A nuclear magnetic resonance (NMR) probe circuit is used with a sample coil tuned to a primary frequency f.sub.1. The circuit is arranged to have a plurality of points of electric field minima at the f.sub.1 frequency. One or more additional frequencies may be coupled to the circuit at these points, without interaction with f.sub.1. 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 f.sub.1. The impedance may be made adjustable to allow tuning of the relative frequency resonances.