Design challenges for a new mostly digital VCO-based delta-sigma modulator

This study presents an architecture and study of closed-loop continuous time delta-sigma modulation utilising only voltage-controlled oscillators (VCOs) and digital elements. It incorporates a VCO-based integrator in which the offset due to the carrier frequency (f(c)) is digitally subtracted. Unlike some prior structures, an extra VCO for reference signal generation is not necessary which can lead to a reduction in power consumption, area and noise. In addition, in comparison to a recently reported structure, the VCO is placed inside the feedback loop, so the input signal swing of the VCO is decreased and the linearity is improved. However, the proposed structure suffers from two noise folding effects at low frequencies which degrade the dynamic range of the modulator. These two noise folding effects will be analysed and it will be shown that these problems can be tackled by the right choice of f(c) and appropriate design. Behavioural simulations are done for the proposed structure to confirm the structure's performance.