8-Path tunable RF notch filters for blocker suppression

The huge growth of the number of wireless devices makes wireless coexistence an increasingly relevant issue. If radios operate in close proximity, blockers as strong as 0dBm may occur, driving almost any receiver in compression (note that 0dBm in 50Ω corresponds to a peak-to-peak voltage of half a 1.2V supply). Thus RF blocker filtering is highly wanted. However, fixed filters are undesired when aiming for multiband, software-defined or cognitive radio transceivers. Passive LC filters show limited Q and tunability. Recently frequency translated filtering has been proposed as a potential solution direction for high-Q filtering. We showed that by applying the “N-path concept”, more than a decade of center frequency range with good linearity, compression point (P
_1dB
>;0dBm, IIP3 >;14dBm) and low noise is feasible for a bandpass (BP) filter. A notch filter with a combination of active and passive mixers is applied in a feedforward path realizing a BP filter. The low input impedance of a transimpedance amplifier with feedback is upconverted to create a notch filter at low frequencies (80MHz) suppressing TX leakage in an FDD system. In this work we explore the possibility to realize a notch filter applying the N-path concept at RF frequencies and in a completely passive way. A single-ended (SE) and a differential 8-path notch filter with passive frequency mixing are presented. The filters are power-matched in the input and output in the passband and provide a low insertion loss, high compression point and also low noise property, thus they can be utilized in front of a receiver to provide rejection of high-power blockers with a large frequency tuning range.