Towards Low Power N-Path filters for flexible RF-channel selection

N-path filters can offer high-linearity high-Q channel selection filtering at a flexibly programmable RF center frequency, which is highly wanted for Software Defined Radio. Relying on capacitors and MOSFET switches, driven by digital non-overlapping clocks, N-path filters fit well to CMOS and benefit from Moore’s law. The basis of this filtering is the linear periodically time variant (LPTV) behaviour of a switch-R-C series circuit, which realizes frequency translated filtering, where a baseband filter characteristic is shifted around the switching frequency. This paper reviews the basic concept of N-path filters and recent developments, with special attention to possibilities to reduce power consumption by increasing the impedance level. The basic operation of the switch-R-C kernel that is at the core of N-path filtering is reviewed in terms of transfer function and noise performance.