Digital quadrature detection

Quadrature detection is universally employed in all modem spectrometers. However, there exist a number of experimental schemes for implementing this, and which of these you are likely to use will be dictated by the spectrometer hardware and perhaps by the age of the instrument (on some modern instruments, the operator can choose between these methods). The differing approaches may be divided into the classical analogue scheme described here for which there are two widely used methods, and the more recent digital quadrature detection (DQD) scheme that is considered later. 

Figure 3.26. Schematic illustration of digital quadrature detection. A single channel is digitised as a suitable intermediate frequency input (ij) and the second channel is generated numerically with an identical amplitude and a precise 90° phase shift.

The two copies of the COSY spectrum and the fi = 0 responses can all be separated without phase cycling if one is prepared to sacrifice digital resolution by increasing the fi-spectral width. The following spectmm was collected without phase cycling, with quadrature detection OFF,... 

JJ Drader, SD-H Shi, GT Blakney, CL Hendrickson, DA Laude, AG Marshall. Digital quadrature heterodyne detection for high-resolution Fourier transform ion cyclotron resonance mass spectrometry. Anal Chem 71 4758-4763,1999. 

An external trigger capability for the digitizer is useful for at least two reasons. One is to be able to sample the magnetization at the correct points in a multiple pulse experiment. (An example would be to sample just the tops of the echoes in a CPMG experiment as described in I.C.2.) The other reason is to allow for synchronization of two digitizers, for example, in quadrature detection. 

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 (if digital signal filters are not employed to eliminate this). 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]. 

The first BioCD took its inspiration from the compact disc. The compact disc was invented in 1970 by Claus Campaan of Phillips Laboratory. The concept is purely digital and uses null interferometers that are far from quadrature, as appropriate for the readout of two binary intensity states. The interferometers were common-path and stable, as required for the mechanical environment of portable compact disc readers. The original BioCD used the same physics as the compact disc, but modified the on-disc microstructures to change from the digital readout to an analog readout that operated in quadrature for sensitive detection of surface-bound proteins7,8. Because the quadrature condition is established by diffraction off of microstructures on the disc, this is called the microdiffraction-class (MD-Class) of BioCD. 

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