21.8 A pulling mitigation technique for direct-conversion transmitters

Despite versatility and low power consumption, direct-conversion transmitters suffer from a fundamental drawback: the local oscillator disturbance by the power amplifier, through unwanted electromagnetic or capacitive coupling [1,2]. As shown in Fig. 21.8.1, the pulled oscillator spectrum is no longer a single-tone sinusoid, which can drastically degrade the transmitter EVM or spectrum mask. To alleviate this, time-consuming and often unpredictable optimization of the floor plan, package, and PCB is required to maximize the isolation between the PA and VCO. Ultimately, this issue may prohibit the use of this architecture for many applications, leading to higher power consumption. Moreover, in many modern radios it is common to have more than one VCO on-chip (Fig. 21.8.1) to support various features such as FDD, carrier aggregation, or coexistence, further exacerbating the problem through multiple-VCO cross-coupling. To address these concerns we propose a calibration scheme that corrects any pulling effect regardless of its source or magnitude. Our approach is fully digital and is automatically calibrated, leading to a reliable and robust solution, and has little impact on power consumption, size, or transmitter noise.