Scalar Load Pull
Today, many cost-effective network analyzers are available on the market. However, 20-30 years ago, these devices meant a substantial financial investment, and there wasn't a network analyzer available at every measuring station. Therefore, load pull analyses were often conducted by means of a load measurement instrument or a spectrum analyzer, as shown in the illustration below.
The HF output power is measured as a function of the input and output impedance. To achieve this, the HF power is varied at the input side to capture the large signal behavior of the DUT. The illustration below shows the representation that is often used for measurement results of an amplifier component:
The load impedance was varied by means of the load pull tuner for three different HF input powers and a defined setting of the source pull tuner. Due to the characterization of the tuner for each tuner setting, the load impedances are known. The illustration shows the curves for a constant output power within a Smith chart (only a section of the Smith chart is shown, since the results that are of interest are contained in a small area). The maximum output power is emitted at a specific load impedance value. Around this impedance value, closed load circuits evolve in the Smith chart at a constant, lower output power. All impedances which constitute a load circuit result in exactly one constant and known output power. In the example, the load circuits for three input powers were recorded using different colors. This graph is required in order to determine the optimum operating point for an amplifier.
We recommend the ATS SW by Maury Microwave (www.maurymw.com) for controlling scalar load pull measuring stations, for characterizing the tuners and for measuring the S parameters of the DUT, as well as for controlling mixed load pull measuring stations with network analyzers. This SW comfortably leads the user through all the necessary calibration routines. An open interface supports the integration of all required measurement instruments (power supplies, spectrum analyzers, power measurement instruments, probe stations).