This paper presents a novel approach to analyze clock jitter influence in the performance of continuous-time sigma-delta modulators. The analysis is based on projecting all the signals and operations over an artificial time-base relative to the sampling clock, on which the sampling period can be considered constant. In the proposed time-base, clock jitter can be modeled as the modulation of the input signal frequency and the integrators gain. Unlike standard techniques, we do not rely on the feedback pulse properties, which enables a broader application of our technique to band-pass or VCO-based sigma-delta modulators. In this paper, we present the basic theory of this technique and its application to some practical examples, showing a good matching between prediction and numerical simulation.