Analysis of VCO based noise shaping ADCs linearized by PWM modulation

Abstract

Nonlinearity is one of the main problems associated with VCO based noise shaping ADCs. Their open loop architecture does not permit correction of the nonlinear voltage to frequency response of the VCO by feedback. Recently, linearization of a VCO ADC by Pulse Width Modulation (PWM) precoding has been proposed. Here, the input signal is encoded by a PWM modulator to drive the VCO with a 2-level signal, thus eliminating the nonlinearity of the VCO. This paper analyzes the remaining inherent distortion in such modulators which originates from subsampling the PWM sidebands.