NMR generalities
NMR experiments are carried out in two steps
![1 - signal acquisition. 2 - signal processing](/resources/images/fiches/bases/rmn/en/rmn2.png)
Signal acquisition
The acquisition of data in NMR experiments from a sample placed in an intense B0 field requires two successive steps:
- Excitation => application of a radio-frequency pulse (ν1) for a given time in the simplest case (“One-Pulse” experiment) => PW (usually lasting a few microseconds). Many multi-pulse experiments exist to specifically edit certain NMR data.
- Detection => of the free induction decay (FID) which decreases over time until the system recovers its equilibrium state. This signal is recorded over a time period => AQ (usually lasting a few seconds).
![Diagram of a FID signal. It is presented as a sinusoid whose amplitude decreases as the detection period AQ. The maximum amplitude at the beginning of the signal corresponds to the PW excitation.](/resources/images/fiches/bases/rmn/en/rmn4.png)
Signal processing: the Fourier transform
The first step is to carry out a Fourier Transform (FT) on the FID, a mathematical operation that transforms a signal that changes with time (FID) into a frequency-dependent signal (spectrum). An NMR spectrum is the Fourier transform of the FID.
![On the left an interferogram (FID) whose signal evolves in time. On the right, after Fourier transformation, an NMR spectrum, whose abscissa corresponds this time to the frequencies.](/resources/images/fiches/bases/rmn/en/rmn5.png)
It is possible to improve certain characteristics of a spectrum by multiplying the FID by an apodization function before carrying out the FT. The most commonly used apodization functions can improve the signal-to-noise ratio or the digital resolution of the spectrum. After acquiring an NMR signal it is therefore possible to apply different treatments on the FID.