offset more than the worse aging rate of the two