With this reduced frequency the dwell time needs to be in the order of milliseconds. So we need the rotating frame to move from the ideal world of theory and to become a practical reality. A detector receives two signals, one coming from the sample under study and another which is a duplicate of the exciting frequency. The detector actually detects the difference between the two frequencies. To fully exploit the power of the pulse, the transmitter is put at the centre of the spectrum.
Signals falling at the left of it appear as negative frequencies.
We have said that the spectrum begins at zero. The FT and its inverse show a number of interesting properties. The first one predicts that, if you complex-conjugate the FID, the transformed spectrum will be the mirror image of the original spectrum. In fact, if you invert the y component of a vector, you obtain its image across a mirror put along the x axis. Some spectrometers already perform this operation when acquiring.