Coupling of digital switching noise to the silicon substrate can severely degrade the analog and RF performance in single-chip transceivers. To predict the degradation of the performance of RF circuits, modeling of the impact of substrate noise is absolutely necessary. Using measurements, this impact is modeled by the cascade of an attenuation through the substrate from the source of substrate noise to the RF circuit and the propagation through the RF circuit to its output. This approach has been validated with measurements on a 0.25 mum CMOS low-noise amplifier (LNA) and reveals insight in the mechanism of impact of substrate noise on RF circuits. In addition, impact of a real digital circuit is measured on a 0.18 mum differential CMOS LNA