Rapidly Tunable Niobium Notch Filters

A major problem limiting broadband digital receivers is the presence of interference signals that saturate the analog-to-digital converter (ADC). This problem can be alleviated using analog notch filters that block these interferers before digitization, provided the notch filters can be adaptively tuned to follow rapidly shifting interferers. Superconducting thin-film filters offer superior notch rejection combined with little insertion loss outside the notch. HYPRES has developed a superconducting broadband digital-RF receiver mounted on a reliable 4K cryocooler. It is natural to combine this niobium (Nb) digital technology with a Nb-based superconducting filter technology, since they can readily be fabricated and integrated together into the same cryogenic assembly. HYPRES proposes to design, simulate, and eventually fabricate and test filter banks consisting of multiple Nb notch filters, which may be rapidly tuned across the relevant RF band. This will be carried out in collaboration with the University of Waterloo. Tuning mechanisms will be based on either of two approaches: current-biased Josephson junctions, and voltage-biased microelectromechanical (MEMS) capacitors. It is anticipated that both approaches may lead to high-Q notch filters in the 2 GHz range that may be tuned ~20% on microsecond times. The filters will also be examined for linearity and power-handling.