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Elucidating how local oscillator (LO) pulling affects a wireless direct-conversion transmitter that uses constant envelope modulation is of concern for global system for mobile communication (GSM). Therefore, this paper presents a phase dynamic model for a phase-locked loop (PLL) under directly modulated self-injection to evaluate the degraded phase noise performance of an LO pulled by a feedback modulation signal from the power amplifier output. Additionally, based on theoretical analysis, the proposed model can instruct system designers on how to optimize PLL parameters, as well as introduce an inner self-injection to minimize the impact of pulling effects. The improved performance is verified by implementing a Gaussian minimum-shift keying transmitter for GSM applications. Furthermore, the theoretical and experimental results correlate well with each other.