Models rush in where simulators fear to tread: extending the baseband-equivalent method

Radio-frequency (RF) receiver circuits have operating characteristics that make them difficult to simulate using traditional SPICE transient analysis. Their gross operation alone, running at gigahertz frequencies, where cycle-by-cycle simulations will overwhelm the fastest computers and the results will rapidly fill disks. Another bottleneck is sifting through the data. For system design work, these problems can be overcome by using Verilog-A behavioral models to remove the high frequency carrier signal from the simulation. Based on the same concepts as the baseband-equivalent models (available in Cadence\´s "rfLib" library) but extended to handle interference the spectral model is based on an algebraic formula derived to model nonlinear amplification of multiple signals. To handle frequency-dependent response, filtering and impedance matching, equivalent models for linear devices are used like their baseband-equivalent counterparts to achieve correct results. This approach, wrapping a nonlinear power-series model with linear frequency-dependent components, is an equivalent circuit alternative to the generally accepted need to use Volterra series for an accurate "black box" nonlinear model. With a bit of math (and no Verilog-A code), the spectral model for an amplifier is developed and compared with the traditional approach for simulation speed and accuracy in reproducing the effects of blocking, intermodulation, and cross modulation.