The sensitivity of a 9-GHz magnetic field modulation, homodyne, electron paramagnetic resonance (EPR) spectrometer has been calculated as a function of the modulation frequency in the range 10 to 10 4 Hz from the measured frequency dependence of the parameters describing a selected matched pair of 1N23WE silicon point-contact crystal mixers. The calculated sensitivity was verified to within 3 dB by direct measurement of the signal-to-noise ratio of samples of a free radical (DPPH) containing a known number of unpaired spins in an EPR spectrometer employing the same crystal mixers. Data are also presented to show that the use of modulation frequencies in excess of 0.2 MHz does not result in improved sensitivity if modern crystal mixers are employed.
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Buckmaster et al. (1965) studied this question.
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