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An experimental characterization of state of the art microwave power amplifiers for high capacity 16 QAM digital radio systems is given. The effects of amplifier nonlinearities on 16 QAM signals are qualitatively demonstrated with the aid of real time signal state diagrams. A quantitative description is obtained by measured degradation of bit error ratio characteristics as a function of output power level. The comparison of the power capability shows advantages for modern high linearity TWTs over GaAs FET amplifiers at slightly superior efficiency. Techniquss for fast and easy characterization of nonlinear amplifiers are derived. Finally, methods to improve amplifier linearity are discussed.