The signal or F.I.D.
The signal given by the receiver coil is known under the name of interferogram : Free Induction Decay or F.I.D.. In opposition to the continue wavelength signal, the time dependent signal we monitor in this case is an emission signal, because the radiofrequency field B1 is turned off upon the signal acquisition. The experiment gives us practically a variable field which is linear at high frequency following the y axis. In fact it is an oscillator or an emitter with the Larmor frequency of the nuclei under study.
The time signal S(t) generated in the receiver coil by the xy component of M weakens under the relaxation process.