A Frequency-Lock Method for the Measurement of Q Factors of Reflection and Transmission Resonators

A new method for the measurement of Q factors of reflection (single-ended) and transmission (double-ended) resonators has been developed. The basis of the technique is the locking of an oscillator to some point away from the center of the resonator response curve. This is accomplished by the introduction of a phase offset into a conventional oscillator-resonator stabilization circuit so as to make the stabilizer sensitive to both the real and imaginary portions of the voltage transfer coefficient of the resonator. From a determination of the phase and frequency shifts, the Q factor may be evaluated. The frequency-lock method shows promise of having significant advantages in speed, convenience, and accuracy over previous techniques. The basic features of this approach were successfully demonstrated in a measurement of the Q factor of an X-band reflection cavity. A circuit for fully automatic Q determinations is proposed. In this mode of operation, the device should be insensitive to changes in the eigen-frequency of the resonator and should be capable of measuring very small changes in Q.