A new detection method for EPR spectroscopy is described that permitssimultaneous acquisition of multiple in- and out-of-phase harmonics of theresponse to magnetic field modulation for both dispersion and absorption (i) conversion of the microwave carrier to an intermediate frequency (IF) carrier; (ii) sub-sampling of the IF carrier by an A/D converter four times in K IF cycles where K is an odd integer greater than one; (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; (iv) feeding the two streams to a computer for the digital equivalent of phase-sensitive detection (PSD). The system is broadbanded, in thefrequency 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 was demonstrated for a nitroxide spin label. A fundamental improvement is achieved by collecting more information than is possible using a single analog PSD.
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