A 0.1-to-1.2GHz tunable 6th-order N-path channel-select filter with 0.6dB passband ripple and +7dBm blocker tolerance

Abstract

Radio receivers should be robust to large out-of-band blockers with small degradation in their sensitivity. N-path mixers can be used as mixer-first receivers [1] with good linearity and RF filtering [2]. However, 1/f noise calls for large active device sizes for IF circuits and high power consumption. The 1/f noise issue can be relaxed by having RF gain. However, to avoid desensitization by large out-of-band blockers, a bandpass filter (BPF) with sharp cut-off frequency is required in front of the RF amplifiers. gm-C BPFs suffer from tight tradeoffs among DR, power consumption, Q and fc. Also, on-chip Q-enhanced LC BPFs [3] are not suitable due to low DR, large area and non-tunability. Therefore, bulky and non-tunable SAW filters are used. N-path BPFs offer high Q while their center frequency is tuned by the clock frequency [2]. Compared to gm-C filters, this technique decouples the required Q from the DR. The 4-path filter in [4] has only 2nd-order filtering and limited rejection. The order and rejection of N-path BPFs can be increased by cascading [5], but this renders a “round” passband shape. The 4th-order 4-path BPF in [6] has a “flat” passband shape and high rejection but a high NF. This work solves the noise issue of [6] while achieving the same out-of-band linearity and adding 25dB of voltage gain to relax the noise requirement of the subsequent stages.