Distortion in Difference Frequency under Two-Tone Signal Input Evaluated with Volterra Series Analysis

For the first step of evaluation of the effect of difference frequency distortion in intermodulation distortion and memory effects, we have derived distortion characteristics at the difference frequency as well as third-order intermodulation. In practical calculation of Volterra kernel in frequency domain, we used measured I-V characteristics, and S-parameters in both fundamental (around 2 GHz) and difference frequencies (down to 3 MHz) of an AlGaN/GaN HEMT. We obtained waving in magnitude of third order-Volterra kernel (H₃) as a function of frequency |Δf|. Furthermore, our simulation predicted how frequency dependence of the transistor around the fundamental frequency affects unbalance of intermodulation distortion under no conversion effect from harmonics and difference frequencies. In terms of H₂ for difference frequency, the magnitude was decreasing by increasing in frequency from 3 MHz to 40 MHz even though the phase of H₂ was almost constant which may results from current collapse of the GaN HEMT.