Digital detection and processing of multiple quadrature harmonics for EPR spectroscopy.
A quadrature digital receiver and associated signal estimation procedure are reported for L-band electron paramagnetic resonance (EPR) spectroscopy. The approach provides simultaneous acquisition and joint processing of multiple harmonics in both in-phase and out-of-phase channels. The digital receiver, based on a high-speed dual-channel analog-to-digital converter, allows direct digital down-conversion with heterodyne processing using digital capture of the microwave reference signal. Thus, the receiver avoids noise and nonlinearity associated with analog mixers. Also, the architecture allows for low-Q anti-alias filtering and does not require the sampling frequency to be time-locked to the microwave reference. A noise model applicable for arbitrary contributions of oscillator phase noise is presented, and a corresponding maximum-likelihood estimator of unknown parameters is also reported. The signal processing is applicable for Lorentzian lineshape under nonsaturating conditions. The estimation is carried out using a convergent iterative algorithm capable of jointly processing the in-phase and out-of-phase data in the presence of phase noise and unknown microwave phase. Cramér-Rao bound analysis and simulation results demonstrate a significant reduction in linewidth estimation error using quadrature detection, for both low and high values of phase noise. EPR spectroscopic data are also reported for illustration.
Center of Biomedical EPR Spectroscopy and Imaging, Davis Heart and Lung Research Institute, Department of Internal Medicine, The Ohio State University, Columbus, OH 43210, USA. Rizwan.Ahmad@osumc.edu
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R
Ahmad
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S
Som
E
E
Kesselring
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P
Kuppusamy
J L
JL
Zweier
L C
LC
Potter
D000465:Algorithms; D003198:Computer Simulation; D004574:Electromagnetic Fields; D004578:Electron Spin Resonance Spectroscopy / Q000295:instrumentation / Q000379:methods*; D016018:Least-Squares Analysis; D008872:Microwaves; D016011:Normal Distribution; D012815:Signal Processing, Computer-Assisted