Direct-conversion transmitter with resistance to local oscillator pulling in non-constant envelope modulation systems

This paper studies the local oscillator (LO) pulling problems associated with a direct-conversion transmitter (DCT) that uses non-constant envelope modulation schemes. The relevant theory provides a time-domain LO model with a modulated outer self-injection to predict the phase fluctuation of an LO that is pulled by the transmitter's modulated output signal. Based on a theoretical analysis, the proposed model can be used to evaluate the degraded quality of the transmission signal in terms of error vector magnitude (EVM) and adjacent channel power ratio (ACPR). To mitigate the effects of LO pulling, a novel approach for reducing the AM-FM and PM-FM distortions by a combination of second-point VCO modulation and inner self-injection is proposed. The improved performance is verified by implementing a Quadrature-Phase-Shift-Keying (QPSK) transmitter for cdma2000 1× applications. The theoretical and experimental results agree closely with each other.