A simulation system is described for computing the overall signal generated in a substrate by a digital system comprising a plurality of gates associated with the substrate, wherein each gate is configured to perform a switching event. Output of a transistor-level model is compared with output of a lumped circuit model for each gate and the substrate, and signal contributions from each gate and switching event are determined based on the comparison. The system determines switching event signals for each of the plurality of gates. The signal contributions and the switching event signals are combined, and a combined lumped circuit model is derived based on a combination of lumped circuit models of the plurality of gates. The overall signal is computed based on the combined gate signal contributions and switching event signals, which are configured as an input to the combined lumped circuit model.