Transistor-level simulation of complex systems involving analog and digital parts is a time-consuming task. The growing interaction of analog and digital devices calls for the use of top-down design methodologies, resulting in behavioral modeling at different levels of abstraction. In this article, an advanced design methodology using a combination of behavioral models and transistor-level models is presented. This methodology is very interesting for complex mixed-signal IC design, improving the design flexibility and reducing the simulation time. To validate the proposed methodology, a Continuous-Time Delta-Sigma Modulator based on a high-speed low-resolution quantizer is modeled, taking into account their nonideaties such as excess loop delay, clock jitter and feedback DAC element mismatch. The main features of the multi-bit quantizer are 3-bit resolution with 4 GHz sampling rate and FOM of about 7 pJ/conv. This modulator samples signals at high-IF, performing directly the analog-to-digital conversion in the modern RF front-end receivers.