Low-pass electronic filter

Figure 11.10. Phase-locked detection of fluorescence lifetime using single reference signal. FID = fluorescence inducing and detecting devices LPF = low-pass electronic filter VCO = voltage-controlled oscillator v, = signal to modulate the output intensity of the excitation light source v/= the fluorescence signal.

To obtain B v), the higher-frequency component B (v) of interferogram F(x) is first removed by passing the signal F(x) through a low-pass electronic filter, and then the Fourier-transform of the resultant lower-frequency component is performed. 

Sample times for the microprocessor-based SLCs vaiy from O.I to 0.4 seconds. Low-pass analog electronic filters are installed on the process inputs to stop abasing errors caused by fast changes in the process signal. Input filter time constants are typically in the range From O.I to I s. Microprocessor-based SLCs may be made part of a DCS by using the communication port (RS-488 is common) on the controller or may be operated in a standalone mode independent of the DCS. 

The shaip cutoff at the limits -i and t, as illustrated by the boxcar function, often occurs in the frequency domain. In this case the boxcar acts as a low-pass filter in applications in electronics. All frequencies below [l] areunaltgEed, in this ideal case all higher ones are suppressed. 

The broadening due to the analyzer depends on the energy at which the electrons travel through the analyzer and the width of the slits between the energy filter and the actual detector. The analyzer contribution to the line width becomes irrelevant at low pass energies, however, at the cost of intensity. 

The signals were recorded as electrical potential in millivolts. The well known Maxwell s formula and an adjustable empirical coefficients were used to obtain the equivalent volume fraction of liquid [43]. Since it is known that kinematic waves exist only in the frequency of some few Hertz, hardware low pass filter with 20 Hz cutting frequency was included for each channel in the electronic unit. The filter was tuned at 82.66 Db. Data were acquired by a computer at 100 Hz. The following comments regarding the above apparatus description are in order ... 

The spectroscopist should also pay attention to the question whether the sampling interval Js, the speed v of the movable mirror and the time constant T of the electronic data recording system are in a proper relation with each other. It is well known (see also Section 5.4) that a low pass filter with time constant t has the complex transfer function (amplitude and phase )... 

Figure 3. (a) Schematic diagram of the Brookhaven transient FM spectrometer, in which EOM = electrooptic modulator and SA = spectrum analyzer, (b) Schematic diagram of the FM detection electronics, in which LO = local oscillator, L pass = low pass filter. [Reprinted with permission from S. W. North, X. S. Zheng, R. Fei, and G. E. Hall, J. Chem. Phys., 104(6), 2129-5. Copyright 1996 American Institute of Physics.]... 

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