Micro-fabricated bandpass filter using intertwined spiral inductor and interdigital capacitor

A bandpass filter (BPF) with a central frequency of 2.31 GHz for future worldwide interoperability for microwave access (WiMax) applications using integrated passive device technology is presented. To realise the BPF, an interdigital capacitor is integrated between the two divisions of a square-shaped spiral inductor. To achieve a compact size, the capacitor is fully embedded inside the inductor. Additionally, the coils of the inductor are intertwined to utilise enhanced mutual inductance for minimising the loss of signal transmission. The strong electromagnetic coupling between the source and load through the interdigital capacitor results in excellent bandpass skirt selectivity with a measured return loss of 26.1 dB. Moreover, it provides an improved stopband response due to the presence of a transmission zero at 5.94 GHz.