A PVT-Resilient, Highly-Linear Fifth-Order Ring-Oscillator-Based Filter

This paper introduces a new class of ring oscillator (RO)-based filters that address linearity and process variance limitation of existing RO-based filters. A highly-linear process-tolerant RO filter topology is achieved by imitating the widely known active-RC topology in the phase domain. We propose utilizing a set of frequency detectors (FDs) and phase detectors (PDs) to extract both the frequency and phase information of an inverter-based RO to synthesize active filters in a way similar to integrator-based active-RC filters, which are synthesized using a set of capacitors and resistors, respectively. A zero compensation technique is proposed to extend the achievable bandwidth of the proposed topology. Also, a delay-locked loop (DLL)-based tuning scheme is introduced to achieve resilience over PVT variations. A prototype 5th-order, 2-22 MHz continuous-time Butterworth filter is presented in 130 nm CMOS technology to demonstrate the proposed topology. The filter consumes 6.2-8.9 mA from a 1 V supply and achieves 26.2 dB in-band IIP3. The filter achieves bandwidth variation less than ±3.5% over a temperature range of −40 °C to 85 °C and supply voltage range of 0.9-1.2 V.