A 0.49–13.3 MHz Tunable Fourth-Order LPF with Complex Poles Achieving 28.7 dBm OIP3

A novel switched-capacitor low-pass filter architecture is presented. In the proposed scheme, a feedback path is added to a charge-rotating real-pole filter to implement complex poles. The selectivity is enhanced, and the in-band loss is reduced compared with the real-pole filter. The output thermal noise level and the tuning range are both close to those of the real-pole filter. These features make the filter suitable for high speed, low noise, and low power applications. A fourth-order filter prototype was implemented in a 180-nm CMOS technology. The measured in-band loss is reduced by 3.3 dB compared with that of a real-pole filter. The sampling rate of the filter is programmable from 65 to 300 MS/s with a constant dc gain. The 3-dB cut-off frequency of the filter can be tuned from 490 to 13.3 MHz with over 100-dB maximum stop-band rejection. The measured in-band third-order output intercept point is 28.7 dBm, and the averaged spot noise is 6.54 nV/√Hz. The filter consumes 4.3 mW from a 1.8 V supply.

I. Introduction

Integrated filters are essential components in various applications such as channel select filtering in radio frequency receivers [1], [2], anti-alias filtering before sampling [3], [4], magnetic disc read channel filtering [5], [6], etc. The desired properties of these filters include high selectivity, low passband ripple, low power consumption, good linearity, low thermal noise level, and a wide tuning range. Integrated-circuit low-pass filters (LPFs) can be implemented using Gm-C blocks [1], [7], active-RC stages [8]–[10], active switched-capacitor (SC) stages [11], [12], source follower blocks [13], [14], ring-oscillator-based integrators [15], and passive-SC circuitry [2], [16]–[19]. Active-RC and active-SC filters, as well as self-coupled source follower based designs, can provide high linearity. Active-RC and active-SC filters are usually implemented using opamps. Due to the need for high-quality opamps in these topologies, implementations of such filters are becoming increasingly difficult in deep submicron technologies. On the other hand, Gm-C filters and ring oscillator based filters are more power efficient and scale well with technology [20], but offer lower linearity.