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Adaptive digital linearization of RF power amplifiers

The performance of an adaptive digital technique for the linearization of RF power amplifiers is investigated. Cubic spline interpolation is used to estimate the amplifier's AM-AM and AM-PM characteristics. Using the computed characteristic coefficients, the baseband input signal is appropriately predistorted to compensate for the amplifier nonlinearity. This method has significantly better suppression of the intermodulation products than other predistortion techniques. The out-of-band power emission is also significantly reduced. The performance of baseband predistortion linearization techniques is adversely affected by modulator and demodulator impairments. A digital correction technique is presented to compensate for these imperfections. In this technique, part of the RF signal is fed to an envelope detector. The detector output and the baseband signal are used to estimate the impairment values, using the Newton-Raphson method. The estimated impairment values are then used to compensate for the modulator/demodulator impairments. Spurious signals can be suppressed by more than 30 dB using this technique.