Ultra-Wide Stopband Low-Pass Filter Using Multiple Transmission Zeros

AA compact microstrip low-pass filter (LPF) with an ultra-wide stopband and its design procedure are proposed in this paper. Using three-stepped impedance stubs(SISs), multiple transmission zeros are introduced in the stopband and can be freely controlled by tuning the impedance ratio of the SISs. Without using the defected ground structures to expand the stopband, three additional open-circuited stubs are used to suppress more spurious passbands of the LPF. To illustrate the concept, an experimental stopband LPF with cut-off frequency (fc) at 0.9 GHz is designed, simulated, fabricated, and measured. A metallic box with absorbent material is used to load the lowpass filter with wide stopband. The measured results are found in good agreement with the simulated results, and indicated good performance: broad stopband (> 17.6fc) with attenuation higher than 22.4 dB, low-passband insertion loss (<;0.5 dB), and compact size.