Load-pull is the colloquial term applied to the process of systematically varying the impedance presented to a device under test (DUT), most often a transistor, to assess its performance and the associated conditions to deliver that performance in a network.
Load-pull is required when superposition is no longer applicable, which occurs under large-signal operating conditions that make linear approximations unusable.
Load-pull is the most common method globally for RF and MW power amplifier (PA) design, transistor characterization, semiconductor process development, and ruggedness analysis.
While there are in theory no physical limits on the frequency of which load-pull can be performed, most load-pull systems are based on passive distributed networks using either the slab transmission line in its TEM mode or the rectangular waveguide in its TE01 mode.
Lumped tuners can be made for HF and VHF frequencies, whereas active load-pull is ideal for on-wafer mm-wave environments, where substantial loss between the tuner and DUT reference-plane limits maximum VSWR.