Filters cascading

Alias reduction for hybrid filter banks. One possible problem of all cascaded filter banks specific to hybrid filter banks needs to be mentioned. Since the frequency selectivity of the complete filter bank can be derived as the product of a single filter with the alias components folded in for each filter, there are spurious responses (alias components) possible at unexpected frequencies. Crosstalk between subbands over a distance of several times the bandwidth of the final channel separation can occur. The overall frequency response shows peaks within the stopbands. 

In [Edler, 1992] a solution to this problem has been proposed. It is based on the fact that every frequency component of the input signal influences two subbands of the cascaded filter bank, one as a signal component and the other as an aliasing component. Since this influence is symmetric, a compensation can be achieved using a butterfly... 

Edler, 1992] Edler, B. (1992). Aliasing reduction in sub-bands of cascaded filter banks with decimation. Electronics Letters, 28 1104 - 1105. 

The materials and structures associated with primary sensors contain dissipative, storage and inertial elements. These translate into the time derivatives appearing in the differential equation that models the sensor system. Hence another major defect is represented by the time (or frequency) response. The means to neutralise this imperfection involves filtering, which may be thought of in terms of pole-zero cancelation. If the device has a frequency response H s) then a cascaded filter of response G s) = 1/H s) will compensate for the non-ideal time response. The realisation of such a filter in analogue form presents a major obstacle that is greatly diminished in the digital case. 

Cascading effects Check valve Clogged (of filter) Consensus standard Conservation vent Dike, berm Discharge valve Division (in electrical area classification) Downspout Expansion joint Explosion proof Faucet... 

The complexity increases even more if one starts incorporating multisensor functions, Figure 13. Even if for improved performance the filter elements would consist of cascaded microresonators selecting 8 wavelengths, the complete multi-sensor with 96 microresonators could still fit on a few mm2. In this way an optical nose or optical tongue with only a modest power supply and without optical peripheral equipment would become feasible. 

Chu, S. T. Little, B. E. Pan, W. Kaneko, T. Kokubun, Y., Cascaded microring resonators for crosstalk reduction and spectrum cleanup in add drop filters, IEEE Photon. Technol. Lett. 1999, 11, 1423 1425... 

It must also be emphasized that the major mass of a heterodispersed aerosol may be contained in a few relatively large particles, since the mass of a particle is proportional to the cube of its diameter. Therefore, the particle-size distribution and the concentration of the drug particles in the exposure atmosphere should be sampled using a cascade impactor or membrane filter sampling technique, monitored using an optical or laser particle-size analyzer, and analyzed using optical or electron microscopy techniques. 

Better collection methods are needed for stack measurements. Virtual Impactors or other devices that avoid the bounce-off problems of cascade Impactors should be developed. Methods for collection of very large amounts of size-segregated particles suitable for detailed organic analyses are needed. Better low-blank filter and collection-surface materials that can withstand high stack temperatures are needed. Careful attention should be given to the determination of vapor-phase species of volatile elements and compounds. 

Most of the published composition/size distribution data have been obtained by analyzing cascade impactor samples. Some of these data suffer from poor size classification as a result of particle bounce or reentrainment, seriously limiting size resolution. Even when this problem is overcome, the data obtained with conventional cascade impactors are not capable of resolving many details of the distribution of submicron particles. These instruments typically classify only those particles larger than 0.3-0.5 tam aerodynamic diameter. All smaller particles are collected on a filter downstream of the impactor. Some measurements of the variation of composition with size below this limit have been attempted by aerodynamically classifying resuspended ash ( ). These data suffer from incomplete disapregation as well as poor classification of the smaller particles. 

Improved control devices now frequently installed on conventional coal-utility boilers drastically affect the quantity, chemical composition, and physical characteristics of fine-particles emitted to the atmosphere from these sources. We recently sampled fly-ash aerosols upstream and downstream from a modern lime-slurry, spray-tower system installed on a 430-Mw(e) coal utility boiler. Particulate samples were collected in situ on membrane filters and in University of Washington MKIII and MKV cascade impactors. The MKV impactor, operated at reduced pressure and with a cyclone preseparator, provided 13 discrete particle-size fractions with median diameters ranging from 0,07 to 20 pm with up to 6 of the fractions in the highly respirable submicron particle range. The concentrations of up to 35 elements and estimates of the size distributions of particles in each of the fly-ash fractions were determined by instrumental neutron activation analysis and by electron microscopy, respectively. Mechanisms of fine-particle formation and chemical enrichment in the flue-gas desulfurization system are discussed. 

Sampling. Four aerosol samples were collected isokinetically on July 26, 1979, at ports on the inlet duct of the ESP at Plant A two were taken with 62-mm fluoropore filters and two were taken with the University of Washington MKV Cascade impactor (1 ). At plant D, samples were collected over a 6-day period at ports both in the outlet duct of the ESP (i.e., upstream of the FGD system) and at the 91-m level of the stack. Eleven fluoropore filter, 1 MKV, and 4 MKIII impactor samples at each location, giving a total of 22 filter, and 8 MKIII samples. A single MKV sample was also collected in-stack at reduced pressure at plant D during the 6-day period. Polycarbonate material coated with apiezon L vacuum grease and 62 or 47-mm-diam, l-pm pore Fluoropore filters were used as back-up filters in the MKV impactor. 

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