Abstract
Design procedure of a novel transmission line low-pass filter is presented. The filter has a flexible rejection response for a prescribed cut-off frequency and an in-band ripple factor. It is compact by using a Z-shaped coupled-line section, which is loaded with open-circuited stubs. All of the transmission line elements are identical in electrical length, and the impedances can be calculated by solving certain specific equations according to prescribed in-band specifications. The simulated results show that the circuit can generate low-pass responses with third-, fifth-, and seventh-order filtering functions. The filter order depends on the number and position of the loading stubs. For verification, two sample filters with cut-off frequency of 0.5/0.3 GHz are fabricated using a three-/five-layer PCB process, which generate seventh-order low-pass filtering responses with good selectivity. The experimental results agree well with the predictions.
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