Theoretical Analysis of Highly Linear Tunable Filters Using Switched-Resistor Techniques

Abstract

In this paper, an in-depth analysis of switched-resistor (S-R) techniques for implementing low-voltage low-distortion tunable active-RC filters is presented. The S-R techniques make use of switch(es) with duty-cycle-controlled clock(s) to achieve tunability of the effective resistance and, hence, the RC time constant. The characteristics of two S-R networks utilizing one set (S-1R) and two sets (S-2R) of switch and resistor combinations are analyzed. It will be shown that the S-2R network outperforms the S-1R counterpart in terms of finite-slew-rate-induced distortion, frequency translation, and noise performance. In order to extend the tuning range, an S-R bank scheme is also described. The theoretical analysis was verified by an experiment on a 100-kHz first-order S-R filter prototype, implemented using discrete elements, where several advantages of the S-2R over the S-1R networks are demonstrated. Simulations of 10-MHz low-pass filters based on the S-1R and S-2R techniques in a standard 0.18-m CMOS process are also included for performance comparison in practical on-chip filter implementations.

Low-power fine-tuning switched-resistor reconfigurable filter

“…However, this will trade with the distortion degradation. To extend the tuning range without altering the distortion performance is possible using the resistor-bank scheme [1] with a cost of die area increasing. The transistor ratio of the PMOS transistors in the constant-g m bias circuit was chosen to be p = 0.25.…”

“…The switched-resistor technique was first developed on the active-RC structure [1] due to its superior linearity. However, one significant constraint is the requirement of opamp unitygain bandwidth much larger than the lter cut-off frequency, which limits usable frequency and leads to high power consumption.…”

“…Nevertheless, there is a critical issue, which is the RC time-constant variation due to process uncertainty, temperature drift, and aging. In order to deliver a low-voltage fine tuning solution, Switched-Resistor techniques seem to be a potential candidate [1]. A lossless active-RC integrator employing two sets of switched-resistor (Switched-2R) is shown in Fig.…”

“…The Switched-2R has some attractive properties over Switched-1R counterpart for it requires opamps with lower slew-rate specification thereby reducing power consumption. In addition, it has lower frequency translation gain so the demanded clock frequency can be minimized [1].…”

A Tunable Ionic Liquid Based RC Filter Using Electrowetting: A New Concept

“…(viii) This filter can be assembled in oil instead of air, which would provide more contact angle change giving a larger cutoff frequency range. This liquid drop RC filter system has low tunability range compared to traditional solid state electronic RC filter systems (16). However, liquid drop RC filter systems have many potential applications where traditional solid-state electronic RC filters cannot be used.…”