On digital resonance estimation of a magnetically perturbed microwave cavity

This paper shows a new digital frequency control system for a microwave spectrometer to investigate ferromagnetic resonance spectra at 11.5 GHz. In order to fulfil the measurement conditions, the measuring frequency and resonance frequency of the cavity must be equal. However, the determination of the cavity´s resonance frequency containing a ferromagnetic sample with field variant characteristics is not possible directly within acceptable measuring times. For the digital control system the model of the cavity is a static and nonlinear channel with unknown and timevariant transmission characteristics. A narrow frequency modulation (Δf<50 kHz) causes an output sequence used for an adaptive recursive algorithm. The resonance frequency of the cavity is regulated by a parameter estimator which uses a least-squares fit performed on a digital signal processor. The transient behavior and the characteristics of the digital control system are excellent. Approximately, the deviation from resonance is less than 2 kHz. As an example, the real part of the susceptibility of a small, diluted, and single-domain hexaferrite sample could be detected for the first time with signal amplitude equal to 220 kHz peak to peak