Planar three-resonator bandpass filters with cross coupling

New designs of planar three-resonator bandpass filters with cross coupling having a large variety of frequency characteristics are considered. In addition to better one-sided selectivity, such filters implement symmetric amplitude–frequency characteristics with an attenuation pole at each side of the passband. These filters can also have constant delay time in the passband. Concatenation of the proposed three-resonator filters leads to multiresonator filters with high selectivity determined by two attenuation poles at each side of the passband. It is found that the type of connection of the middle resonator in the circuit of a three-resonator filter with cross coupling results in substantial differences in amplitude–frequency characteristics. The results of modeling of different frequency characteristics are presented. It is shown that filters have small dimensions, which are comparable to overall dimensions of microwave ceramic filters. For example, dimensions of a three-resonator stripline filter designed for a frequency of 1.9 GHz are 9.4 × 5 × 2 mm for the permittivity of the filter material ε_r = 92 and dimensions of a six-resonator filter are 10 × 9.4 × 2 mm.