Amplifier tuned

From eqn. (85), it follows that the demodulation signal should preferably be measured by means of a phase-selective voltmeter or lock-in amplifier tuned to the frequency 2cuL A complete set-up for demodulation measurements at varied d.c. potential ( demodulation polaro-graphy ) has been described by Struijs et al. [27]. Also, the theoretical shapes of plots of JLF and QLF against E have been extensively discussed [27]. It is characteristic for all kinds of second-order techniques that the response signal as a function of d.c. potential exhibits the double-peak shape, one peak being a maximum, the other a minimum. Figure 23... 

Figure 1. Conventional, transmission-based CD spectrophotometer. S, light source M, monochromator (wavelength selection) L, polarizer (linear) P, electro-optic circular polarizer D, light detection (photomultiplier) LI, lock-in amplifier (tuned amplifier) PC, data procurement and display.

For the measurement of differential absorption spectra in the visible and ultraviolet spectral region, a light modulation technique was introduced over two decades ago [7]. The principle of this measurement also governs the measurement of infrared VCD. This technique involves the high frequency modulation of the exciting beam between left and right circular polarization states via a photoelastic modulator, and measuring the differential absorption with a lock-in amplifier tuned to the modulation frequency. 

When Walsh started to think about using AAS for analytical purposes back in 1952, one of his key conclusions was that, in order to carry out absorption measurements on luminous atomic vapors, it would be necessary to employ a modulated light source and a synchronously tuned detection system, so that any radiation emitted by the sample would produce no signal at the output of the detection system [2]. This modulation principle, using either an AC-operated radiation source or a chopper in the radiation beam, and a selective amplifier tuned to the same modulation frequency, has ever since been applied in all commercially available atomic absorption spectrometers. It has been considered one of the major advantages of... 

In an infrared spectrometer equipped with a linear polarizer and a PEM the CD of the sample is then extracted from the detector signal by means of a lock-in amplifier tuned to the operating frequency of the photoelastic modulator. 

Weak spectral absorption is, however, characterised by small changes in a large background power and is therefore ill suited to superheterodyne detection. That background is also detected and overloads the sensitive intermediate frequency amplifier tuned to the beat frequency that is derived from the mixing process. Instead it is customary to modulate the absorption in some way and to observe the modulation on the detected signal rather than the absorption itself... 

Since the foundations of atomic absorption spectroscopy were laid by Walsh a number of improvements in instrumentation and techniques have been made. Russell, Shelton, and Walsh modulated the hollow cathode signal and used an amplifier tuned to the modulating frequency so measurements could be made without interference from flame emission. Sullivan and Walsh developed very high-intensity hollow cathode lamps that led to lower detection limits. Willis proposed the use of nitrous oxide-acetylene flame as a means of overcoming certain interferences and produce a higher population of free atoms in the flame. 

A second type of spectral interference is due to thermal emission from the flame of the sample. For interference to occur in this case the thermally excited spectral lines also must lie within the spectral band pass of the monochromator slit width, and this also results in an erroneously enhanced signal. In this case, however, the interference can be eliminated by use of a modulated source and an amplifier tuned to the frequency of modulation since the undesired signal will be a steady (dc) signal. 

One of the greatest advantages of AAS, namely, its specifity, is based on the use of element-specific radiation sources that emit the spectrum of the analyte element in the form of very narrow spectral lines. While the quality of an instrument in other spectrometric techniques frequently depends on the resolution of the monochromator or on its spectral bandpass (the range of radiation that passes through the exit slit), these factors are not of primary importance in AAS. If the element-specific radiation is modulated and the amplifier tuned to the same frequency, AAS is selective and free of spectral interferences caused by overlapping of atomic lines of different elements (see Sec. 1.6). 

For a sufficiently small modulation amplitude (a/o)o 1), the first terms in each bracket are dominant. Therefore we obtain for the signal S(nf2) behind a lock-in amplifier tuned to the frequency (Fig. 6.3) ... 

The signal that is induced in a pickup coil may be recovered by using a lock-in amplifier tuned to frequency... 

Fig. 8.36a-c. Optical-optical double resonance. The common level is the lower pump level in (a) and the upper pump level in (b). (c) illustrates the different phases of the double-resonance signals in case (a) or (b) monitored with a lock-in amplifier tuned to the chopping frequency f of the pump laser... 

Spectrometers have been built around this phenomenon to simplify complicated spectra. For example, instead of Stark modulation, the pump source may be modulated, and this modulatates the signal absorption by means of the double-resonance effect. A narrowband amplifier tuned to the modulation frequency is employed along with the usual phase-sensitive detector. A simplified version of such a spectrometer is shown in Fig. 12. Clearly, as the signal frequency is swept, only transitions with a common energy level with the fixed pump will be modulated and observed, thus greatly simplifying the spectrum. For... 

The intensity of the fluorescent light was monitored by a photomultiplier and the time-dependent signal was isolated by an amplifier tuned to 462 kHz. The radiofrequency modulation of the fluorescent light was readily confirmed by this method, but the small percentage modulation of the incident light made phase-sensitive detection essential for detailed studies. This technique had the advantage that the amplitudes of the in-phase and quadrature components... 

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