Source modulation

The remainder of this text will now focus on the complementary source modulation (CoSM) method of gas detection. 

Figure 5. Schematic of a Complementary Source Modulation method of correlation spectroscopy.

Chambers P., Austin E.A.D., Dakin J.P., Theoretical analysis of a methane gas detection system using the complementary source modulation of correlation spectroscopy, Measurement Science and Technology 2004 15 (8) 1629-1636. 

Photomultipliers are generally used to convert the spectral radiation to an electrical current and often phase-sensitive lock-in amplifiers are used to amplify the resulting current. AES and AFS require similar read-out systems because both methods are measuring small signals. The difficulty associated with both these methods is the separation of the signal for the atomic transition of interest from the background radiation emitted by excited molecular species produced in the atom reservoir. AFS phase locks the amplifier detection circuit to the modulation frequency of the spectral source. Modulation of the source is also used in AAS. 

Fischer, M.A. and Black, H.S., Modification of membrane composition, eicosanoid metabolism, and immunoresponsiveness by dietary omega-3 and omega-6 fatty acid sources, modulators of ultraviolet-carcinogenesis, Photochem. Photobiol., 54, 381, 1991. 

Since source modulation and synchronous signal demodulation are practically essential in both AAS and AFS, and most other instrumental requirements are common to both techniques, it should come as no surprise to hear that the majority of AFS determinations have been performed using instrumentation designed primarily for AAS.42... 

Kang, B.H, Jaluria, Y and Tewari, S.S., "Mixed Convection Transport from an Isolated Heat Source Module on a Horizontal Plate", J. of Heat Transfer, Vol. 112, Aug. 90, pp. 653-61, 1990. 

Sometimes one has to live with this problem, and some of the finest work, particularly on transient molecular species, has been achieved using source modulation. This includes some of the earliest work on neutral free radicals, by Kewley, Sastry, Winnewisser and Gordy [5], who studied the SO and CS species. With the benefit of hindsight, we know these radicals to be very long-lived. We shall illustrate the details of source modulation by describing two other experiments. The first is the classic work of Woods [6] and Dixon and Woods [7] who obtained the first microwave spectrum of a molecular ion, namely CO+. The second is an example of a high-temperature microwave cell for the study of refractory materials. 

Why is source modulation employed in atomic absorption spectroscopy ... 

The BioPerl, BioJava, BioPython, BioRuby, BioCorba, and BioPipe [20] family of packages are facilitated by the Open Bioinformatics Foundation to provide open source modules and scripts for life science research in the major programming languages of bioinformatics. 

Why is source modulation used in aU)inic absorption spcctr( scop ... 

This problem is overcome most simply by modulation of the radiation source. Modulation means that the source radiation is switched on and off very rapidly. This can be done by using a rotating mechanical chopper placed directly in front of the source. A chopper is shown in Figs. 6.8 and 6.14. The mechanical chopper is a circle of metal sheet with opposite quadrants cut out. Every quarter turn of the chopper alternately blocks and passes the source radiation. Another way to modulate the source is by pulsing the power to the lamp at a given frequency. When modulated, the signal from the source is... 

Figure 3.11b Comparison of the spectral linewidth and maximum deviation of the FM source frequency. The frequency deviation at the MMW frequency would be typically 250 kHz and linewidth HWHM > 1-2 MHz. The carrier sweeps between the deviation limits at the source modulation rate, 1 kHz typically...

Since the number of detectors for the Winkler, GRIS mask and NAE concepts was small, the number of independent sky pixels would also have been small. The purpose of these imaging systems was therefore not so much to make sky maps, as to position individual sources. The mask/antimask systems of the GRIS mask and NAE would also have provided rapid and clean source modulation for background subtraction. The use and design of small-array masks is discussed by Sembay and Gehrels (1990). 

Fig. 1. The expected SNR for an off-axis source modulated by a slat collimator, as a function of spatial resolution (continuous line no spatial selection, dashed line spatial selection). The collimator transmission in this example is 0.44.

The analysis has the following objectives eheeking that each source module implements its module design speeification, eorreetness of real arithmetic will be informally considered,... 

Schematic of an MWCNT-based field-emitting photocathode. The incident 532 nm optical source modulates the emission current via the embedded pin diode, while the CNTs impart high-field enhancement [241].

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