Signal random modulation

The power supply to the hollow cathode source is modulated and an ac detection system is used. This arrangement prevents any radiation from the flame or resonance detector from producing an output signal. Random noise is less troublesome than in a conventional spectrophotometer. The resonance detector must, of course, produce a cloud of atomic vapor of the same element being aspirated into the flame. The hollow cathode source also must emit resonance lines of the same element. Analytical calibration curves closely parallel those obtained with conventional atomic absorption systems and sensitivities and detection limits are similar. 

In applying RAIRS to CO adsorption, the contribution from CO molecules in the gas phase to the absorption spectrum at CO pressures above 10-3 mbar completely obscures the weak absorption signal of surface adsorbed CO. Beitel et al. found it possible to subtract out the gas phase absorption by coding the surface absorption signal by means of the polarization modulation (PM) technique applied to a conventional RAIRS spectrometer, p-polarised light produces a net surface electric field which can interact with adsorbed molecules, whereas both polarization states are equally sensitive to gas phase absorption because gas phase molecules are randomly oriented. By electronic filtering a differential spectrum is computed which does not show contributions from the gas phase and which has much higher surface sensitivity than a conventional RAIRS setup. 

Burke and his students (11) have published a proposal for solving the non-linearity problem associated with CC and the consequent correlation noise. They used a constant frequency multiple injection signal while this occurred, this frequency was modulated. Before each injection, a random number was generated to determine the magnitude and sign of the deviation from the carrier frequency for the next injection time. Thus, the next... 

An alternate approach, developed in a speech coding context [McAulay and Quatieri, 1986a, McAulay and Quatieri, 1992], uses a harmonically-dependent set of sine waves with a random phase modulation. Yet another related technique [Marques and Almeida, 1988], represents the signal by a sum of adjacent narrowband sines of uniformly-spaced center frequency, with random amplitude and frequency modulation. 

Figure 5 Field modulation detected stochastic (FMDST) ENDOR sequence. The upper level indicates monitoring of the EPR signal prior (baseline) and (usually) after (signal) application of the rf pulse. This pulse is applied for time and at frequency v, which is a randomly selected frequency among the total munber of possible values determined by the rf range and number of spectral points. There is also a delay time, fj, that can range from zero (sample concurrent with apphcation of the rf pulse) to > fu (sample immediately after application of the rf pulse, or later) this...

The signal in Eq. (57) contains a phase shift term modulated by molecular flow or drift, along with the term that attenuates the signal because of random particle migration. To evaluate Eq. (57), we express the particle position in the spin phase in terms of the instantaneous velocity v, (r) and integrate by parts to obtain... 

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