Switched-RC radio frequency N-path filters

Abstract

Tunable on-chip Radio Frequency (RF) filtering is highly desirable for cost effective wireless communication devices. As mobile wireless devices increasingly support multiple RF-bands, tunable RF filtering is wanted. The tremendous growth of wireless communication combined with scarcely available spectrum asks for new, more programmable radio hardware. This has led to the concept of a cognitive radio capable of smart dynamic spectrum access (DSA) which asks for software-defined radio (SDR) hardware, with flexibly programmable tunable filtering. In this thesis N-path switched-RC circuits are explored, aiming for RF pre-filtering for wireless transceivers. The filter concept fits well to SDR as the center frequency is programmable by the switching frequency, i.e. via a digital clock. As new CMOS technologies provide higher density capacitors and MOS switches with low on-resistance and low parasitic capacitance, N-path filtering benefits from Moore’s law. To demonstrate feasibility, a 4-path differential switched-RC bandpass filter, an 8-path single-ended, as well as an 8-path differential bandstop (notch) filter have been implemented in 65nm CMOS technology. A mathematical analysis is presented to describe the filtering behavior as well as various imperfections for the N-path bandpass and notch filters. The implemented bandpass filter provides an in-band input referred third order intercept point IIP3in-band>+14 dBm with compression point P1dB,in-band>0 dBm at a noise figure NF 17 dBm, P1dB,in-band>2 dBm and NF 20 dB. Especially the compression point and IIP3 are much higher than what is typically achieved with RF CMOS receivers. The N-path filtering technique can also be applied for spatial filtering purposes. A 4-element phased-array system has been implemented in 65nm CMOS technology. 8-phase passive mixers translate the Spatial- and frequency-domain filtering from baseband to RF frequencies at the antenna inputs. As a result a remarkable input compression point P1dB of up to +10 dBm is achieved for out-of-band/beam blockers.