Optimum design of microstrip parallel coupled-line band-pass filters for multi-spurious pass-band suppression

A modified configuration for the parallel coupled-line band-pass filters is presented which allows it to largely suppress the spurious pass-bands. The proposed filter configuration is composed of two attached open-circuit stubs at the open-end of each section of the conventional coupled-line filter. A least-square-based algorithm is developed for the design and optimisation of the proposed filter. First, the two-port impedance matrix of a coupled line is determined from its four-port matrix. Then, the transmission matrix of a coupled line and subsequently that of the multi-section filter are obtained, which lead to its scattering parameters. Finally, an error function is constructed by the specified insertion and return losses of the filter. To demonstrate the characteristics of the proposed filter, a third-order band-pass filter centred at the operating frequency of 1.7 GHz with 5% fractional bandwidth is designed, optimised, fabricated and tested. Simulated and measured data show that for such a filter prototype, a rejection level exceeding 20 dB is obtained in the first four spurious pass-bands, while keeping the desired pass-band attenuation less than 2 dB. In addition, the filter circuitry size is reduced in comparison with the conventional parallel coupled-line filters, and an impedance matching function is also embedded in the filter configuration.