This paper suggests an answer to the problem of simulating heterogeneous wireless systems composed of baseband and RF blocks, as the former demand for SPICE-like engines, while the latter are more efficiently solved with frequency-domain tools as harmonic balance. The proposed time-frequency hybrid technique splits the circuit’s node voltages or mesh currents into fast and slowly varying state variables treating the former with the popular envelope transient harmonic balance technique and the latter with a pure time-marching engine. This way, the circuit’s hardest nonlinearities—and their rich harmonic content responses—are appropriately computed in the natural time domain, while the more moderate ones—and their corresponding smooth narrowband RF modulated signals—are efficiently processed in the frequency domain. Simulation tests performed in two illustrative application examples, revealed evident gains in computation speed over state-ofthe-art RF simulation tools without any noticeable loss in accuracy.