Design and Modeling of Compact IPD Bandpass Filter Based on Cross-Coupling Transformer Structure

A compact IPD band-pass filter base on a cross-coupling structure is introduced for Wi-Fi applications. The cross-coupled structure uses a transformer instead of a traditional inductor to reduce insertion loss. The transformer structure is analyzed, and the transformer is equivalent to two ideal inductors for coupling, which explains why the transformer itself carries transmission zeros and controls the transmission zeros generated by cross-coupling. In order to further improve the performance of the filter, a ternary filter circuit is cascaded. The simulation shows that it can achieve an insertion loss around 2.0 dB and a return loss exceeding 20 dB in the frequency range from 2.4 GHz to 2.5 GHz, and the out-of-band rejection exceeds 40 dB after 10 GHz.