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This paper aims to study the local oscillator (LO) pulling effects on a wireless direct-conversion transmitter (DCT) that uses constant envelope modulation. A phase dynamic model for a phase-locked loop (PLL) under directly modulated self-injection is proposed to predict the degraded phase noise performance for a LO pulled by a feedback modulation signal from the power amplifier output. In addition, the model can analyze the deterioration of the transmit signal quality due to such pulling effects. Comparison between the theoretical and experimental results shows excellent agreement.