Measuring method of active impedance of crystal oscillator circuits

Abstract

AbstractAlthough crystal oscillators are used widely in such electronic devices as communication equipment and measuring apparatus, it is not possible to determine if the circuit oscillates or how the adjustment must be made until the crystal is connected to the circuit. To remedy this situation and to improve the design method for the crystal oscillators, a method has been developed for measuring the two‐terminal impedance of the active circuit seen from the crystal terminal or the equivalent capacitance when negative resistance of a specific value is obtained. In principle, a known resistance is connected in series with the crystal terminal so that negative resistance is cancelled. Then the resultant pure capacitance is measured with a bridge circuit. A measurement system consisting of a bridge circuit which is controlled by a computer has been constructed. Representative oscillator circuits have been measured. It is found that a usable frequency range can be found from the measured frequency characteristics of the excitation current and the equivalent capacitance, and that the static capacitance of the active circuit seen from the crystal terminal can be adjusted to a desired value via an adjusting capacitance.