IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-27, NO. 4, DECEMBER 1978 An Active Frequency Technique for Precise Measurements on Dynamic Microwave Cavity Perturbations CEVDET AKYEL, STUDENT MEMBER, IEEE, RENATO G. BOSISIO, AND GEORGES-EMILE APRIL Abstract-A new method for the dynamic measurement of the passive parameters of a microwave cavity is described. Both the resonant frequency (.fts) and the loaded Q factor (QL,) are measured simultaneously from active frequency signals generated by a closed- loop circuit containing the test cavity. Resonant frequency deviations of up to 20 MHz are measured with an accuracy of better than + 0.25 percent, and loaded Q-factor variations (from 500 to 7000) are measured with an accuracy of better than + 1.2 percent. The unperturbed cavity resonance was at 2452.0249 MHz, and the time of measurement of each cavity parameter was < I s. I. INTRODUCTION HE MEASUREMENT of the resonant frequency (fos) and the Q factor (QL,) of a perturbed microwave cavity has been of increasing interest in the measurement of the dielectric properties of materials [1]-[4]. In addition, such measurements are also useful for nondestructive on-line measurements of physical and chemical parameters that have to be monitored in various industrial processes [5]-[8]. In this latter application, open-cavity rather than closed- cavity resonators must be used. More recently, automatic tracking techniques have also been used for measuring passive-cavity parameters [9], [10] in dynamic processes. Similar tracking systems were used to measure the permitti- vity of materials [11]. The technique now presented meas- ures both the resonant frequency shift and the Q factor by means of a phase-locked microwave loop circuit generating a number of microwave frequency signals. The active signals are measured with an automatic microwave frequency counter. The proposed technique of measurement is entirely digital; therefore, it is very accurate and reliable. In addition, the rate of measurement is limited largely by the count rate of the automatic frequency counter such that the required time of measurement for the cavity Q factor and the resonant frequency can be less than t s. Il. METHOD AND THEORY OF MEASUREMENT The closed-loop circuit for measuring the passive frequency shift of microwave cavity resonances by active frequency techniques was described in previous papers [7], [12]. The block diagram in Fig. 1 illustrates how changes in the loaded Q factor of the microwave cavity may be measured simultaneously with the frequency shift by the use Manuscript received May 15, 1978. The authors are with the Laboratoire d'Hyperfrequences, Departement de Genie Electrique, Ecole Polytechnique de Montreal, C.P. 6079 Succur- sale "A", Montreal, P.Q. H3C 3A7, Canada. External Perturbation Preamplifier & Limiter L-q licroprocessor 14-- - -J L _ _ _ _ _ -J Fig. 1. System block diagram. of the same automatic microwave frequency counter. An electronic phase shifter introduces a periodic phase shift 'de the closed-loop circu't. The corresponding frequency shift of the oscillating system is measured with the frequency counter. A microprocessor then adjusts the mechanical phase shifter in the closed loop such as to minimize the amplitude of the measured frequency shifts. At this setting of the closed-loop system the average active frequency (fo) corresponds to the passive resonant frequency of the cavity, as shown in Fig. 2, and the active frequency shift Aj'o can then be used to calculate the loaded Q factor of the cavity as described below. A. Frequencies Generated by a Step Pliase Modulatioti Inside the Reentratit Loop Under steady-state conditions it can be shown that the system in Fig. I will oscillate when the total phase shift in the reentrant loop is equal to an integral number N of 27r rad, and the total gain is greater than unity. The above phase condition can be written, with no perturbation of the cavity, as follows: o)T + Ojoi, wo, QL) + OMo + OA +O, = 27rN (1) o-)'T + O#)', wo, QL) + OW + OA + O'e = 27rN (2) 364 %r-%"nl8-9456/78/1200-0364$00.75 (D 1978 IEEE