Characterisation, analysis and injection of two-tone third-order intermodulation products in an amplifier

A new mathematical description of the relationship between injected and output third-order intermodulation (IM3), to predict the injection signals required for predistortion correction of a power amplifier (PA), without manual optimisation is presented. A new concept of interactions and an iteration algorithm are also presented in order to improve this technique. The method is demonstrated with a commercial 2 GHz PA driven hard in a two-tone test with a range of input power and tone-spacing. The measurement results show that IM3 in a two-tone test can be suppressed by at least 60 dB, when the interactions between the upper and lower IM3 generation and the correction process are properly accounted for. The technique can deal with memory effects by cancelling unequal upper and lower sidebands.