Problems with Signal Detection

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Another type of probe is based on the principle of the sudden cooling of the heated element. When foam comes in contact with a heated electrical element, the hot surface detects sudden cooling, which is translated to an output signal. The major problem with the use of a heated element is fouling of the media the sensitivity decreases while it is used, so such detectors may not be reliable in practice. 

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 ... 

Using on-line mass spectroscopy [65] carbon dioxide and formic acid were demonstrated as soluble products of methanol oxidation. The former gives the most intense MS signal according to the fact that it is the main product. There are two main problems to detect formic acid as such. In the presence of carbon dioxide most of the m/e signals of HCOOH overlap with signals of the major product. Besides this, in the presence of methanol, formic acid reacts to form the methyl ester ... 

Many endpoint detection systems, based on mechanisms, such as those based on reflected optical light [9], spindle motor current [10], pad temperature [11,12], have been used to resolve this problem, with limited success. Some systems may work with blank wafers or wafers with relatively low pattern density (at the STI level, for example), but for the PMD or ILD levels no useful results have been reported. The presence of a pattern at the PMD or ILD levels adds a great deal of complexity to the signals. Currently, use of an endpoint detection system to control the final post-CMP thickness is still a fertile topic for research and development. 

For relaxation studies of biomolecules in solution (which is no specialty of the authors of this chapter), it is often essential to use inverse-detection schemes to obtain reasonable sensitivity. Furthermore, besides problems with poor sensitivity, the carbon-13 and nitrogen-15 spectra are often too crowded to allow measurement of individual relaxation rates for different I nuclei, either by direct detection or by indirectly detecting the protons. If this is the situation, one can spread out the I nuclei signals for better resolution of individual resonances by detecting a two-dimensional H-I correlation spectrum. Relaxation experiments of this type can be considered a modification of the double polarization-transfer IS correlation experiment [7, 17] ... 

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