Temperature compensated crystal resonator found in a dual-mode oscillator

Abstract

A device utilizing a quartz crystal resonator with an orientation substantially equal to 21.93.degree./34.10.degree.. The crystal resonator is capable of vibrating simultaneously in two thickness modes, namely the B-mode and the C-mode. Because of the nature of the difference between the B- and C-modes, the B-mode may be used as an indication of the resonator temperature in order to compensate the C-mode frequency signal. A digital technique for temperature compensation by using the crystal itself as a sensor and a feedback loop varies the heater on the surface of the crystal. The temperature sensor compensation system contains a quartz resonator with a heater affixed thereon. The resonator is arranged as part of the oscillator to generate both B-mode and C-mode frequency signals. The C-mode signal is used as a time standard or frequency reference. Initially, the frequency of the B-mode is counted. The count is started at the same time the frequency count of the C-mode is initiated. The C-mode is then counted to a predetermined frequency value. When the predetermined count is reached for the C-mode, a signal is issued to stop the count of the B-mode frequency. The eight least significant bits of the resultant B-mode count are passed to a D-to-A converter and then used to control a switching regulator which, in turn, alters the temperature of the heater disposed on the crystal resonator. This change in temperature alters the B-mode frequency and, thus, provides a feedback loop to stabilize the C-mode frequency signal.