US Patent 7187176 NMR probe circuit with nodal impedance bridge

A nuclear magnetic resonance (NMR) probe circuit is used with a sample coil tuned to a primary frequency f1. The circuit is arranged to have a plurality of points of electric field minima at the f1 frequency. One or more additional frequencies may be coupled to the circuit at these points, without interaction with f1. The probe circuit also uses an impedance coupled between two of the minima points that affects the frequency response at the additional frequency or frequencies, without affecting the frequency response at f1. The impedance may be made adjustable to allow tuning of the relative frequency resonances.