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

• Published in: IEEE journal of solid-state circuits Vol. 46; no. 5; pp. 998 - 1010
• Main Authors: Ghaffari, A, Klumperink, E A M, Soer, M C M, Nauta, B
• Format: Journal Article Conference Proceeding
• Language: English
• Published: New York, NY IEEE 01.05.2011; Institute of Electrical and Electronics Engineers; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Applied sciences; Architecture; Baluns; Band pass filters; Band-pass filter; Bandpass filters; Bandwidth; Capacitors; Circuit properties; Circuits; Clocks; CMOS; cognitive radio; commutated capacitors; Design. Technologies. Operation analysis. Testing; Dielectric, amorphous and glass solid devices; Electric, optical and optoelectronic circuits; Electronic circuits; Electronics; Equations; Exact sciences and technology; Frequency filters; frequency translated filter; Harmonic analysis; high linearity; high-Q; inductorless filter; Integrated circuits; linear periodically time variant circuit; LPTV; Mathematical model; Miscellaneous; N-path filter; Noise; Noise levels; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; software-defined radio; Switches; tunable filter; N-path filter; Tunable circuit; Balun; Clock; Q band; Modeling; frequency translated filter; linear periodically time variant circuit; inductorless filter; CMOS; cognitive radio; Impedance matching; Complementary MOS technology; high-Q; Band pass filter; RLC circuit; Transfer function; high linearity; Non ideality; High Q factor; Q factor; Band-pass filter; LPTV; Mismatching; RC filter; Switching; Linear system; Intercept point; Aliasing; Tunable filter; Insertion loss; Equivalent circuit; software-defined radio; commutated capacitors
• ISSN: 0018-9200; 1558-173X
• DOI: 10.1109/JSSC.2011.2117010

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, P 1dB =2 dBm and the noise figure is 3-5 dB, while the power dissipation increases from 2 mW to 16 mW (only clocking power).