A Wideband Method for the Rigorous Low-Impedance Loadpull Measurement of High-Power Transistors Suitable for Large-Signal Model Validation

This paper presents a rigorous method for low impedance loadpull measurements using a 6-section Tschebyscheff transforming water-cooled pre-matching test fixture. The transformers were designed to be able to accurately determine S-parameters that represent each fixture half at the fundamental frequency of operation fo and its second and third harmonics, 2fo and 3fo. A two-tier non-50 Ω TRL calibration technique was used to establish the measurement reference planes. In order to accurately establish the calibrated system impedance, a simulated Time Domain Reflectometry (TDR) technique was employed. The calibration of the loadpull system was confirmed by calculating the difference between the measured and calculated transducer gain (ΔGt). This difference provides a measure of the measurement uncertainty. The largest uncertainty, in areas of interest on the Smith chart, was found to be 0.25 dB. The fixture was successfully utilized to measure the loadpull performance of one of Motorola's LDMOS, 90 Watt, 1.9 GHz high power transistors.