Tunable Filters and Interference Rejection System for Interferer Suppression at RF and Microwave Bands

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Virginia Tech


Contemporary wireless systems have advanced toward smart and multifunctional radios such as software-defined or cognitive radios which access a wideband or multiband spectrum dynamically. It is desirable for the wireless systems to have high frequency selectivity early in the receiver chain at RF to relax the dynamic range requirements of subsequent stages. However, integration of high selectivity RF band-pass filters (BPF), or band-stop filters (BSF) is challenging because of limited quality factor (Q) of passive components in integrated circuit (IC) technology [1].

This proposed research achieves the followings:

  1. Developing, and demonstrating innovative integrated band-pass filter that relaxes the performance tradeoffs in conventional LC filters to maximally increase filter reconfigurability in frequency tuning range (2-18 GHz), selectivity (Q=5~100) with superior dynamic range (DR>100 dB) at RF to microwave frequency range [2].

  2. Implementing active notch filter system comprised of a Q-enhancement band-pass filter (BPF) and an all-pass amplifier. The notch response is synthesized by subtracting the BPF output from the all-pass output. In the proposed synthetic notch filters, the BPF is responsible for defining selectivity while stop-band attenuation is primarily dependent on the gain matching between the BPF and all-pass amplifier. Therefore, notch attenuation is controllable independently from the bandwidth tuning, providing more operational flexibility. Further, the filter dynamic range is optimized in the all-pass amplifier independently from the selectivity control in the BPF, resolving entrenched tradeoff between selectivity and dynamic range in active filters [3].

  3. Demonstrating the mode reconfigurable LC filter that works in either BPF or BSF for a flexible blocker filtering adaptive to the dynamic blocker environments.

  4. Implementing a novel feedback-based interference rejection system to improving the linearity of the BPF for high Q cases, in which the BPF Q is set to a specific value and further increase in Q is achieved using feedback gain. And finally, the second LC tank is added to increase the out of band rejection in band-pass characteristics.



RF and microwave frequencies, band-pass filter, band-stop filter, interference rejection system