ELECTRON PARAMAGNETIC RESONANCE DETECTION BY TIME-LOCKED SUBSAMPLING

A detection method for electron paramagnetic resonance spectroscopy is described that permits simultaneous acquisition of multiple in- and out-of-phase harmonics of the response to magnetic-field modulation for both dispersion and absorption: (i) conversion of the microwave carrier to an intermediate frequency (IF) carrier; (ii) subsampling of the IF carrier by an analog-to-digital converter four times in K IF cycles where K is an odd integer; (iii) dividing the digital words into two streams, odd indexes in one and even in the other, followed by sign inversion of every other word in each stream; and (iv) feeding the two streams to a computer for the digital equivalent of phase-sensitive detection (PSD). The system is broadbanded, in the frequency domain, with narrow banding for improved signal-to-noise ratio occurring only at the PSD step. All gains and phases are internally consistent. The method is demonstrated for a nitroxide spin label. A fundamental improvement is achieved by collecting more informa...