Electron spin echo system having magnetic field shift during generation of the echo signal

A method and means of generating an electron spin echo signal having a relatively higher output frequency than the input frequency of the applied 90 DEG and 180 DEG RF pulses applied to an electron spin echo sample located in a static magnetic field by increasing the magnitude of the Zeeman magnetic field by a predetermined amount after the application of the 90 DEG and 180 DEG pulses. Also the spin echo sample is situated in a microwave cavity which has two resonant frequency modes, the first of which has a relatively high Q while the second has a relatively low Q. The 90 DEG and 180 DEG RF pulses are applied to the system at the first resonant frequency having the high Q while the spin echo signal is coupled out of the system at the second resonant frequency having the low Q.