Speeding up Amplifier Design by Combining LSNA and New Tuner Technology
Pailloncy, G; Vanden Bossche, M
NMDG NV

Nowadays, Power Amplifier (PA) designers mainly use load-pull systems to extract specifications like Output Power, Inter Modulation Distortion (IMD), Power Added Efficiency (PAE) or Adjacent Channel Power Ratio (ACPR) under different source and load impedance conditions. Such a system, based on passive or active tuners, has several drawbacks: preparing the setup with the tuners is time-consuming and cumbersome, while the tuning process is slow due to the power meters which are typically used and the precise movement of the passive tuners. Furthermore, the broadband power measurements do not give full insight in the component. Recently, an alternative using a Large-Signal Network Analyser (LSNA), was introduced [1] to overcome these drawbacks and to speed up amplifier characterisation. This LSNA is a kit, running on top of a VNA and is referred to as a VNAPlus. Extending the capabilities of a VNA, the LSNA accurately measures the amplitude and phase of the incident and reflected waves at the ports of the device under test (DUT), both at the fundamental and harmonics.

Presently, calibration of a classic load-pull system requires off-line characterisation of all parts of the measurement setup, typically using a vector network analyser, including pre-characterisation of tuner for a limited set of impedances. With the alternative LSNA solution, such a calibration is simplified but one still needs to characterize the tuners separately. In fact, as the reflectometers used to capture incident and reflected waves are lossy components, they would degrade the tuner performance - in terms of gamma presented to the DUT – if connected between the tuner and the DUT. Positioning the tuner in between the DUT and the reflectometer, deembedding is required for each tuner position. Besides the cumulation of errors due to the combination of S-parameters blocks, tuner resolution is restricted to the limited set of impedances obtained during pre-characterisation, unless proper interpolation functions are used. Finally, measurement speed is reduced due to the required tuner stabilization for each position.

In this article, a simple, fast and accurate power amplifier characterisation technique is introduced which combines the VNAPlus and sensing tuners which integrate a quasi lossless coupler-like structure in between the DUT port and the tuner hardware. With this new type of tuner, the LSNA provides direct feedback of the impedances presented to the DUT at both the fundamental and harmonics. Because pre-characterisation of the tuner is no longer required and only one standard (in-situ) calibration is needed, the system is ready to measure in minutes instead of hours. Furthermore, measurement accuracy is improved and tuning resolution is determined by the tuner itself, not by its pre-characterisation.

This article demonstrates that with the unique combination of a VNAPlus and a sensing tuner, PA designers will benefit from a fast, easy-to-use and accurate load-pull system. Additionally it is shown that this system enables real waveform engineering through harmonic load-pull while providing full insight in the device behaviour.

[1] Frans Verbeyst, Marc Vanden Bossche, "Speed up power amplifier design by fast source-pull, real-time load-pull and accurate measurement-based behavioural models", Microwave journal - Editorial, p. 24 - 30, November 2005.