Compact (<0.5mm2) K-band metamaterial bandpass filter in MCM-D technology

A bandpass filter based on complementary split ring resonators (CSRRs) and implemented in MCM-D technology is presented for the first time. It is a thin-film microstrip structure with CSRRs etched in the ground plane and interdigital capacitors in the signal strip. In addition, the structure includes grounded stubs. The combination of CSRRs and such stubs is useful to generate a transmission zero above the left-hand passband of the filter, and thus improve frequency selectivity. The filter, consisting of two CSRR cells, has a central frequency of f_o=19 GHz and a fractional bandwidth of FWB=18%. Inband return losses (RL=-15 dB) are satisfactory, while insertion losses (IL=-7.4 dB) are affected by the very small size of the resonant elements. Filter size is as small as 918times450 mum, i.e. 0.094 lambda_gtimes0.045lambda_g, where lambda_g is the guided wavelength at f_o. The implementation of these metamaterial-based structures in MCM-D technology opens new directions for the synthesis of ultra-small filters operating at millimetre wavelengths