Pursley Randall H, Salem Ghadi, Pohida Thomas J, Devasahayam Nallathamby, Subramanian Sankaran, Krishna Murali C
Signal Processing and Instrumentation Section, Division of Computational Bioscience, Center for Information Technology, National Institutes of Health, 12 South Dr, Bldg 12A-2025, Bethesda, Maryland 20892-1002, USA.
Rev Sci Instrum. 2005 May;76(3):1-6. doi: 10.1063/1.1903163.
The application of direct time-locked subsampling (TLSS) to Fourier transform electron paramagnetic resonance (FT-EPR) spectroscopy at radio frequencies (rf) is described. With conventional FT-EPR spectroscopy, the high Larmor frequencies (L(f)) often necessitate the use of intermediate frequency (IF) stages to down convert the received free induction decay (FID) signal to a frequency that can be acquired with common data acquisition technology. However, our research focuses on in vivo studies, and consequently utilizes a FT-EPR system with a L(f) of 300 MHz. This relatively low frequency L(f), in conjunction with the advent of bandpass sampling analog-to-digital conversion and signal processing technologies, has enabled us to omit the IF stage in our FT-EPR system. With this in mind, TLSS techniques have been developed to directly sample the 300 MHz FID signal at a sampling rate of 80 MHz providing a signal bandwidth of 20 MHz. The required modifications to the data acquisition and processing system specific to this application are described. Custom software developed to control the EPR system setup, acquire the signals, and post process the data, is outlined. Data was acquired applying both coherent averaging and stochastic excitation sequences. The results of these experiments demonstrate digital down conversion of the 300 MHz FID signal to quadrature baseband. Direct FID TLSS eliminates many noise sources common in EPR systems employing traditional analog receiver techniques, such as the IF mixer stage in single channel systems, and the quadrature baseband mixer stage in dual channel systems.
描述了直接时间锁定子采样（TLSS）在射频（rf）傅里叶变换电子顺磁共振（FT-EPR）光谱中的应用。在传统的FT-EPR光谱中，高拉莫尔频率（L(f)）通常需要使用中频（IF）级将接收到的自由感应衰减（FID）信号下变频到可以用普通数据采集技术采集的频率。然而，我们的研究集中在体内研究，因此使用了一个L(f)为300 MHz的FT-EPR系统。这个相对较低的频率L(f)，结合带通采样模数转换和信号处理技术的出现，使我们能够在FT-EPR系统中省略IF级。考虑到这一点，已经开发了TLSS技术，以80 MHz的采样率直接对300 MHz的FID信号进行采样，提供20 MHz的信号带宽。描述了针对此应用对数据采集和处理系统所需的修改。概述了为控制EPR系统设置、采集信号和对数据进行后处理而开发的定制软件。应用相干平均和随机激发序列采集数据。这些实验的结果表明，300 MHz的FID信号被数字下变频到正交基带。直接FID TLSS消除了采用传统模拟接收器技术的EPR系统中常见的许多噪声源，例如单通道系统中的IF混频器级和双通道系统中的正交基带混频器级。
