Low Noise Coupling and Signal Detection for EPR Spectroscopy

电子顺磁共振（EPR）方法和仪器的最新发展有望革命性地增强EPR在材料科学、化学、物理、生物和医学等领域的应用。一个关键缺失的发展是改进了含样品EPR谐振器与微波源和探测器系统的耦合。在初步工作中，建立了EPR谐振器与传输线三种耦合类型的基本工程设计方程：互感耦合、串联电容耦合和并联电容耦合。这些方程为开发新的、更优的谐振腔耦合技术提供了准确的基础。针对多用途环隙型EPR谐振器（LGR），搭建了测试夹具，对LGR原型耦合网络进行了测试。实验验证了方程的预测，并证明变容二极管是LGR耦合网络中有用的调谐元件。一个原型，新颖的，通用的谐振器已经构建。它产生了有用的金属酶的低温光谱。对原型谐振器和耦合装置进行的设计方程和测试结果直接指向拟议拨款期的具体目标。现在提出在s波段（2-4 GHz）建立一个谐振器，实现预测的最佳低噪声耦合方案，并结合便于连续波和脉冲EPR的样品变化和灵活性的设计特征。该设计将符合方便的变温（包括低温）操作。将使用砷化镓变容二极管和场效应晶体管放大器，因为它们在低温下具有良好的性能。几个关键的微波元件可以位于靠近样品的小电路板上，其本身也可以与样品和谐振器一起冷却，目标是在电子元件中实现最低的热噪声。仔细注意材料的性能将导致一个坚固和可用的组装。第二阶段的工作将是开发一种全新的方法，将谐振器、耦合组件、调谐装置和微波桥元件集成在一个小型混合微波天线上或附近。这种混合微波桥电路具有较低的损耗，从而提高了室温下EPR的信噪比。通过将电路冷却到低温，可能会大大增加信噪比。扩展到x波段（9ghz）区域，这需要把所有东西都做得更小、更精确，将在拟议的拨款期间开始。将在x波段演示变容管调谐的使用，并建立一个测试夹具，以获得谐振器的灵敏度和耦合系统性能对各种结构特征的实际操作经验。

Recent developments in the methodology and instrumentation of electron paramagnetic resonance (EPR) hold promise of revolutionary enhancement of applications of EPR to problems in materials science, chemistry, physics, biology, and medicine. One key missing development is an improvement in the coupling of the sample- containing EPR resonator to the microwave source and detector systems. In preliminary work basic engineering design equations were developed for three types of coupling of an EPR resonator to a transmission line: mutual inductive coupling, series capacitive coupling, and parallel capacitive coupling. These equations form an accurate basis for developing new, more optimum resonator coupling techniques. Focusing on the versatile loop-gap type~of EPR resonator (LGR), a test fixture was built in which were tested prototype LGR coupling networks. The tests verified predictions of the equations and demonstrated that a varactor (a voltage-variable capacitor) diode is a useful tuning element in LGR coupling networks. A prototype, novel, general-purpose resonator has been constructed. It yields useful low-temperature spectra of metalloenzymes. The design equations and the results of tests conducted on prototype resonators and coupling devices point directly to the specific aims of the proposed grant period. It is now proposed to build a resonator at S-band (2-4 GHz) implementing the predicted best low-noise coupling scheme, and incorporating design features that facilitate ease of sample change and flexibility for both continuous wave and pulsed EPR. The design will be consistent with convenient variable temperature (including cryogenic) operation. GaAs varactor diodes and field-effect transistor amplifiers will be used, since they have good performance at cryogenic temperatures. Several of the critical microwave components can be located in close proximity to the sample on a small circuit board, which itself can be cooled alo ng with the sample and resonator, with the goal of the lowest thermal noise in the electronic components. Careful attention to materials properties will result in a robust and usable assembly. The second phase of the work will be the development of a totally new approach to integrating the resonator, coupling assembly, means for tuning, and the microwave bridge element all on or close to a small hybrid microwave subs~te. Such a hybrid microwave bridge circuit will have lower losses resulting in improved EPR signal-to-noise ratio at room temperature. By cooling the circuitry to cryogenic temperatures there is potential for a large increase in signal-to-noise. Extension to the X-band (9 GHz) region, which requires making everything smaller and building it more precisely, will begin during the proposed grant period. The use of varactor tuning will be demonstrated at X-band, and a test fixture will be built to obtain practical operating experience with the sensitivity of the resonator and coupling system performance to various construction features.