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/sup rd/ overtone c-modes over the operating temperature range (e.g. -55/spl deg/C to +85/spl deg/C). The stability of MCXO depends primarily on the frequency vs. temperature hysteresis of the resonator. The hysteresis of the 10 MHz 3/sup rd/ overtone MCXO resonator has been a few parts in 10/sup 8/. 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/sup rd/ overtone SC-cut resonator packaged in a TO-5 metal can (/spl sim/13% the volume and /spl sim/57% the height of the HC-40 package) is reported. Hysteresis over the -55/spl deg/C to +85/spl deg/C temperature range was found to be a few parts in 10/sup 8/, comparable to that of the 10 MHz 3/sup rd/ overtone resonator.