Tuned Filter

Such relays are normally instantaneous, highly sensitive and operate at low spill cuiTents. Since they detect the residual current of the system, the current may contain third-harmonic components (Section 23.6) and operate the highly sensitive relay in a healthy condition. To avoid operation of the relay under such conditions, it is a normal practice to supply the relay coil with a tuned filter, i.e. a series L-C circuit to filter out the third-harmonic components. The capacitance of the filter circuit may also tame a steep rising TRV (Section 17.10.3) during a momentary transient condition and protect the relay. 

DPFGSE sequence with double tuned filters has been first proposed by Ogura et al. These filters demonstrate very high filtering efficiency for isotope-filtered, isotope-edited NOESY spectra.40... 

The simplest double tuned filter can be constructed by a concatenation of two X-half filters and removal of redundant 180° pulse pairs (Fig. 17.4d) [22]. Alternatively, it can also be realized by keeping the 180° pulse pairs and adding short spin-lock periods (to dephase the 1H-13C magnetization which is orthogonal to the spin-lock axis, Fig. 17.4e) [23], or it is based on the gradient-purging scheme of Fig. 17.4b, resulting in the double filter shown in Fig. 17.4f [18]. 

Harmonic filters are broadly classified into passive and active filters. Passive filters, as the name implies, use passive components such as resistors, inductors, and capacitors. A combination of passive components is tuned to the harmonic frequency that is to be filtered. Figure 4.22 is a typical series-tuned filter. Here the values of the inductor and the capacitor are chosen to present a low impedance to the harmonic frequency that is to be filtered out. Due to the lower impedance of the filter in comparison to the impedance of the source, the harmonic frequency current will circulate between the load and the filter. This keeps the harmonic current of the desired frequency away from the source and other loads in the power system. If other harmonic frequencies are to be filtered out, additional tuned filters are applied in parallel. Applications such as arc furnaces require multiple harmonic filters, as they generate large quantities of harmonic currents at several frequencies. 

Applying harmonic filters requires careful consideration. Series-tuned filters appear to be of low impedance to harmonic currents but they also form a parallel resonance circuit with the source impedance. In some instances, a situation can be created that is worse than the condition being corrected. It is imperative that computer simulations of the entire power system be performed prior to applying harmonic filters. As a first step in the computer simulation, the power system is modeled to indicate the locations of the harmonic sources, then hypothetical harmonic filters are placed in the model and the response of the power system to the filter is examined. If unacceptable results are obtained, the location and values of the filter parameters are changed until the results are satisfactory. When applying harmonic filters, the units are almost never tuned to the exact harmonic frequency. For example, the 5th harmonic frequency may be designed for resonance at the 4.7th harmonic frequency. 

FIGURE 4.22 Series-tuned filter and filter frequency response. 

Sometimes, tuned filters are configured to provide power factor correction for a facility as well as harmonic current filtering. In such cases the filter would be designed to carry the resonant harmonic frequency current and also the normal frequency current at the fundamental frequency. In either case, a power system harmonic study is paramount to ensure that no ill effects would be produced by the application of the power factor correction/filter circuit. 

In principle, Fourier transformation allows you to be much more specific, and to pick out or reject, e.g., a single frequency or a specific set of frequencies. For example, one can tune in to a particular frequency (the digital equivalent of a sharply tuned filter, or of a lock-in amplifier, see section 8.4), or selectively remove noise at, say, 60,120, and 180 Hz, while leaving signals at other frequencies unaffected. However, this requires that the selected frequency or frequencies precisely coincide with those used in the Fourier transformation, in order to avoid the so-called leakage to be described in section 7.4. 

Occasionally, unbalanced filters may be tolerable — for example a single DM choke (i.e. on one line only). Or sometimes, in very low-power applications, just a plain decoupling capacitor (e.g. Cl) may suffice. Sometimes tuned filter stages are seen in... 

Cavities, combiners, and duplexers are all selective-tuned filters. They are composed of three types of passive filters ... 

Microscope with Multichannel Detection In this scheme, an unfocused laser beam illuminates the area of the sample. An optical filter isolates the selected Raman line, and the multichannel detector produces a picture of the sample. At the time of this publication, the optical filter was a monochromator used as a filter. [Since then, this same concept has been implemented with dielectric filters [57], AOTFs (acousto-optic tuning filter) [58], and LCTFs (liquid-crystal tuning filter) [59,60] these technologies will certainly be covered in later chapters on microscopy and imaging.]... 

An alternative to the angle-tuned filter approach is to employ a stationary dielectric bandpass filter in combination with a tunable laser source [16]. The fixed filter eliminates... 

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