Effects of Modulation during the Evolution Period

an amplitude-modulated signal can always be considered as the sum of two signals modulated in phase by opposite frequencies. Equation 10.5 permits one type of modulation to be converted to the other provided both phase components or both amplitude components are known. 

We have already observed phase modulation in the acquisition of data in a ID spectrum, where finite rf power results in off-resonance magnetizations giving rise to a phase error proportional to the off-resonance frequency (see Eq. 2.55), and similar effects occur in the t2 dimension of 2D spectra. Here our interest is in modulation that arises in the f, dimension. A simple example (but with no utility as a 2D pulse sequence) is 90°, t2, which resembles a ID experiment in which 

Because both sin fi]tj and cos modulations appear, Fourier transformation with respect to t] gives 

Equation 10.5 indicates how amplitude modulation may be converted to phase modulation, which provides considerable flexibility in data processing. In order to obtain both sine and cosine components of amplitude modulation, we usually 

FIGURE 10.11 Example of a phase-twisted 2D NMR line. Courtesy of Ad Bax (National Institutes of Health). 

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