Sensitivity of Marginal Oscillator Spectrometers

Abstract

A linearized theory for the signal and noise characteristics of a marginal oscillator NMR spectrometer has been developed. The model modifies Watkins' theory to include the mixing of 1/f noise into the oscillator passband. Signal response, noise spectrum, signal‐to‐noise ratio, and noise figure are all calculated and compared with accurately calibrated experimental data. The calculated signal response (absolute) is within 12% of experiment. The results show that there are two distinct regions of spectrometer operation, one in which the noise spectrum is white and one in which the noise spectrum is dominated by 1/f mixer noise. In the region of predominately white noise, the calculated noise factor is 2.9 for our circuit, compared with the experimental value of 3.3. Guidelines for optimum selection of rf level and modulation frequency are presented.