Calibration Error Reduction in Millimeter-Wave Load-Pull Systems Measuring Highly Reflective Loads

This article investigates sources of calibration error inherent in the load-pull (LP) measurement of millimeter-wave transistors when high reflection coefficient loads are applied. In these conditions, the calibration error on relative metrics such as power gain and power added efficiency (PAE) can be significant and strongly dependent on the reflection coefficient. Using both simulations and experiments, the article tries to discriminate the source of uncertainty between noise and "operator" errors. While the former can be minimized by maximizing the dynamic range and improving the filtering on the receiver, the latter, linked to the small mechanical perturbations that occur when changing the system configuration from calibration to measurement, is unavoidable in most LP systems. To address this issue, the article proposes a method, based on the TRL calibration, that consists of load-pulling the thru and line calibration standards using the measurement system in its measurement configuration and calculating their scattering parameters using least-squares on the measured dataset. This enables a recalculation of the calibration coefficients previously obtained when the measurement system was configured for calibration and allows for a significant reduction in calibration error. This method has been tested and applied to measurements of mm-wave devices in both active and passive LP systems, demonstrating a significant impact on the measured performance metrics. Calibration error, at 82.5 GHz, is shown to have resulted in a power gain inaccuracy as high as 0.7 dB at a reflection coefficient of 0.7, which led to an underestimation of PAE of 8.9% for a gallium arsenide pHEMT.