Frequency demodulation technique

Equations (33) and (34) demonstrate that the motion quantities 5 (displacement) and v (velocity) are encoded in phase and frequency modulation of the detector output signal, purely referenced to the laser wavelength A. Tobeableto recover the time histories s(t) and v t) from the modulated detector signal, adequate phase and frequency demodulation techniques, or both, are utilized in the signal decoder blocks of a laser vibrometer. 

Thus, the contributions of several orders can be accessed experimentally by means of a detection device tuned to the proper frequency. Applications of this principle have thus far been limited to the so-called second-order techniques second harmonic a.c. polarography [25], faradaic rectification [26], and the recently developed demodulation technique [27],... 

Further development in this field will be governed by questions related to the yet unsolved fundamental and technical issues as well as the quest for more user-fnendly implementations of the techniques. Although the STM and NC-AFM technologies already allow operation at most advanced levels, still most technical implementations operate at technical rather than physical hmits. This specifically applies to the NC-AFM where improvements in the detection optics and electronics, frequency demodulation, control software, and electronics still have to be made [275]. An equally important technical issue is the further development and widespread implementation of advanced atom tracking and feed-forward... 

Theory. If two or more fluorophores with different emission lifetimes contribute to the same broad, unresolved emission spectrum, their separate emission spectra often can be resolved by the technique of phase-resolved fluorometry. In this method the excitation light is modulated sinusoidally, usually in the radio-frequency range, and the emission is analyzed with a phase sensitive detector. The emission appears as a sinusoidally modulated signal, shifted in phase from the excitation modulation and partially demodulated by an amount dependent on the lifetime of the fluorophore excited state (5, Chapter 4). The detector phase can be adjusted to be exactly out-of-phase with the emission from any one fluorophore, so that the contribution to the total spectrum from that fluorophore is suppressed. For a sample with two fluorophores, suppressing the emission from one fluorophore leaves a spectrum caused only by the other, which then can be directly recorded. With more than two flurophores the problem is more complicated but a number of techniques for deconvoluting the complex emission curve have been developed making use of several modulation frequencies and measurement phase angles (79). 

At present, two main streams of techniques exist for the measurement of fluorescence lifetimes, time domain based methods, and frequency domain methods. In the frequency domain, the fluorescence lifetime is derived from the phase shift and demodulation of the fluorescent light with respect to the phase and the modulation depth of a modulated excitation source. Measurements in the time domain are generally performed by recording the fluorescence intensity decay after exciting the specimen with a short excitation pulse. 

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... 

Another technique that is useful at frequencies of less than —10 kHz is phase-selective demodulation. A distinct advantage of this technique is that it enables the separation of the resistive (in-phase or real ) and capacitive (90°-out-of-phase or quadrature ) components of the cell impedance. This is accomplished through the process of cross-correlation [13] (selecting that component of es that correlates with the phase of i). The excitation voltage waveform is multiplied by a square wave that is in phase with the cell current waveform. 

Tvvo vidcl used approaches are used for lifetime measurcnienis. ilie lime-domain approach and the frt i/iu niy-domain approach. In tinte-domain measurements. a pulsed source is employed and the time-depcndcnr decay of fluorescence is measured. In the frequency-domain method, a sinusoidallv modulated source is used to excite the sample. The phase shift and demodulation of the fluorescence emission relative lo the excitation waveform provide the lifetime information. ( onimercial instrumentation is available to implement both techniques. ... 

The chopper modulation approach is an amplitude modulation technique, which uses square wave as a carrier signal. It works by shifting the sensor signals to higher frequencies to suppress the l//noise. The demodulation of modulated sensor signal is required to recover the original signal [51,58], Offset and noise compensation should be performed for better performance [51,58],... 

An alternate approach to balanced double beampolarizing interferometer is to modulate the polarization at a very high frequency and demodulate the detector signal so that the difference interferogram is observed only. This technique has been exploited very successfully by Nafie using circularly polarized light and modulating the direction of rotation with electro-optical crystals. 

In the technique of Golden et the infrared radiation first is chopped at a frequency and focused on the electrode surface (Figure 17). The reflected beam passes then through a photoelastic modulator and through a polarizer P. The radiation is thus alternately modulated between s and p states. A double synchronous detection, which is necessary to demodulate the reflected... 

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