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” 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.