Influence of Transistor Compact Model Accuracy on Phase Noise Simulation

When designing oscillatory circuits, intuition about phase noise is often hindered by the complex underlying mathematics and proprietary device compact models. While these models are very accurate, their complexity (and sometimes encryption) can lead to a black-box behavior. For the investigation of circuit topologies regarding noise it is desirable to work with more accessible, general compact models, decoupled from a specific technology node. We investigate how differences in compact model accuracy influence the simulation results of phase noise properties. For two exemplary circuits phase noise properties are calculated using a highly accurate foundry model and a simple SPICE level 1 model. Comparison shows that qualitatively correct results can be expected even for the simple compact model, especially when the susceptibility of oscillator phase as a function of time is investigated. However, simulations of statistical properties (e.g. the phase noise spectrum) are based on a fragile balance of positive and negative phase contributions, and are shown to vary more severely for different models.