Chzhan M, Kuppusamy P, Zweier J L
Department of Medicine, Johns Hopkins Medical Institutions, Baltimore, Maryland 21224, USA.
J Magn Reson B. 1995 Jul;108(1):67-72. doi: 10.1006/jmrb.1995.1103.
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.
虽然对生物样品的电子顺磁共振（EPR）光谱学和成像的兴趣推动了许多在高频射频（RF）和L波段频率下工作的谐振器和微波桥设计的发展，但仍存在一些重要的技术问题。其中两个问题是谐振器共振频率中样品引起的漂移的出现以及低频EPR测量灵敏度的固有局限性。为了解决这些问题，开发了一种用于获得固定频率EPR测量的方法，该方法利用低相位噪声固定频率振荡器作为微波源以及电子可调谐陶瓷凹腔L波段谐振器，该谐振器通过自动频率控制（AFC）锁定到振荡器。本文所述的可调谐微波谐振器采用压控压电致动器，通过改变谐振器的等效电容来调节共振频率。文中给出了设计考量和数据，以及示意图和测试结果。所报道的方法在广泛的样品和环境条件下以及长时间内都能提供真正的固定频率操作。将这种可调谐谐振器设计与超低噪声固定频率振荡器结合使用，应能提高对大型有损耗生物样品进行EPR测量时的灵敏度。
