Development of an electronically tunable L-band resonator for EPR spectroscopy and imaging of biological samples.

While interest in the EPR spectroscopy and imaging of biological samples triggered the development of numerous resonator and microwave-bridge designs operating at high RF and L-band frequencies, a number of important technical problems remain. Two of these problems are the occurrence of sample-induced drifts in resonance frequency of the resonator and the intrinsic limitations in sensitivity of low-frequency EPR measurements. In an effort to address these problems, an approach was developed for obtaining fixed-frequency EPR measurements, utilizing a low-phase-noise fixed-frequency oscillator as a microwave source and an electronically tunable ceramic re-entrant L-band resonator, which was locked to the oscillator via automatic frequency control (AFC). The tunable microwave resonator described in this paper employs a voltage-controlled piezoelectric actuator for adjustment of the resonance frequency via variations of the equivalent capacitance of the resonator. Design considerations and data, as well as schematic drawings and test results, are presented. The reported approach provides true fixed-frequency operation under a wide range of sample and environment conditions and over prolonged periods of time. The use of this tunable resonator design in conjunction with an ultra-low-noise fixed-frequency oscillator should enable enhanced sensitivity in EPR measurements of large lossy biological samples.