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 o ut-of-phase harmonics of the response to magnetic-field modulation for both dispersion and absorption: (i) conversion of the microwave carri er 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 in version of every other word in each stream; and (iv) feeding the two s treams to a computer for the digital equivalent of phase-sensitive det ection (PSD). The system is broadbanded, in the frequency domain, with narrow banding for improved signal-to-noise ratio occurring only at t he PSD step. All gains and phases are internally consistent. The metho d is demonstrated for a nitroxide spin label. A fundamental improvemen t is achieved by collecting more information than is possible using a single analog PSD. (C) 1998 American Institute of Physics. [S0034-6748 (98)03707-1]