Monday, 25 February 2008
Published in Stan's Library
Click here to read the full article
Permalink via DOI: 10.3247/SL2Nmr08.002
Friday, 22 February 2008
High-frequency (HF) oversampling, however, has also some problems of its own. The digital decimation from the HF range to the audio range and the contextual digital filtering (a combination of CIC and FIR filters) need to be properly implemented in the hardware in order to be completely transparent to the User.
If a plain FT is applied to this FID, we will get a spectrum with a lot of wiggles in the baseline analogous to the convolution with a sinc function centred in the middle of the spectral window. This can be explained by recalling the time shift theorem of the Fourier Transform which says that if the time domain signal is shifted by n points, the frequency domain spectrum corresponds to the standard spectrum (when the FID has not been shifted) multiplied by exp(-i2*pi*w*n). In other words, we have introduced a very large first order phase correction in the spectrum. For example, if the FID is right shifted by 60 points (death time = 60 points), f-spectrum will exhibit a first order phase distortion of 60 * 360 = 21600 degrees.
However, the fact remains that Bruker FID’s are not time corrected. Evidently, Varian also uses oversampling and digital filtering and their FIDs are time corrected, that is, they start at time = 0. If the digital filter is known in advance, which is always the case, the group delay should be compensated in the spectrometer, therefore, in my opinion, this death time or group delay is a bug in the spectrometer. For this reason, and going back to the title of this article, any input given as to why Bruker FID’s are not time corrected, would be greatly appreciated.