Apparatus for automatically adjusting the frequency of piezoelectric resonators in the form of bars or plates

Apparatus for automatically adjusting the frequency of a piezoelectric resonator in the form of a bar or plate includes circuitry for applying a voltage to the resonator to cause it to resonate, measuring the frequency of the resonator continuously during a frequency adjusting cycle, continuously comparing the measured frequency with a desired frequency to obtain a difference frequency value, storing the difference frequency value at the beginning of the adjusting cycle, deriving from the difference frequency value a threshold value equal to one-half of the difference frequency value, comparing the threshold value with the frequency difference value, removing material by a sand blast from a first extremity of the resonator as long as the difference frequency value is greater than the threshold value and thereafter removing material from the other extremity of the resonator until the difference frequency value is nil.