Analysis of the impact of interferers on VCO-based continuous time delta-sigma modulators

Abstract

Analog to digital converters (ADCs) used in receivers, face desired and undesired signals. Undesired signals are located out-of-band (OOB) of interest and are stronger than the desired signal and degrade the performance of the ADCs. Continuous time (CT) delta-sigma modulators (DSMs) can provide inherent anti-aliasing and hence they relax the requirement of the dynamic range. In this paper the behavior of CT-DSMs, utilizing voltage controlled oscillators (VCOs), for receiver applications are studied. The results indicate that, due to the frequency modulation occurring in the VCOs, when interfering signals are applied at the half of the sampling frequency (fs), integer multiples of the carrier frequency (fc) and fc/2,fc/3…, modulation tones appear in the bandwidth of the desired channel. This leads to degradation of the signal to noise and distortion ratio (SNDR). An analytical expression for the output signal of the VCO-based integrator in the presence of the interferers has been derived. In addition, behavioral simulations are done for both cascade of integrators in the feedback structure (CIFB) and cascade of integrators in the feedforward structure (CIFF) as well as multi-stage noise shaping (MASH) structures. It will be discussed how the designer should choose the suitable structure.