IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 47, NO. 5, OCTOBER 1998 1259 Measurements of Time-Domain Voltage/Current Waveforms at RF and Microwave Frequencies Based on the Use of a Vector Network Analyzer for the Characterization of Nonlinear Devices— Application to High-Efficiency Power Amplifiers and Frequency-Multipliers Optimization Denis Barataud, Caroline Arnaud, Barbara Thibaud, Michel Campovecchio, Jean-Michel Nebus, and Jean Pierre Villotte Abstract—A new time-domain waveform measurement system based on the combination of an harmonic source and load- pull setup with a modified vector network analyzer (VNA) is presented. It allows the visualization, the measurement, and the optimization of high-frequency currents and voltages at both ports of nonlinear microwave devices. Measurements of GaAs field effect transistor (FET’s) and GaInP/GaAs heterojunction bipolar transistor (HBT’s) at -band were performed to demon- strate the great capabilities of the system. On one hand, voltage and current waveforms at both ports of transistors, working as power amplifiers, were optimized for maximum power-added effi- ciency. On the other hand, time-domain waveforms of transistors operating as frequency multipliers were optimized for maximum conversion gain. Such results prove the capabilities offered by this new nonlinear time-domain measurement system to aid in designing optimized power amplifiers or frequency multipliers. They also provide valuable information for nonlinear transistor model validation. Index Terms— Active-loop technique, experimental optimiza- tion, frequency multiplier, load-pull/source-pull, microwave and RF transistors, modified-vector network analyzer, time-domain characterization, voltage/current waveforms. I. INTRODUCTION T HE design of nonlinear microwave circuits requires mod- ern microwave computer-aided design (CAD) tools asso- ciated with accurate large signal nonlinear models of tran- sistors. To verify nonlinear models of semiconductor devices and to optimize their operating conditions, an accurate and appropriate measurement tool is increasingly necessary. In the last years, waveform measurement systems allowing the large signal characterization of microwave transistors in a 50 Manuscript received May 21, 1998; revised November 30, 1998. The authors are with the IRCOM-Facult´ e des Sciences-UMR, Limoges C´ edex, 87060 France. Publisher Item Identifier S 0018-9456(98)09836-2. environment were reported [1]–[5]. A nonlinear measurement system, based on the use of two calibrated and synchronized microwave transition analyzers combined with a multihar- monic active load-pull setup, was recently presented [6]. In this article, a new measurement system based on the use of a modified conventional vector network analyzer (VNA) is presented. A harmonic source and load-pull system coupled to the calibrated modified VNA is very attractive because optimum operating conditions of the device under test (DUT) can be easily, quickly, and methodically obtained as explained in this paper. The proposed measurement system can be viewed as a time- domain load-pull system because it allows the control and the measurement of current/voltage waveforms at both ports of the DUT. To prove the great capabilities of this new system, two kinds of measurement have been performed. The first one consisted in the optimization of voltage/current waveforms at both ports of the DUT to obtain maximum power added efficiency. The second one consisted in the optimization of operating conditions of the DUT working as a frequency doubler. To our knowledge, the measurement, with a modified VNA of microwave time-domain voltages and currents has never been reported before. II. DESCRIPTION OF THE TIME-DOMAIN WAVEFORM MEASUREMENT SYSTEM A. Problem Formulation Let us consider an arbitrary two-port device described by incoming and outgoing power waves (Fig. 1). 0018–9456/98$10.00  1998 IEEE