High selectivity and ultra-wide stopband microstrip lowpass filter using high-low impedance resonators

A compact microstrip lowpass filter is designed to obtain high selectivity and extra-wide stopband up to 10
^th
harmonic suppression. To improve the selectivity and make the filter structure compact, folded high-low impedance resonators are used. For achieving high relative stop bandwidth (163%), modified hexagonal shaped resonators and open stubs as suppressing cells are employed. The proposed filter has 3 dB cutoff frequency of 1.85 GHz. An ultra-wide stopband bandwidth of 2.04 GHz to 20 GHz has been achieved at 20 dB suppression level. A sharp transition range of 0.275 GHz is obtained between the cutoff frequency and -40 dB attenuation level. The LC equivalent circuit model of the proposed filter is developed and simple circuit analysis method is used to extract the values of parameters.