Signal and Noise Characteristics of the PKW Marginal Oscillator Spectrometer

Results are presented of an experimental investigation into the signal and noise characteristics of a Pound‐Knight‐Watkins marginal oscillator spectrometer used for nuclear magnetic resonance studies. Both the wide band and the narrow band spectral noise distribution of the oscillator have been examined. Noise curves for various oscillator tubes are presented. The signal and noise response as a function of the modulation frequency of a Watkins' calibrator and the rf oscillator level has been studied. The noise distribution studies show that at low and high rf oscillation levels 1/f frequency components are always present. At intermediate levels a region of white noise is obtained between 20 cps and 1 kc. Over the range of rf levels investigated (0.6 V rms and below), the results show that for maximum signal/noise ratio the oscillator should be operated at the highest rf level. Some gain in signal/noise is obtained by using modulation frequencies in the 500 cps region. At different modulation frequencies NMR traces confirm these results.