A simple design procedure for microstrip resonator filters with a single pair of transmission zeros

Current microwave communication systems need high-performance narrow-band bandpass filters of size who is compatible with devices including MIC or MMIC components. These needs are expressed especially in communication satellites where payload dimensions and weights are important criteria. Multiple coupled resonator filters are good candidates in this respect. Thanks to their ability to place transmission zeros near cut-off frequencies, high selectivity with a restricted number of resonators is obtained. This characteristic is of great interest in the case of narrow-band filters where losses are strongly related to the number of resonators. Nevertheless, making these filters with coupled microstrip resonators presents several difficulties due to the technology. In fact, the discontinuities presented in this topology and the velocity phase differences on parallel coupled lines make the synthesis of this kind of filter tedious. With the aim of answering this problem, a complete design procedure is presented in this paper which allows this kind of filter to be designed easily on any commercial microwave circuit simulator software. In order to validate particular elements incorporated in the procedure, a four order X-band filter was designed. This filter is intended to be inserted in a small size satellite transponder.