Highly compact tunable stop-band active filter for advanced communication systems

In this paper a novel electronically tunable active stop-band filter design is reported. This filter is designed in the UHF band to avoid the degradation of the receiver sensitivity when interfering signals at fixed or changing frequency are coupled to the antenna of the receiver front-end. The presented solution shows low insertion loss far from the notch frequency while rejecting the interfering signals by more than 40 dB regardless of their frequencies. The filter architecture is simple; it is based on a main transmission line segment to which a quarter-wavelength line segment is coupled. The fact of loading the end of the coupled line by a novel tunable active capacitor circuit introduced in this paper allows tuning simultaneously the filter frequency and the band-stop depth while reducing the power consumption and maintaining the device electrically stable. Moreover, by using the semi-lumped approach, the filter is compacted of more than 70 % in comparison to the conventional quarter-wavelength coupled line. The concept is demonstrated using low cost FR4 substrate and surface mount technology. The complete design has been fully simulated and experimental prototypes realized. Measurements are demonstrating the benefits of the proposed design.