Tunable High-Q N-Path Band-Pass Filters: Modeling and Verification

A. Ghaffari, Eric A.M. Klumperink, M.C.M. Soer, Bram Nauta

    Research output: Contribution to journalArticleAcademicpeer-review

    256 Citations (Scopus)
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    Abstract—A differential single-port switched-RC N-path filter with band-pass characteristic is proposed. The switching frequency defines the center frequency, while the RC-time and duty cycle of the clock define the bandwidth. This allows for high-Q highly tunable filters which can for instance be useful for cognitive radio. Using a linear periodically time-variant (LPTV) model, exact expressions for the filter transfer function are derived. The behavior of the circuit including non-idealities such as maximum rejection, spectral aliasing, noise and effects due to mismatch in the paths is modeled and verified via measurements. A simple RLC equivalent circuit is provided, modeling bandwidth, quality factor and insertion loss of the filter. A 4-path architecture is realized in 65 nm CMOS. An off-chip transformer acts as a balun,improves filter-Q and realizes impedance matching. The differential architecture reduces clock-leakage and suppresses selectivity around even harmonics of the clock. The filter has a constant 3 dB bandwidth of 35 MHz and can be tuned from 100 MHz up to 1 GHz. Over the whole band, IIP3 is better than 14 dBm, dBm and the noise figure is 3–5 dB, while the power dissipation increases from 2 mW to 16 mW (only clocking power).
    Original languageEnglish
    Pages (from-to)998-1010
    Number of pages13
    JournalIEEE journal of solid-state circuits
    Issue number5
    Publication statusPublished - 1 May 2011


    • METIS-278754
    • high Q
    • tunable filter
    • linear periodically time variant circuit
    • commutated capacitors
    • inductorless filter
    • Software Defined Radio
    • frequency translated filter
    • LPTV
    • N-path filter
    • EWI-20415
    • high linearity
    • Cognitive Radio
    • Band-pass filter
    • CMOS
    • IR-77956


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