Analysis of an Enhanced-Q N-Path Filter with Improved Even-Order Harmonic Rejection

摘要

Based on a feedback system, an N-path bandpass filter robust to even-order harmonic mixing (BPF-REHM) is presented. BPF-REHM consists of a conventional differential N-path filter around a gain stage. Despite assuming a single-ended antenna, this work aims at partial rejection of even-order harmonics before the signal travels through the baseband circuitry. Linear periodically time-varying analysis of the new filter is presented. Mathematical derivations verify an enhanced-Q filtering behavior in addition to even-order harmonic rejection at RF nodes. Drawback of BPF-REHM, i.e., input impedance mismatch, is addressed by proposing a two-stage LNA. The second stage of the new LNA, which is the BPF-REHM, suppresses even-order harmonic blockers while the first stage matches the input to the source and provides enough gain to accomplish an acceptable overall noise performance. The design example is a 500 MHz four-path filter simulated with the 90 nm CMOS. It achieves > 22 dB even harmonic rejection with an enhanced-Q filtering characteristic thanks to the new technique. Besides, this design achieves 21 dB gain, 2 dB NF, and 2.5 dBm out-of-band IIP3 at 50 MHz offset while consuming 8.3 mW of power.