A Novel Approach for Mitigation of RF Oscillator Pulling in a Polar Transmitter

A novel technique for mitigation of self interference in a highly integrated SoC transmitter is presented. The interference originates from the internal power amplifier (i.e., aggressor) that leads to injection pulling of the local RF oscillator (i.e., victim). The characteristic of injection pulling was found to be dependent on the AM signal applied to the power amplifier. A hypothesis describing the mechanism of injection pulling of the local oscillator is presented. A mathematical model is developed to study the characteristics of this self interference verified then by measurements. Based on this model, a digitally controlled delay circuit is proposed and implemented in a digital polar GSM/EDGE transmitter that makes the system less susceptible to injection pulling through automatic phase adjustment between the aggressor and the victim. Compliant EVM and spectrum performance is measured on SoC fabricated in 65-nm CMOS showing the effectiveness of the proposed solution.