Traditional load pull was the first system to automatically measure device performances under various source and load impedances. These test benches are using pre-characterized passive tuners for impedance matching and scalar power detectors for measurements.
Based on the pre-characterization of the impedance tuners, the measurements are relying on the set of measured or interpolated s-parameters of the tuners at different impedance conditions. Raw measurements from scalar instruments (Power meters) are de-embedded, in real time, to the DUT reference plane using the loss calculation of tuners and other components in the setup.