Low Level Tracking Marginal Oscillator for in situ Magnetic Field Measurements at Low Temperatures

A low level NMR spectrometer which operates satisfactorily over the frequency range between 1 and 100 MHz has been developed using standard integrated circuits and field effect transistors. The high gain automatic level control allows rf levels of less than 1 mV to be achieved without adjustment; the system has been used to make in situ magnetic field measurements in superconducting magnet systems with the narrow 1G NMR spectra observed in doped alkali halide specimens at 4.2 K. In conjunction with the marginal oscillator, an automatic tracking system which utilizes capacitor switching is described.