Wide-stopband, compact microstrip diplexer with common resonator using stepped-impedance resonators

A compact microstrip diplexer with a common resonator has been proposed in this paper. By exploiting the variable frequency response of stepped impedance resonator, the common resonator can be shared by two bandpass filter channels of a diplexer if its fundamental and higher frequencies are properly designed. In addition a diplexer design method for suppressing spurious responses in the stopband by choosing the constitutive resonators with the same fundamental frequency, but staggered higher order resonant frequencies. The design concept is demonstrated by three pole parallel-coupled bandpass filter. The bandpass filter is composed of three different stepped-impedance resonators for which a general design guideline has been provided in order have the same fundamental frequency and different spurious frequencies by proper adjusting the impedance and length ratios of stepped impedance resonators. The measured results are in good agreement with the simulated predictions, showing that better than -25 dB rejection levels in the stopband up to 8f01 and 5.5f02 are achieved by Chevbyshev filters.