A miniaturized microstrip ring resonator lowpass filter with sharp attenuation

A miniaturized circuit of a microstrip ring resonator lowpass filter with sharp attenuation is investigated in this paper. The miniaturization of the circuit is realized by using a simple specific transmission line with two open stubs. The ratio of the original ring length and miniaturized ring length is 3:2. However the miniaturized circuit is equivalent to the original circuit at three frequencies f₀, 1.5f₀ and 2f₀. Thus, the width of the stopband is maintained as for the original circuit. Therefore, the filter characteristic is not distorted after miniaturization. The theoretical result of a miniaturized circuit shows a broader stopband and a sharper cut off frequency response than the original one, though it also shows smaller minimum attenuation in stopband and larger maximum attenuation in passband compared to the original one. Experimental result is shown for verification of the present study.