Multi-frequency large-signal analysis using describing functions

Describing functions are developed as a large-signal analysis tool for multi-frequency and multi-dimensional nonlinearities. The implementation and analysis method is discussed along with limitations. Describing functions are then used to analyze a GaN transistor in large-signal operation at X-band with a low frequency excitation on the drain for supply modulation. Describing functions agree well with harmonic balance over RF input power into saturation of the FET device and are used to characterize the FET baseband impedance with a second RF tone.