The term “load pull” consists of varying or “pulling” the load impedance seen by a device-under-test (DUT) while measuring the performance of the DUT. Similarly, “source pull” is Involves changing the source impedance instead of the load impedance. This is typically achieved by using a passive instrument called a tuner, which allows the user to “tune” the impedance seen at the input or output of the DUT. Load and source pull are used to measure a DUT in actual operating conditions. This method is important for large signal, nonlinear devices where the operating point may change with power level or tuning. Load or source pull is not usually needed for linear devices, where performance with any load can be predicted from small signal S-parameters.
Scalar Load Pull
When using a scalar, also known as a traditional load pull setup, the aim is to determine the source and load impedance conditions which enable the DUT to reach the targeted performances (Pout, Efficiency, Gain, etc …). Starting from the target definition Power performances versus Zsource and Z load, ISO-circles plot can be plotted. This leads to the determination of optimal operating conditions. The traditional load pull architectures are mainly based on power meter use, both for input and output power measurements. Using an automated tuner and a proper calibration of the path between the power sensors and the DUT reference planes enables users to measure the value of the power seen by the DUT.