X-Band MMIC-Based Tunable Quasi-Absorptive Bandstop Filter

— This letter reports on the design and experimental testing of a new class of tunable monolithic microwave integrated circuit (MMIC)-based quasi-absorptive bandstop filters (ABSFs). An asynchronous tuning method is used to improve the design robustness and obtain an ultrahigh stopband attenuation over a wide tuning range. RF tuning is facilitated by GaAs pHEMT varactors. The proposed filter configuration exhibits the following unique features: 1) ultrahigh stopband rejection (>70 dB) despite using low quality factor lumped element (LE) resonators; 2) miniaturized footprint enabled by the GaAs MMIC process and the use of LE-based impedance inverter elements; and 3) frequency agility. An X-band filter prototype was manufactured using a commercially available GaAs process. It exhibits a high isolation notch whose center frequency can be tuned between 8.7 and 10.5 GHz while having >40-dB attenuation and a maximum attenuation of 73.6 dB.