Comparison between CW and PFT

As any time domain function F (r), a square wave rf pulse of width tp can be approximated by a Fourier series of sines and cosines with frequencies w/2 tp (n — 1, 2, 3, 4, 5.) [14, 7]. An rf pulse of width t thus simulates a multifrequency transmitter of frequency range A = 1/4 (p (eq. (2.14)). Accordingly, an rf pulse of 250 ps simultaneously rotates the M0 vectors of all Larmor frequencies within a range of at least A = 1 kHz. It simulates at least 1000 simultaneously stimulating transmitters, the resolution in the Fourier transform depending on the number of FID data points (eq. (2.16)), not the stimulation time t-. 

During the 250 ps needed to stimulate the Larmor frequencies within a range of 1 kHz by a single rf pulse, only 0,5 10 6 of a 1 kHz scan is stimulated in aCW experiment using a 500 s/1 kHz sweep. A more realistic comparison accounts for the time required for Fourier transformation NMR information at a spectral width of 1 kHz can be obtained 

Furthermore, in the 500 s required for sweeping a 1 kHz spectrum, at least 1000 2K interferograms can be accumulated before being Fourier transformed. The signal noise of the PFT NMR spectrum is thus increased by a factor of 10 /lO, according to eq. (2.1). 

In summary, PFT NMR is much more sensitive for equivalent measuring times and much less time consuming for equivalent signal noise ratios in comparison to CW NMR. 

The measuring and calculation time (Fourier transformation and phase correction for the 8 K interferogram/ 4 K spectrum) was less than 100 s in (b) and (c). 

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