Frequency-Selective Limiters Using Triple-Mode Filters

This paper presents the first frequency-selective limiter (FSL) with both power-activation and user-prompted override that also features a reconfigurable threshold power and frequency tuning with closed-loop feedback. The FSL is based on a tunable absorptive bandstop-to-all-pass filter with integrated electronics. Three distinct modes are available: user-defined bandstop, power-activated bandstop, and all-pass mode. In the user-defined bandstop mode, the FSL presents a measured notch of 45-55 dB at an user-defined center frequency within 1.5-2 GHz tuning range. The out-of-band insertion loss is less 0.8 dB. In the power-activated mode, the FSL switches intrinsically between its bandstop and all-pass responses based on the incident power level at any user-defined power threshold and frequency within the 1.5-2 GHz range. In the all-pass mode, the measured loss is less than 3.1 dB at the user-defined center frequency and less 0.8 dB elsewhere. The measurements are conducted including the two SMA connectors and the associated connector-to-microstrip transitions. The threshold power level is tunable in the -45 to +8 dBm range. The filter is designed and fabricated with high-quality substrate-integrated cavities with an extracted resonator unloaded quality factor ( $Qu$ ) of 300. The unique features and the measured RF performance of the presented FSL are state-of-the-art results.