Quadrature detection aliasing

When the operator suspects the presence of an aliased signal, the testing procedure for such aliasing also depends on the type of quadrature detection being used. For either type of quadrature phase detection system, the sw parameter can be significantly increased. The position of the putative aliased resonance then changes in relation to the rest of the signals. 

In Section 3.2.3 it was shown that a resonance falling outside the spectral window (because it violates the Nyquist condition) will still be detected but will appear at an incorrect frequency and is said to be aliased or folded back into the spectrum. This can be confusing if one is unable to tell whether the resonance exhibits the correct chemical shift or not. The precise location of the aliased signal in the spectrum depends on the quadrature detection scheme in use and on how far outside the window it truly resonates. With the simultaneous (complex FT) scheme, signals appear to be wrapped around the spectral window and appear at the opposite end of the spectrum (Fig. 3.24b), whereas with the sequential (real FT) scheme signals are folded back at the same end of the spectrum (Fig. 3.24c). If you are interested to know why this difference occurs see reference [7]. 

Because the reference frequency for the detector cannot be set within the spectrum without aliasing real lines in a single phase detection experiment, it is difficult to perform selective saturation experiments wherein only a small part of the spectrum is irradiated. We will see that it will be trivial to irradiate anywhere in the spectrum with quadrature detection. For such selective irradiation experiments, usually to saturate an unwanted water line, the reader is referred to the papers by Redfield (1976) and Hoult (1976). 

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