A test method allows investigating the dependence of microwave gain, power and linearity behavior of transistors, in which modulated RF signal is injected, as a function of source and load impedance at the signal RF carrier frequency and its harmonics as well as at the modulation (baseband) frequency, using passive programmable tuners. A calibration method generates data allowing the test setup to simultaneous and independent tune at RF and baseband frequencies without the need of on-line vector signal measurement. This allows optimizing baseband frequency impedances for nonlinear amplifier performance with modulated signal, such as ACPR, EVM, IMD etc. Baseband tuning is done using low frequency programmable impedance tuners inserted in the DC branch of the bias networks, which operate as frequency diplexers.