The signals emitted by the excited atoms in the sample are received by the spectrometer and Fourier transformed by the software of the data station computer. The process of receiving the NMR signals is called an acquisition. Data is said to be acquired. A distinction should be made between the two terms; “FID” (time domain) and its associated “spectrum” (frequency domain).

When an acquisition is carried out, “raw” data is acquired and the received signal is called an FID (Free Induction Decay). A typical FID is illustrated in the figure below.

Before an FID can be usefully analyzed it must first be transformed to the frequency domain. This is achieved by applying a Fourier transformation. A Fourier transformation is a mathematical operation which converts the FID into a frequency spectrum. An  FID is a signal whose intensity varies with time whereas a spectrum displays how intensity varies with frequency. Fourier transformation is the most important one of several processing operations that is normally carried out on raw data.