Design of parallel-coupled microstrip filters with suppression of spurious resonances using substrate suspension

Substrate suspension is used to suppress the spurious response of microstrip bandpass filters at twice the passband frequency (2f_o). It is known that a proper height of substrate suspension can be used to equalize the even- and odd-mode phase velocities for coupled microstrip lines. In this paper, this property is applied to design the coupled stages of a parallel-coupled line filter so that the spurious response at 2f_o can be completely suppressed. The individual image impedance for each coupled stage is changed accordingly. Required filter design formulas are derived for a series of coupled stages having different image impedances. Several filters made on a substrate of relative high dielectric constant are designed and fabricated. The measured results show that a rejection level of better than -40 dB to the spurious resonance at 2f_o can be obtained.