Theory, analysis and design of high order reflective, absorptive filters

In this study, design methodology and equations necessary for low-loss high-order absorptive filters in the reflective-type configuration are introduced, derived and described in detail. Initially, a simple single, absorptive filter is described and its use in radio frequency front-ends is discussed. Based on this, improvements to the single, first-order absorptive filter are discussed and novel circuits that cater for lower loss higher-order absorptive filters are proposed. The necessary conditions for the achievement of the proposed higher-order absorptive filters from the proposed circuits are derived in order to establish the potential of the proposed circuits. As an experimental verification, first-, second- and third-order reflective-type absorptive filters are fabricated and their performances measured. In particular, in the case of a third-order absorptive filter, it is shown that the depth of the introduced attenuation is at least 27.4 dB across the frequency range of 2.17-2.27 GHz. Further, the use of the obtained absorptive filters was put to a test by inserting them in series with a commercially available band-pass filter.