Band-reject filters

Fig. 4.57. Example of sensitive Raman equipment. The band pass filter, BP, cleans the laser radiation. The high NA objective lens LI focuses the laser on the sample and collects the Raman scattered radiation within a large solid angle. The band-rejection filter, BR, blocks elasti-...

Figure 1.2. Raman spectrum of room-temperature chloroform obtained with 514.5 nm light. Rayleigh scattering at zero Raman shift is heavily attenuated by a band reject filter and is actually several orders of magnitude more intense than the Raman scattering. The x axis is shown in three different scales but is normally plotted as Raman shift in reciprocal centimeters relative to the laser frequency (19,435 cm in this case). Although the Stokes Raman to the right is actually a negative frequency shift, convention assigns Stokes Raman shifts as positive numbers.

Figure 9.1. Schematic of Raman spectrometer based on an acousto-optic tunable filter (AOTF). BP, bandpass filter BR, band reject filters APD, avalanche photodiode. (Adapted from Reference 9, with permission.)...

Vengsarkar AM, Lemaire PJ, Judkins JB, Bhatia V, Sipe JE, Ergodan TE (1996) Long-period fiber gratings as band-rejection filters. J Lightwave Technol 14 58-65... 

Blue LED LED Driver Emission FiIter (Band Rejection Filter)... 

RGURE 18-11 Fiber-optic Raman spectrometer with spectrograph and CCD detector The bandpass filter (BP) is used to isolate a single laser line. The band-rejection filter (BR) minimizes the Rayleigh-scattered radiation. 

At least one filter is always installed directly after an isolator, since the ferrite material of the isolator generates harmonic signals. If an ordinary bandpass or band-reject filter is not to be used, a harmonic filter will be needed. 

FIGURE 13.84 Typical frequency response for a single cavity band reject filter. 

FIGURE 13.85 Frequency response for a two-cavity band-reject filter with identical resonant frequencies. 

A branched combiner is a simple combination of a tee junction and the required number of filters to ensure a sufficient amount of isolation. For example, an FM branched combiner containing a three-cavity bandpass filter in series with a two-cavity band-reject filter maybe used to combine two closespaced (0.8 MHz) frequencies. This close spacing requires maximum filtration. 

Refer again to Fig. 13.80, curve B (response curve for a three-cavity bandpass filter) and Fig. 13.86 (response curve for a two-cavity band-reject filter). 

The balanced combiner, like the runout combiner, uses a hybrid ring as the basis for its operation. Each leg of the ring contains an identical set of either bandpass or band-reject filters, hence the term balanced. 

FIGURE 13.94 Impedance diagram for a filter system consistingofa three-cavity bandpass filterandatwo-cavity band-reject filter in series. 

Band elimination lilten A filter having a single continuous attenuation band, with neither the upper nor lower cut-off frequencies being zero or infinite. A band elimination filter may also be referred to as a band-stop, notch, or band reject filter. 

The band-rejection filter has the reverse function of a band-pass filter and consists of a high-pass filter, followed by a low-pass filter (Fig. 3-20). 

Figure 6.11 illustrates several results of this simulation. The frequency response in Fig. 6.11a shows the band rejection filter characteristic required for the compensation of the force Pi lying between about 70 Hz and 329 Hz. In Fig. 6.11b the effect of the closed-loop force compensation is illustrated within the time domain, over the time interval of Os... 0.4s. During the interval... 

An appropriate filter would consist of a high-pass filter and a series of bandpass filters or band-reject filters. The high-pass filter would be designed to pass from just below 5 kHz and the band-pass filters would be tuned to pass 5 kHz, 15 kHz, 25 kHz, etc. A simple system as shown in Figure 9.10b could be tried It consists of an RC high-pass filter and a series-resonant set... 

MJ Pelletier, RC Reeder. Characterization of holographic band-reject filters designed for Raman spectroscopy. Appl Spectrosc 45 765-770, 1991. 

The simple, first-order, filter approach may not always be adequate. In fact, if too large a time constant is chosen, it may interfere with control. A more sophisticated approach is to determine experimentally the noise-frequency spectrum and to design notch filters (i.e., band rejection filters) fijr the dominant frequencies. 

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