Hysteresis measurements of 20 MHz third overtone SC-cut MCXO resonators

The microcomputer compensated crystal oscillator (MCXO) utilizes dual c-mode excitation of an SC-cut resonator for self-temperature sensing and compensation. The dual mode operation requires well-behaved fundamental and 3^rd overtone c-modes over the operating temperature range (e.g. -55°C to +85°C). The stability of MCXO depends primarily on the frequency vs. temperature hysteresis of the resonator. The hysteresis of the 10 MHz 3^rd overtone MCXO resonator has been a few parts in 10⁸. This resonator is packaged in an HC-40 metal can, which makes the resonator the largest single component in the MCXO. The size of this resonator has limited efforts to reduce the size of the MCXO. In this paper, results of hysteresis measurements of a 20 MHz 3^rd overtone SC-cut resonator packaged in a TO-5 metal can (~13% the volume and ~57% the height of the HC-40 package) is reported. Hysteresis over the -55°C to +85°C temperature range was found to be a few parts in 10⁸ , comparable to that of the 10 MHz 3^rd overtone resonator