Wideband Active Load–Pull by Device Output Match Compensation Using a Vector Network Analyzer

Abstract

This work investigates wideband active load-pull (WALP) on microwave electron devices with a standard vector network analyzer (VNA), solely using calibrated frequencydomain relative measurements. Differently from the methods requiring full time-domain waveform acquisition capabilities, this approach removes any instantaneous bandwidth (BW) requirement, allowing to target modulated signals with arbitrarily large BWs. A VNA-based measurement setup is presented, and a novel mathematical framework is developed in order to enable the synthesis of an arbitrary wideband load profile using suitable numerical algorithms. A linear pre-compensation of the LS output match of the device, here implemented by different methods, is shown to significantly improve the speed and the stability of the basic secants' iterative method used to reach the target. Performance results, investigating the convergence properties of the approach and the effects of the setup dynamic range, are reported for a Gallium Nitride (GaN) high-electronmobility-transistor (HEMT) for up to 120-MHz output BW in the sub-6 GHz range.Index Terms-Active load-pull, modulated signals, power amplifiers, vector network analyzer, device characterization. I. INTRODUCTIONT HE ever-growing demand for high data rates in modern mobile communication standards, e.g., 5G, requires the operation of power amplifiers (PA) in radio frequency (RF) transmitters across extremely wide bandwidths (BWs), i.e., tens or hundreds of MHz. In this respect, novel characterization techniques are required to evaluate the behaviour of electron devices used in PA circuits under operating conditions that closely approximate typical telecom applications. Wideband active load-pull (WALP) has been proposed as a promising technique to characterize microwave transistors and to optimize PA performance using excitations that mimic typical communication standards [1]-[4]. The method allows to set, for a given device-under-test (DUT), a user-prescribed load reflection coefficient profile across a wide modulation BW at fundamental and harmonic frequencies. The approach, at the cost of increased complexity, overcomes the main disadvantages of traditional passive load-pull. WALP can provide full coverage of the Smith Chart compensating for insertion loss, and can extend the narrow relative BW due to delay

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