Abstract
An ultra-compact multi-band balanced (differential) bandstop filter (BSF) topology with all lumped elements has been introduced for the first time in this study. The proposed filter is a four-port structure where each symmetrical bisection is a series cascade of K N -band cells. Each N -band cell comprises of N parallel resonant circuits connected to the same node, through capacitors. K series-coupled N -band filter sections in the symmetrical bisection exhibit a K th order N -band BSF response in differential mode (DM) whereas common mode (CM) signal is minimally affected and maintains a high CM rejection ratio (CMRR). The advantages associated with this engineered structure can be summarised as follows: (i) it can be extrapolated to realise an arbitrary number of DM stopbands; (ii) it does not need any additional component in the symmetry plane to suppress the CM noise; and (iii) it occupies a very small area. For demonstration, a second-order dual-band differential BSF with resonant frequencies 1.15 and 1.36 GHz is designed and fabricated. It offers a rejection level of about 25 dB for each DM stopband and maintains a CMRR value of about 25 dB for both bands. Moreover, it covers an area of only 0.003, where is the guided wavelength at the centre frequency between the two bands.
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