Abstract: A planar type thickness shear mode quartz resonator which can be operated by an oscillation circuit free of any coils or condensers in the tuning circuit and in which the ratio between the diameter and the thickness of the resonator is so designed as to yield stable oscillation in the third overtone. The difference between the negative resistance of the oscillation circuit and the crystal impedance of the quartz resonator is greater in the third overtone than the difference in other oscillation modes.

Abstract: An overtone crystal oscillator includes a crystal vibrator, a CMOS circuit connected in parallel to the crystal vibrator, and two load capacitors connected to the input and output of the CMOS circuit, respectively. The capacitors have respective capacitances C.sub.g and C.sub.d determined to satisfy at least one of following two conditions wherein the first condition is that an oscillation starting resistance R.sub.KN of a desired N-th overtone is sufficiently larger than an effective resistance R.sub.QN of the crystal vibrator of the N-th overtone, the oscillation starting resistance being expressed as:R.sub.KN =K/(.omega..sup.2 .multidot.C.sub.g .multidot.C.sub.d)(.omega.: oscillating frequency, K: constant determined by mutual conductance of the CMOS circuit) and the second condition is that an effective resistance R.sub.Qn of the crystal vibrator of another n-th overtone except the desired N-th overtone satisfies the following relationR.sub.Qn >N.sup.2 .multidot.R.sub.KN /n.sup.2.