Processing digitally filtered data (RD)

An extra processing step is required before FT for digitally filtered data (Bruker data from the Avance series of spectrometers such as DRX and ARX, Tecmag NT-NMR data and some JEOL data).  This is necessary because the data have been digitally filtered and "decimated" prior to saving the FID. The initial points of the FID are zero, as shown below. A circular left shift must be performed before the FT. The number of points to shift is calculated from parameters found in the file header (the Decimation Number and the DSP firmware version found in the Bruker file header), but the user can change the number of points.  (In a circular left shift, points are removed from the left end of the data and added to the right end.)

This FID is typical of digitally filtered data. Note the odd appearance of the beginning of the FID.

If an FT is performed on the above FID, a seriously distorted spectrum results. The "wiggles" can be removed with very large linear phase correction (many 100s of degrees) which is not very convenient.

The first 61 points at the beginning of the FID need to be removed, which is done as a circular left shift, so that the points removed from the beginning of the FID are added onto the end.  This is performed with the NUTS command RD.  

When RD is typed, this screen is displayed. NUTS determines the number of points to rotate the data from the Decimation Number and Firmware Version, which are imported with the data.

If NUTS has been able to determine a default value from parameters in the data header, a non-zero value will be displayed in the Number of Points box.  Usually the user should just click on OK.  If NUTS has not been able to determine the correct number, this value will be zero, and the user must input the appropriate number.  This can be determined by examining the beginning of the FID.

The user can experiment with different values for Number of Rotations.  After FT and phasing, look at the value for linear phase correction (choose Processing Parameters from the View menu).  When the correct value is used, the linear correction should be fairly small (less than 360 degrees).

The resulting FID now has its maximum at the beginning of the FID. 

FT of the FID after RD gives a reasonable looking spectrum. In some cases, as here, there is some baseline distortion at the ends of the spectrum, as seen below.

Any apodization should be applied before RD is executed. 

If the acquisition time was short enough that the FID has not decayed to zero, then doing an RD can result in a discontinuity, and the resulting spectrum will be distorted.  Applying a window function before the RD operation, to bring the end of the FID smoothly to zero, will avoid this.

It is often desirable to discard the distorted ends of the spectrum.  This can be done with the "extract" or xt command.  See details.

Last updated 4/30/08.