Instrumentation multiplexing

An improved signal/noise ratio because all signals are seen simultaneptisiy along with the instrument s own noise (called the multiplex or Fellgett advantage). 

These two transducer pairs are activated alternating. For this purpose an ultrasonic instrument is combined with a two channel multiplexer. Figure 8 presents a modified standard instrument USN52 which also implies a modified software. This system performs four measurements per second - alternating the velocity and the thickness are determined. The probe can be scanned over the surface and in every position both, the velocity and the wall thickness are indicated Using the serial interface of the instrument finally a two-dimensional map of velocity or thickness can be generated. 

Fig. 8 Ultrasonic instrument USN52 with a two channel multiplexer...

Measurement at 500 m/min was considered so promising that it was decided to manufacture and test a prototype for the four channel scanning system. In such a system multiplexing of signals from the four transducers to one ultrasonic instrument was a possibility. Alternatively four independent instruments (one for each transducer) could be used in the scanning system. 

A variation on depth profiling that can be performed by modern scanning Auger instruments (see Sect. 2.2.6) is to program the incident electron beam to jump from one pre-selected position on a surface to each of many others in turn, with multiplexing at each position. This is called multiple point analysis. Sets of elemental maps acquired after each sputtering step or each period of continuous sputtering can be related to each other in a computer frame-store system to derive a three-dimensional analysis of a selected micro volume. 

The chemical world is often divided into measurers and makers of molecules. This division has deep historic roots, but it artificially impedes taking advantage of both aspects of the chemical sciences. Of key importance to all forms of chemistry are instruments and techniques that allow examination, in space and in time, of the composition and characterization of a chemical system under study. To achieve this end in a practical manner, these instruments will need to multiplex several analytical methods. They will need to meet one or more of the requirements for characterization of the products of combinatorial chemical synthesis, correlation of molecular structure with dynamic processes, high-resolution definition of three-dimensional structures and the dynamics of then-formation, and remote detection and telemetry. 

In this section we will review the application of near-IR system instrumentation to the most commonly encountered fluorescence measurements such as steady-state spectra, excited state lifetimes, anisotropy, microscopy, multiplexing, high-performance liquid chromatography (HPLC), and sensors. 

Farrens and Song<40) have replaced the original spark source with a picosecond diode laser in a multiplexed dual wavelength T-formatfluorometer.(41)With an overall instrumental response width of ca. 300 psec full-width half-maximum (FWHM), near-IR fluorescence lifetimes as low as 75 psec in the case of l,l -diethyl-4,4 carbo-cyanine iodide (DCI) (excitation 660 nm) and decay components as low as 48 psec in the case of 124 kDa oat phytochrome (excitation 752 nm) were reported. 

D. J. S. Birch, A. S. Holmes, J. R. Gilchrist, R. E. Imhof, S. M. A1 Alawi and B. Nadolski, A multiplexed single-photon instrument for routine measurement of time-resolved fluorescence anisotropy, J. Phys. E Sci. Instrum. 20, 471-473 (1987). 

The use of hber optics and hber-optic multiplexing can increase the number of analysis points, and hence can reduce the overall costs related to a single analyzer. This approach has been used successfully with NIR instrumentation, where typically up to eight points can be handled. As noted earlier, the use of hber optics with IR Fourier transform instruments has in the past been limited. New hber materials with improved optical throughput are available, and also with the considered use of IR lasers, the role of hbers for IR applications is expected to increase. Although in the past commercial multiplexers have been available for mid-lR hber systems, their use has not been widespread. 

Acquisition times commonly vary from seconds to minutes, often with negligible time between acquisitions, even when measuring multiple locations simultaneously (multiplexing). The dedication of different areas on the charge coupled device (CCD) detector to each measurement point makes this possible. The detectors used for MIR and NIR instruments cannot be multiplexed in the same fashion and must measure multiple samples sequentially. 

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