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Design of a Direct Conversion Transmitter to Resist Combined Effects of Power Amplifier Distortion and Local Oscillator Pulling

This work elucidates how the combined effects of power-amplifier distortion and local-oscillator (LO) pulling adversely impact a wireless direct-conversion transmitter (DCT) that adopts a time-varying envelope modulation. An analytical model is developed to evaluate the deterioration of DCT output signal quality, including error vector magnitude, adjacent channel power ratio, and spectral regrowth. Additionally, an integrated approach for PA linearization and anti-LO pulling is designed based on an open-loop digital-predistortion method and the proposed analog feedback compensation mechanism. Experimental results demonstrate that a quadrature-modulation-based DCT that incorporates the proposed approaches can significantly improve the LO spectral purity, while achieving a high linearity and efficiency performance simultaneously. These attributes are highly desired for third-generation systems such as cdma2000 1x and W-CDMA.