Synchronous detection

FIGURE 13.46 Nondispersive Infrared analyzer based on (d) interference filters and (b) gas correlation cechnictues. M = mirror, D = detector, S source, F = filter disk. WO = motor, FB = baud pass filter. SD = synchronous detection. C = correlation cell. N nitrogen filter. 

Both methods obtain the necessary sensitivity by modulating the electrode potential between two values which define two distinct states of the electrode surface thus the chemistry to be observed is directly modulated and may be detected with great sensitivity by an appropriate form of synchronous detection. In the case of EMIRS, the modulation frequency is made sufficiently high compared to the wavelength scanning rate to enable a phase sensitive detection system to be used whereas, for SNIFTIRS, the electrode potential is held for a sufficient period at each potential to accumulate data from several interferometric scans and, after an adequate number, the two sets of data are ratioed. 

Since the CL reaction is being produced by an electrochemical stimulus, greater control is gained over the initiation, rate, and course of the CL reaction. Indeed this control can be to the extent that the CL reaction may be switched on and off, allowing for synchronous detection, effective background correction, and ready automation with computer control. By careful selection of the electrode material, surface treatment, and applied potential, an additional degree of electrochemical selectivity can also be introduced. 

The entire analysis of synchronous detection, or lock-in amplification as it is sometimes called, can be conveniently analyzed by straightforward application of the Fourier transform techniques, transform directory, and convolution theorem developed in Section IV of Chapter 1. 

Some of the most useful polarization-dependent NSOM methods, however, involve modulation of the polarization from the probe (see Figure 3.20),224 coupled with synchronous detection of the near-field signals. Such methods allow for multiple imaging modalities so that topography, absorption dichroism, and/or birefringence information can all be readily obtained. 

The most common way to deal with the problem of stochastic drift is to modulate the exposure of the analyte to the sensor and to synchronously detect the sensor response. When the analyte is off (i.e., the sensor is zeroed ), the sensor signal can be recorded as the baseline value. Drift-corrected signals can be obtained by subtracting the baseline signal from that recorded when the analyte is on. If the frequency of the on/off modulation is much higher than the frequency of the baseline drift, then this scheme results in dramatically improved stability in the measured data. An implicit requirement in this measurement strategy is that the response kinetics of the sensitive film/analyte combination be sufficiently fast to allow on/off modulation at the desired frequency. 

At this point we have expressed the Hamiltonian, the density operator and the evolution operator in Fourier space. We have introduced an effective Hamiltonian, defined in the Hilbert space of the same dimension 2 as the total time-dependent Hamiltonian itself, and we have shown how to transform operators between the two representations. The definition of the effective Hamiltonian enables us to predict the overall evolution of the spin system, despite the fact that we can not find time-points for synchronous detection, f, where Uint f) = exp —iWe//t In actual experiments the time dependent signals are monitored and after Fourier transformation they result in frequency sideband... 

Additional gain in the signal-to-noise ratio can also be obtained by synchronous detection [63] or by acquiring multiple echoes during the free precession period of the observable single-quantum coherence (MQ-QCPMG-MAS experiment proposed by Vosegaard et al. [64]). 

Because of its important place in modern chemical instrumentation, an entire chapter is devoted to Fourier transformation and its applications, including convolution and deconvolution. The chapter on mathematical analysis illustrates several aspects of signal handling traditionally included in courses in instrumental analysis, such as signal averaging and synchronous detection, that deal with the relation between signal and noise. Its main focus,... 

Ml, II and the VCO form a phase-locked loop feedback system. The multiplier makes a synchronous detection of (V2 - Vi) at the frequency/h. The output of II drives the VCO so that the output frequency/h constantly adjusts to the frequency where the admittance of the motional arm Ys of the sensor is real, i.e., to the series resonance frequency/s. Therefore, the oscillator output frequency/out =/h is continuously tracking/. 

The first laser beam can be amplitude modulated ( lkHz) with an optical chopper (Fig. 2), which modulates the concentration of the excited metal atoms, M. Because the excited metal atoms have a much higher reactivity, the concentration of product molecules is also modulated. The modulated fluorescence excited by the second laser is then detected by a PMT and a lock-in amplifier. A monochromator or an optical filter is used to analyze the emission and to control the optical bandwidth detected by the PMT. This photochemical modulation and synchronous detection of the fluorescent signal is a very powerful technique for increasing the signal-to-noise (S/N) ratio. 

The tediousness and slowness of the technique has been overcome by Goodman [85] with the computer-automated instrument, shown in Fig.9. Its basic components are a light source, a monochromator, a chopper to interrupt the light at chopping frequencies of typically 100-600Hz, the sample, a capacitive pick-up probe and a lock-in amplifier for synchronous detection of the SPV signal, which is typically only a few mV in magnitude. The... 

Thermal Lensing, Detection, Fig. 3 Signal recovery by intensity modulation of the excitation beam (upper) and synchronous detection of the same frequency component by a lock-in amplifier (bottom)... 

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