Band-pass

FIGURE 13.55 (sensor based on an elecmcalty tunable micromachined Eabry-Perot interferomeced and (b) the siitfcing of the pass band controlled by the tuning voltage. ... 

Tuni 7g The interferometer is tuned by electrode voltage control. The band-pass center wavelength is displaced accordingly. The FWHM of the transmission pass band is approximately 70 nm at the 4.2 pm wavelength. The stable tuning range of L is... 

Figure 13.55 shows the principle of a gas sensor based on an electrically tunable micromachined Fabry-Perot interferometer, and the shifting ol the pass band controlled by rhe tuning voltage. 

A pracdcal unit for good performance must have a low frequency cut-off below the lowest significant frequency being generated and must not have pass bands greater than the cut-off frequency, which coincides with significant har-... 

Fig. 3.20. Idealized continuum and actual fluxes measured in 50-A-wide bands of A7 stars in the Hyades open cluster with Teff = 8000 K, plotted against inverse wavelength in turn-1. Horizontal lines above the spectrum show the locations of the Johnson U, B, V pass bands and the vertical boxes show schematically the corresponding properties of the Stromgren system with central wavelengths in A. (In that system, there are actually two H/3 pass bands, one narrow and one broad, so that comparison of the two gives a measure of the strength of the line.) Some prominent spectral features are marked.

In the general multiple-crystal beam conditioner case, there is no universal formula for broadening. Rather, the duMond diagram is constructed for the beam conditioner and the shape of the passed band in and is determined. The specimen crystal is then represented on the duMond diagram and scanned... 

The first filter in the chapter is one of the most popular. The schematic of the fourth-order Butterworth response low pass filter is shown in Fig. 3.1. The frequency response of the filter to an AC sweep is shown in Fig. 3.2. Note the flat response in the pass band and the stop band frequency of 100 kHz. 

Advantages Moderate parts count, flat response in the pass band Disadvantages Filter Q greater than that of other Alter types... 

The schematic in Fig. 3.44 of the Chebyshev band pass filter utilized the predicted values from the MathCAD file, where lab resources allowed. Close approximations were used, to which the circuit performance was extremely sensitive. Any deviations from the values predicted in the MathCAD file resulted in gain in the pass band. Using SPICE to test possible circuit realizations greatly reduces the time to implement hardware. SPICE will predict if a given circuit realization will perform as desired with available parts, before actual hardware measurements are made. This is helpful because Chebyshev circuit realization can be difficult small changes in the circuit elements can result in undesired performance. The simulated AC results from IsSpice, PSpice, and Micro-Cap are shown in Figs. 3.45, 3.46, and 3.47, respectively. The measured breadboard AC response of the filter is shown... 

The basic principle of the experiment of Canter, Mills and Berko (1975) was to collide low energy positrons with a surface and to look for coincidence between a Lyman-a photon and a delayed gamma-ray arising from the subsequent annihilation of a 13S positronium. The presence of the Lyman-a signal was verified by the use of three interference filters with pass bands centred on, just above, and just below, 243 nm. An enhanced coincidence rate was found with the 243 nm filter in place. A similar Lyman-a gamma-ray technique has been adopted by all subsequent workers in this field (e.g. Laricchia et al., 1985 Hatamian, Conti and Rich, 1987 Ley et al., 1990 Schoepf et al., 1992 Steiger and Conti, 1992 Hagena et al., 1993 Day, Charlton and Laricchia, 2000). 

Fig. 5.38 Microwave ceramic components (a) metallized ceramic engine block for 40 MHz pass band filter at 1.4 GHz (b) 11.75 GHz oscillator incorporating ceramic dielectric resonator together with various resonator pucks.

Fig. 5.39 (a) Schematic of a 2/4 strip line resonator pass-band filter exploiting LTCC technology the approximate overall dimensions are 15x10x1 mm. (b) Equivalent circuit Q is the pad-to-strip-line capacitance and L and C the inductance and capacitance of the stripline. 

To tailor the stop- or pass-band to requirements more than one line will be involved (coupled resonators), and the geometries will depart from the simple form illustrated. 

By reference to the equivalent circuit explain why the device is a pass-band filter and suggest a design modification which would more closely define the band, and why. [Answer 5 GHz]... 

A filter is required to pass a certain selected frequency band, or to stop a given band. The passband for a piezoelectric device is proportional to k2, where k is the appropriate coupling coefficient. The very low k value of about 0.1 for quartz only allows it to pass frequency bands of approximately 1% of the resonant frequency. However, the PZT ceramics, with k values of typically about 0.5, can readily pass bands up to approximately 10% of the resonant frequency. Quartz has a very high Qm (about 106) which results in a sharp cut-off to the passband. This, coupled with its very narrow passband, is the reason why the frequency of quartz oscillators is very well defined. In contrast PZT ceramics have Qm values in the range 102—103 and so are unsuited to applications demanding tightly specified frequency characteristics. 

Fig. 18.2. ADAS408 generates recombination, ionization and radiated power data from parametric representations of the various coefficients. Electron temperature and density ranges are specified on the processing screen along with atomic masses. This code can accept special filter files which describe the soft X-ray pass band of arbitrary window/detector combinations and modify the calculated radiated power. ADAS408 delivers output data in the ADAS data format adfl 1 - a principal data class accessed by plasma modeling codes...

Aluminum has the advantage that it adheres well to common oxide substrates, is easy to deposit, is only 17% less conductive (for an equivalent thickness) than Au, and is far less dense. The lower density is significant because reflections of AWs from Au IDT fingers in delay-line applications can cause appreciable pass-band ripple in the IDT frequency response. Al s main disadvantage is the relative ease with which it corrodes this problem is sometimes addressed, particularly for (non-sensor) commercial applications of SAW devices, by passivating the A1 using a relatively impermeable layer of a material such as SiaN4 or AIN. 

Fiber (passive) A device that passes only a particular range of frequencies. A bandpass filter passes a specified band of frequencies (its bandwidth is often expressed as a percentage of the center of the pass band) a notch filter removes a narrow band of frequencies high- and low-pass filters pass signals higher or lower than a specified cut-off frequency, respectively. 

For comparison, the frequency response of a two-port SAW resonator is shown in Figure 6.11 (page 364). Note that it resembles the response of the delay line, with the addition of a sharp spike, where the insertion loss is considerably lower, at the center of the pass band. The similarity of the delay line and resonator frequency responses is a consequence of both devices using the same transducer pattern, while the spiked region of much lower insertion loss is a result of the ridge-reflector array utilized to set up a standing wave. Unlike the highest point of the delay-line spectrum, there is no 6-dB theoretical insertion loss limit for the peak of the resonator spectrum — loss can approach 0 dB. 

Figure 6.11 Frequency response of a SAW resonator, [after Ref. 2] the spike in the center of the pass band is the point of resonance.

Multicavity interference band-pass filter sets (Omega Optical, Inc., Brat-tleboro, VT). Wavelengths (center of pass band half band width) Hoechst fluorescence (filter set XF06) ex. = 365 12.5 nm, em. = 450 32.5 nm FITC immunofluorescence, (filter set XF22) ex. = 485 11 nm, em. = 530 15 nm... 

The isolation of Cu Kv. radiation may be taken as an example. Its wavelength is 1.542 A, which means that cobalt and nickel can be used as filter materials since their K absorption edges (1.608 and 1.488 A, respectively) effectively bracket the Cu Kti line. Their linear absorption coefficients p are plotted in Fig. 7-29(a), which shows that balancing can be obtained by making the nickel filter somewhat thinner than the cobalt one. When their thicknesses. v are adjusted to the correct ratio, then = l co co except in the pass band, and a plot of //.y versus /. has... 

It should be emphasized that the beam entering the counter is never physically monochromatic, as it is when a crystal monochromator is used. Radiation with a great many wavelengths enters the counter when either filter is in place, but every wavelength transmitted by one filter has the same intensity as that transmitted by the other filter, except those wavelengths lying in the pass band, and these are transmitted quite unequally by the two filters. Therefore, when the intensity measured with one filter is subtracted from that measured with the other filter, the difference is zero for every wavelength except those in the pass band. 

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