Overview
Large signal models can provide an ideal form for nonlinear device characterization. Such models are especially beneficial in the design of complex or multi-stage circuits because they can enable analysis of performance for varying drive and impedance conditions. Much work has been devoted to large signal model development at microwave frequencies. While the models can be accurate over certain regions of the device operation, they may fail in other areas. Since they are typically extrapolated from DC and small-signal measurements, verification under actual large-signal operation is generally required with mixed results. In many cases of practical interest, the device is used over a wider range of voltage and current than can even be characterized under DC and linear S-parameter conditions, which can lead to errors.
X-parameters* are a measurement of a device under actual operating conditions which include large-signal operation. The vector load impedance dependency can now be included with scattering parameters for non-50 ohm devices using traditional load pull tuners.
Problem
Engineers charged with designing today’s complex and multi-stage circuits are faced with a key question: what matching circuit do I need to apply to get the maximum efficiency or power output from my design? Load pull offers a means of answering this question. Load-pull tuners were designed to aide in this process. Used with a power meter, they allow the engineer to change their device’s load impedance and map the contours of the efficiency curves and power-added-efficiency (PAE) on a Smith Chart. The resulting information allows the engineer to determine the impedance needed to achieve maximum power and PAE of a single stage device.
The problem with traditional load pull data is that it is really a one-port, scalar measurement often done at the fundamental frequency only. It does not provide phase information and, if harmonic interactions are included, does not include the cross-frequency phases of the harmonics. Consequently, it does not provide enough information to reconstruct the terminal waveforms of the device when they are highly distorted under large-signal conditions. While the traditional load-pull measurement works well for simple amplifiers, the same cannot be said of complex or multi-stage circuits. It simply is not sufficient to be used independently in the simulator as a component model.