A Tunable NMR Spectrometer for the vhf-uhf Range

Abstract

A nuclear magnetic resonance detection system for the vhf‐uhf frequency range is described in detail. It consists of a radio frequency bridge built around a coaxial hybrid T. The sample is contained in a tunable L‐Cresonator coupled with an adjustable impedance transformer to a 50 Ω airline. The system is thus fully tunable over wide ranges of frequency from a remote location. For example, using a two‐turn copper ribbon coil, the range 270–710 MHz is covered: The system may be set to any desired point in this range within a few minutes. The system was used with standard lock‐in detection to obtain the absorption derivative ∂χ″/∂H. An absolute noise measurement was made at a radio frequency of 300 MHz and an audio detection frequency of 200 Hz. This showed that the root mean square noise at the lock‐in output was between four and seven times the value corresponding to the rf Johnson noise at the bridge. The system was used to observe a proton resonance in a Bitter type solenoid at 125 kG.