Ku-band differential lossy filter manufactured in thin-film on alumina technology

This paper presents the design and the fabrication of a compact and robust Ku-band differential bandpass filter, manufactured in thin-film on alumina. The 3-port device includes a balancing network which converts a single-ended input into a differential output. The goal is to achieve a flat in-band Insertion loss, keeping constrained the device size. An 8
^th
order lossy filter with 2 transmission zeros is synthetized using resistive cross-couplings to increase the IL flatness at the expenses of the absolute insertion loss. The balanced to unbalanced conversion is performed by a microstrip Lange coupler and a wideband Schiffman phase shifter. The device exhibits 0.7dB IL flatness in the operative bandwidth 12.75GHz-14.8GHz (FBW 14.5%) and an overall footprint of 14x26mm
²
. The device demonstrates also high robustness to thermal variations in the temperature range between -10 and 70°C.