The Modern Pulsed Mode for Signal Acquisition

Further improvement in S/N had to await the development of faster computer microprocessors, which was exactly what happened during the 1980s. Armed with very fast and efficient microcomputers with large memories, chemists discovered they could now generate NMR signals in an entirely new way. 

Have you ever been sitting in a quiet room when suddenly a fixed pitch noise (sound waves) causes a nearby object to vibrate in sympathy This is an example of acoustic resonance, in which the frequency of the sound waves exactly matches the frequency with which the object naturally vibrates. If the pitch of the sound changes, the resonance ceases. 

In a pulsed-mode NMR experiment, which is performed at both constant magnetic field and constant rf frequency, rf radiation is supplied by a brief but powerful computer-con-trolled pulse of rf current through the transmitter coil. This monochromatic (single-frequency) pulse, centered at the operating frequency Vq, is characterized by a power (measured in watts and controlling the magnitude of B[) and a pulse width (tp), the duration of the pulse measured in microseconds. However, as a direct consequence of the uncertainty principle [Eq. (1.6)], this brief pulse acts as if it covers a range 

The exact value of vg is designed to be slightly offset from the range of nuclear precession frequencies to be examined (the spectral width, SW, in hertz). Therefore, the SW can be no greater (and preferably less) than Av, leading to the relationship 

one important lesson is that the receiver circuit must be sampled often enough to make the pattern recognizable. The Nyquist theorem tells us that in order for a computer to determine the frequency (Av,) of each component of the wave function by this method, it must acquire at least two points 

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