Low-Pass Selective Filters with Increased Selectivity
The characteristic function of the polynomial (CMAC and C) filters available, when requirements for increased selectivity is to be satisfied, a question arises as to what is better from the circuit complexity point of view: to increase the order of the filter (the order of the polynomial) or to introduce transmission zeros on the ω-axis so making the transfer function rational. This dilemma is regularly avoided in the literature and no solutions are offered which would help the designer to come to “the cheapest” physical system. Here we propose an algorithm for introduction of arbitrary (up to the maximum possible) number of transmission zeros located on the ω-axis based on the minimum stopband attenuation. It is shown by examples that by introduction of transmission zeros benefits are collected in both the passband and the stopband compared to polynomial solutions extended to the same circuit complexity. In the stopband the selectivity is raised while in the passband the reflected power is decreased (the attenuation in all but the upper end of the passband is reduced). Special extension of this chapter is related to the transfer functions with multiple transmission zeros including the ones exhibiting maximally flat attenuation in the origin.
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