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These design approaches are found in very narrow band applications (less than 0.1%) at low frequencies (Less than 3 GHz). The idea is to maintain the electromagnetic field inside the dielectric material and away from the lossy metallic wall to improve the Q factor.
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Dual-mode filters can be based on low-loss high-permittivity square or cylindrical dielectric blocks. Thus, the filter structure can be miniaturized, and elliptic function and linear phase performance can also be obtained. The quasiplanar structure is used when the following components are printed on the same substrate, allowing high integration density.ĭual-mode filters (canonical symmetric design), as described in Figure 7.38, are based on square or circular waveguide cavities that support two orthogonals in a single cavity (Zaki et al., 1987). The pure metal insert structure is used if insertion loss is of paramount importance. The filter function is mostly determined by the ladder-shaped pattern. The latter structure is called a quasiplanar filter.
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The inserts can either be made from pure metal or they consist of a ladder-shaped metal pattern etched on a thin supporting low-permittivity substrate. Selected Design Choices of Band-Pass Filter with Nonplanar Metallic Waveguide, Finline, Coaxial Line, and Dielectric WaveguideĮ-plane filters are usually referred to as finline and metal insert filters that consist of ladder-shaped inserts in the E-plane of metallic waveguide (Vahldriek, 1989). The coupling between the two modes is accomplished by a topological perturbation that takes place along the symmetrical axes with respect to the input and output lines.įIGURE 7.38. The dual-mode ring and square-patch resonators can simultaneously induce two resonant modes that are orthogonal in space and are excited by the two orthogonally arranged input and output lines. In addition, they are good candidates for narrow band designs. The interdigital combline, and hairpin-line have side-to-side inter-resonator coupling schemes, and filters can be made compact if spurious responses are suppressed. The parallel coupling can be made stronger to achieve a larger bandwidth. Directcoupled resonator filters are of excessive length that can be reduced by using a parallel-coupled geometry. Most frequently used configurations for planar band-pass filters are direct-coupled, parallel-coupled, interdigital, comb-line, hairpin-line, dual-mode ring, and square-patch resonators, some of which are shown in Figure 7.37. Field theory-based design techniques may alleviate this problem to some degree.
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Fabrication tolerances and material uncertainties as well as inaccurate design techniques may all contribute to the need for tuning. One of the most difficult problems for band-pass filters (especially for narrow-band types) is the need for postfabrication tuning. In addition, low-loss and lightweight dielectric filters are constructed for millimeter-wave applications. Coaxial line and dielectric resonator filters are most frequently used in the lower microwave range if high Q values or small size are needed. With the emergence of commercial broadband systems in the millimeter-wave range, quasiplanar filters have made a comeback. In the higher microwave domain and well into the millimeter wave region, waveguide filters are most commonly used. Some Q-enhanced planar filters are possible in conjunction with strip line and suspended microstrip structures (up to 30 GHz). Usually, they are limited to the lower microwave domain (up to 15 GHz) and only if insertion loss and slope-selectivity is not a major concern. Planar resonators are known to suffer from high ohmic losses, in particular at high frequency, which excludes them from the construction of high-performance, narrow band filters. A basic feature of the geometry of a band-pass filter is that it may consist of resonators coupled along the transmission path of signal, and its number of poles is related to the number of resonant modes of the filter. Band-pass filters may be built from all common transmission line media, ranging from waveguide to microstrip line. This type of filter is the most frequently used. Ruediger Vahldieck, in The Electrical Engineering Handbook, 2005 7.6.2 Band-Pass FiltersĪ band-pass filter may also be called a band-select filter as it selects a specific frequency range to pass a signal unattenuated.