VCO-ADCs with a Quadrature Band-Pass Noise-Transfer-Function

This work describes a systematic way to implement a band-pass noise-transfer-function (NTF) in an analog-to-digital converter implemented with a voltage-controlled-oscillator (VCO-ADC). The approach is based on the pulse frequency modulation (PFM) interpretation of a VCO-ADC, where the NTF is defined by a suitable pulse shaping filter inserted after the VCO. In this work, we explain how this pulse shaping filter can be defined to implement a quadrature band-pass NTF. In addition to describing the best theoretical way to achieve this, we also propose a hardware implementation using a digital delay line whose outputs are decoded with two lookup tables (LUTs). This enables tuning the NTF in software. Transient simulations were made to validate the implementation and robustness of the proposed approach and a case study of the power consumption was made for the case of a 65-nm CMOS process. Due to the mostly digital circuit implementation, the proposed structure is suitable for even newer ultra deep submicron CMOS processes.