The Transmitter and Receiver System

Artifacts may be roughly categorized into those due to inherent limitations (e.g. pulses cannot excite unlimited bandwidths even if all hardware components work perfectly) and those that result from improper set-up of the experiment or nonideal functioning of the NMR spectrometer system. In this chapter we will mainly focus on the latter two. These artifacts are more likely to appear in multiple-pulse experiments. Quite often, they are avoided by clever programming of the experiments (e.g. interleaved acquisition of data for NOE spectra, use of pulsed-field gradients instead of phase-cycling). 

In order to reveal sources of artifacts and noise we will give a brief description of spectrometer hardware and probehead technology. Of course, spectrometer manufacturers do their best to construct hardware with optimal performance. However, experience shows that all hardware components may occasionally fail. It is one of the goals of this chapter to present ways to recognize malfunction quickly and to locate the source of problem. 

The schematic transmitter and receiver pathways of a modem NMR spectrometer are displayed in Fig. 3.2. 

Defects in the transmitter path may cause total signal loss, increased I, noise or diamond patterns. Problems with the frequency control unit or the synthesizer will lead to no signal or amplitude or phase instabilities. Random variations in amplitude or phase will increase q noise. A more subtle point arises when the phase presetting time is too short such that the 

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