This figure illustrates the mode spectrum of a quartz resonator, showing the fundamental mode, third overtone, fifth overtone, and some of the spurious responses (i.e., unwanted modes). Some of the unwanted modes have steep frequency vs. temperature characteristics. In oscillator applications, the oscillator usually selects the strongest mode. Occasionally, as the temperature changes, at a certain temperature, the frequency of an unwanted mode coincides with the oscillator frequency, which causes an “activity dip”. At the “activity dip”, excitation of the unwanted mode results in extra energy dissipation in the resonator, a decrease in the Q, an increase in the equivalent series resistance, and a change in the frequency of the oscillator. When the resistance increase is sufficiently large, the oscillation stops and temperature changes away from the “activity dip” temperature restarts the oscillation. Unwanted modes can be controlled by proper design and fabrication methods. Maintaining the correct relationships among electrode and resonator plate dimensions and maintaining the parallelism between the major faces of the resonator plate can minimize the unwanted modes.