A base-band (kHz to MHz range) very low frequency active impedance tuner allows controlling the reflection factor at a multitude of frequencies in the base-band for systematic evaluation of the effect of bias networks on nonlinear quantities of a RF transistor, like EVM and ACPR. It comprises an electronically controlled active heterodyne feedback loop, frequency mixing, filtering and a remotely controlled digital electronic tuner. The digital electronic tuning feedback module operates at standard octave band GHz range, which allows controlling impedances over several decades in the base-band frequency range.