Free-induction decay (FID)

The free-induction decay (FID) is the raw signal detected by the NMR. A very simple FID might look like this:

Note that the signal is recorded as a function of time (x-axis); this is why FID's are also referred to as time-domain signals. The irregularity in the signal (particularly noticeable at the long-time end) is random noise. At longer accumulation times, this will be less apparent.