Vanadium oxide bandstop tunable filter for Ka frequency bands based on a novel reconfigurable spiral shape defected ground plane CPW

This paper proposes and validates a new principle in coplanar waveguide (CPW) bandstop filter tuning by shortcutting defected ground plane (DGS) inductor shaped spirals to modify the resonant frequency. The tunable filter is fabricated on a high-resistivity silicon substrate based on a CMOS compatible technology using a 1 μm x 10 μm long and 300 nm thick vanadium oxide (VO2) switch by exploiting its insulator to metal transition. The filter is designed to work in Ka band with tunable central frequencies ranging from 28.2 GHz to 35 GHz. The measured results show a tuning range of more than 19 %, a low insertion loss in the neighboring frequency bands (below 2 dB at 20 GHz and 40 GHz in on/off-states) while a maximum rejection level close to 18 dB in off-state, limited by the no RF-ideal CMOS compatible substrate. The filter has a footprint of only 0.084 · λ0 x 0.037 · λ0 (where λ0 represents the free space wavelength at the highest resonance frequency) thus making it the most compact configuration using CPW DGS structures for the Ka frequency band. In addition, a more compact filter concept based on the Peano space filling curve is introduced to increase the tuning range while minimizing the DGS area