Frequency-domain method advantage

The second chapter by Peter Verveer and Quentin Hanley describes frequency domain FLIM and global analysis. While the frequency domain technique for fluorescence lifetime measurement is sometimes counterintuitive, the majority of the 10 most cited papers using FLIM have taken advantage of the frequency domain method as stated by these authors. The global analysis of lifetime data in the frequency domain, resolving both E and /d has contributed significantly to this advantage. 

While publications on fluorescence lifetime imaging microscopy (FLIM) have been relatively evenly divided between time and frequency domain methods, a majority of the 10 most highly cited papers using FLIM have taken advantage of the frequency domain method [1, 2-9]. Both techniques have confronted similar challenges as they have developed and, as such, common themes may be found in both approaches to FLIM. One of the most important criteria is to retrieve the maximum information out of a FLIM... 

Figure 9.7. Noise content of a fiberoptic oxygen sensor signal (a) in the time and (b) in the frequency domains. Time domain signals require broad frequency bandwidths. Frequency domain signals require very limited-frequency bandwidths. Noise is reduced by band limiting the signal, an advantage of frequency domain methods.

The design of feedback controllers in the frequency domain is the subject of this chapter. The Chinese language that we learned in Chap. 12 is now put to use to tune controllers. Frequency-domain methods are widely used because they have the significant advantage of being easier to use for high-order systems than the time- and Laplace-domain methods. 

The methods based on RET are conunonly used for determining intramolecular donor-acceptor distances in macromolecules. The use of lanthanide chelates as donors and prompt fluorophores as acceptors is especially advantageous because of the large difference in their lifetimes. The present example shows the use of the frequency-domain method as an alternative in determining the relevant lifetimes in connection with a recently published work on RET [16],... 

Prom our point of view, each approach has its advantages. The first two model-based approaches have a more intuitive time domain performance specification than traditional frequency domain design methods. However, the frequency domain methods require less structural information about the process dynamics. Chapters 6 and 7 present a new frequency domain PID design approach that we feel combines these advantages. This new... 

There is significant debate about the relative merits of frequency and time domain. In principle, they are related via the Fourier transformation and have been experimentally verified to be equivalent [9], For some applications, frequency domain instrumentation is easier to implement since ultrashort light pulses are not required, nor is deconvolution of the instrument response function, however, signal to noise ratio has recently been shown to be theoretically higher for time domain. The key advantage of time domain is that multiple decay components can, at least in principle, be extracted with ease from the decay profile by fitting with a multiexponential function, using relatively simple mathematical methods. 

We shall conclude this chapter with a few speculative remarks on possible future developments of nonlinear IR spectroscopy on peptides and proteins. Up to now, we have demonstrated a detailed relationship between the known structure of a few model peptides and the excitonic system of coupled amide I vibrations and have proven the correctness of the excitonic coupling model (at least in principle). We have demonstrated two realizations of 2D-IR spectroscopy a frequency domain (incoherent) technique (Section IV.C) and a form of semi-impulsive method (Section IV.E), which from the experimental viewpoint is extremely simple. Other 2D methods, proposed recently by Mukamel and coworkers (47), would not pose any additional experimental difficulty. In the case of NMR, time domain Fourier transform (FT) methods have proven to be more sensitive by far as a result of the multiplex advantage, which compensates for the small population differences of spin transitions at room temperature. It was recently demonstrated that FT methods are just as advantageous in the infrared regime, although one has to measure electric fields rather than intensities, which cannot be done directly by an electric field detector but requires heterodyned echoes or spectral interferometry (146). Future work will have to explore which experimental technique is most powerful and reliable. 

More detailed information can be obtained from noise data analyzed in the frequency domain. Both -> Fourier transformation (FFT) and the Maximum Entropy Method (MEM) have been used to obtain the power spectral density (PSD) of the current and potential noise data [iv]. An advantage of the MEM is that it gives smooth curves, rather than the noisy spectra obtained with the Fourier transform. Taking the square root of the ratio of the PSD of the potential noise to that of the current noise generates the noise impedance spectrum, ZN(f), equivalent to the impedance spectrum obtained by conventional - electrochemical impedance spectroscopy (EIS) for the same frequency bandwidth. The noise impedance can be interpreted using methods common to EIS. A critical comparison of the FFT and MEM methods has been published [iv]. 

Especially at the short times, the use of time domain methods, as opposed to their polnt-by-polnt frequency domain equivalents, is advantageous in a number of ways. They can, for example, be considered as truly spectroscopic techniques because of their broad-band nature and their capacity to generate dielectric properties as a continuous function of time or, Ity appropriate transformation, frequency. In the past few years, time domain methods have received fresh Impetus from advances in two different types of method firstly, the d.c. step response technique as used by Reddish and Williams has been up-graded in sensitivity and bandwidth through... 

The following is the personal view of the author, based on his experience with the discussed techniques. The advantage of frequency-domain techniques, described in Sects. 2.1-2.3 is that no external field is required and ZESs are obtained directly. The ability to apply a small field is useful, especially at frequencies above ca. 20 cm in order to distinguish magnetic resonance transitions from molecular vibrations and other phonon-type excitations. The advantage of field-swept techniques is that field-swept spectra tend to have flatter baselines, which increases sensitivity. Cavity and other resonator methods, which are only easily implemented in field-swept experiments, are much more sensitive. Therefore, single-crystal... 

While in lumped-circuit methods the dielectric response is measured in the frequency domain, following the ajpplication of a sinusoidal alternating electrical field, for frequencies below 10 Hz it is advantageous to cany out the measurements in the time domain because it is less time consuming. The polarization or depolarization current following the application of a step-like electrical field is measured as a function of time. 

To understand the reasons for the advantage of the pulsed method,13-17 it is necessary to begin by defining some basic magnitudes that describe an NQR line. In the frequency domain, an NQR line is fully described by its frequency and by a normalized shape function S(w). For example. 

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