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Large-signal adjacent-channel power ratio load-pull contours of a GaAs MESFET and a GaAs HEMT excited by Â¿4-DQPSK modulation are demonstrated for the first time using an automated load-pull system. It is shown that in general there is only a weak relationship between two-tone third-order intermodulation and adjacent-channel power ratio for the (Japanese) Personal Digital Cellular standard. The relationship is both load impedance and device technology dependent insofar as two-tone linearity characterization cannot generally be used to optimize adjacent-channel power. The load-pull system presented here is modulation and device technology independent.