Phase Spectrum

(adapted from Alan Shusterman, Reed College)

The time-to-frequency transformation produces complex (real + imaginary) numbers. Phasing is a process that combines the real and imaginary numbers in their proper ratio.

More simply, an improperly phased spectrum contains a distorted baseline around each peak, high on one side and low on the other (it may even go negative). A properly phased spectrum contains a balanced baseline. The following images show you the difference between bad phasing and good phasing.

Phasing is a three-step procedure:

1. Phase the entire spectrum using SpinWorks' automatic procedure
2. Phase the entire spectrum (but look at the tallest peak) using a frequency-independent (ph0) correction
3. Phase the entire spectrum (but look at a pattern relatively far from the tallest peak) using a frequency-dependent (ph1) correction

It can also be done incrementally, i.e., you can make corrections over and over again. In fact, if you get frustrated, you can start over with SpinWorks' automatic procedure.

First, do the automatic procedure:

• Click Processing: Automatic Phase Correction

Notice that the peaks look pretty good at this point. In fact, if your vertical scale is squeezed small enough, the peaks will look great, but don't be fooled - more phasing is usually required. Now let's perform the manual procedure:

• Click on the interactive phasing button

At this point, an interactive phasing window appears with four slider controls, two for ph0 and two for ph1. A green line is also drawn on the spectrum to mark the tallest peak (this is where you look while you perform the ph0 adjustments).

If your vertical and horizontal scales are squeezed too small, it will be hard to see small changes in the baseline. You should adjust these scales so that you can see the baseline around the tallest peak (and you can see small variations in the baseline).

• ph0 adjustment. Find the tallest peak (marked by green line). Use the coarse and fine controls to adjust the baseline around this peak
• ph1 adjustment. Find a peak relatively far from the tallest peak (see diagram above). Use the coarse and fine controls to adjust the baseline around this peak
• Troubleshooting. You may need to go back and forth between ph0 and ph1 adjustments. You may also need to readjust the vertical and horizontal scales so that you can see what you are doing. You may also need to compromise by adopting a phase setting that seems to work well for several peaks, but not perfectly for any of them.
• Click the Apply and Exit button

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