Source and load pull techniques

Modulation schemes with high crest factors are common for the air-link of all modern mobile communication systems. For such signals highly linear power amplifiers are needed. Furthermore, to improve battery lifetime and to ease thermal management highly efficient power amplifiers are mandatory. Thus, power stage circuits attaining the highest performances with respect to efficiency and linearity are key to modern communication systems. Load-pull setups are very powerful tools to characterize the nonlinear behavior of a microwave transistor under different load conditions to verify or improve large-signal computer models. Load-pull setups enable by their structure also direct access to desired source and load terminations for power amplifier design. Thus, these systems allow for rapid PA design without simulation. After a very brief theory of power amplifier design this chapter presents the basic source and load pull setups, where it is shown that neglecting harmonic components can significantly alter measurement results. Thus, harmonic source/load pull setups are presented to circumvent this problem. This chapter describes different ways to implement variable loads required for source/load pull setups and discusses important system issues such as deembedding, calibration and the limitation of such systems. Examples of passive and active harmonic load pull setups including measurement results finally complete this chapter.