Double electron-nuclear-double resonance (Double ENDOR) requires the application of two rf (RF1 and RF2) fields to the sample. The change in signal intensity of RF1 is observed while RF2 is swept through the spectrum. The two fields are perpendicularly oriented and are controlled by two tunable resonance circuits which can be adjusted independent of each other. In spin decoupling experiments, the amplitude of the decoupling field should be as large as possible. However, in multiple quantum transition studies, both rf fields should be maximized.