A study of excess loop delay in tunable continuous-time bandpass delta–sigma modulators using RC-resonators

Abstract

Excess loop delay is one of the most critical non-idealities of continuous-time delta---sigma modulators as it leads to degradation of the signal-to-noise-ratio or even instability. A comprehensive study of the impact of excess loop delay on tunable continuous-time bandpass delta---sigma modulators using RC-resonators is performed in this paper, both analytically and by simulations. Moreover, a detailed analysis of the conventional compensation techniques for single-band continuous-time bandpass modulators as well as their adaptability to tunable bandpass modulators is performed. The results indicate that only tuning of the scaling coefficients is suitable to compensate for excess loop delay in high-speed tunable bandpass modulators. Based on this result, an approach to the compensation of excess loop delay is proposed which maps the poles of the noise transfer function (NFT) to almost ideal and thus stable positions. Excess loop delay equal to one clock cycle may thus be compensated while the available tuning range of the center frequency depends on the order and the out-of-band-gain of the NFT. A prototype implemented on a printed circuit board proves the feasibility of the proposed approach.