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.
Ahmad
R
R
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
Som
S
S
Kesselring
E
E
Kuppusamy
P
P
Zweier
J L
JL
Potter
L C
LC
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