Recording a Free Induction Decay

Prior to the data acquisition (simulation) a number of experimental parameters have to be set. Because these parameters can effect the appearance of the final spectrum and are common to all pulse sequences they will be discussed in more detail under the following headings  

Spectral width, acquisition time and time domain data points 

Normally a FID is acquired using simultaneous quadrature detection, sequential quadrature detection is explained in section 2.3.1. The FID consists of the values of the X- and y-components of the magnetization measured at discrete intervals during the acquisition period T q. The time interval between two successive pairs of data points is called the dwell time For a given (chosen) spectral width SW [Hz], a total number of data points TD and acquisition time T q, the following relation is valid (Nyquist theorem)  

In Check it 3.2.1.1 the correct measurement of two sets of signals is demonstrated. In addition the effect of the number of time domain data points TD on the digital resolution using the same spectral width recorded at higher magnetic field strengths is illustrated. 

Because NMR is a relatively insensitive technique, a NMR experiment is usually repeated several times (multi-scan accumulation) with the data from successive experiments accumulated in the computer memory in order to improve the signal-to-noise ratio in the final spectrum. It is a common misconception that the statistical noise is destructively superimposed to a constant level the signal intensity of the real resonance signals is directly proportional to the number of scans n while the random noise is directly proportional to the square root number of scans. 

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