Quadrature detection images

A serious problem associated with quadrature detection is that we rely on the cancellation of unwanted components from two signals that have been detected through different parts of the hardware. This cancellation works properly only if the signals from the two channels are exactly equal and their phases differ from each other by exactly 90°. Since this is practically impossible with absolute efficiency, some so-called image peaks occasionally appear in the center of the spectrum. How can you differentiate between genuine signals and image peaks that arise as artifacts of quadrature detection ... 

The image peaks resulting from quadrature detection can be easily distinguished from small genuine peaks since they show different phases and move with changes in the reference frequency. 

Quadrature images Any imbalances between the two channels of a quadrature detection system cause ghost peaks, which appear as symmetrically located artifact peaks on opposite sides of the spectrometer frequency. They can be eliminated by an appropriate phase-cycling procedure, e.g., CYCLOPS. 

Figure 5 JO. The coherence level diagram for a single-pulse ID experiment. The solid line represents the coherence transfer pathway of the detected magnetisation. The dotted line represents the mirror image pathway that is rejected by the quadrature detection scheme.

By convention a pure absorption signal using the quadrature detection procedure is composed of a real part Ik,y and a imaginary part Ik,x- Conversion to spherical operators means that i Ik, i = (Ik,y + i Ik,xV 2 represents a pure absorption signal, while Ik,+i represents a pure dispersion signal. The operator Ik,+i corresponds to the "quad image". 

A very large image signal appears in CRAMPS spectra because the x-component of the magnetization is different from the y-component. Therefore, half of the spectral region is abandoned. However, some quadrature-phase detection CRAMPS (QD-CRAMPS) methods which cancel the image signal have been reported. ... 

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