A novel field-frequency lock for a superconducting spectrometer

Abstract

A conventional field-frequency lock requires excellent phase stability (typically ±1°) and negligible change of leakage (<2% of the signal) if the lock is to be maintained to an accuracy of one part in 10 9, a typical homogeneity for a high-resolution magnet. Such specifications are very difficult to achieve under variable temperature conditions. Another drawback of a conventional lock is that it tends to interfere with the main signal, giving sidebands in the spectrum, if the two Larmor frequencies are at all close to one another. Provided only long-term stability is required, as is normally the case with a superconducting system, a lock having none of the above disadvantages may be constructed on the principle of a phase-locked loop, using a deuterium free induction decay to monitor the magnetic field. Such a lock is capable of “pulling in” in milliseconds from a considerable error in deuterium frequency (e.g., 30 Hz) while maintaining the desired long-term stability. The device has been successfully used with concentrations of heavy water as small as 0.1% of the sample volume.