Towards SET Mitigation in RF Digital PLLs: From Error Characterization to Radiation Hardening Considerations

In this work, we analyze the characteristics of single-event transient (SET) generation and propagation in a digital phase-locked loop (DPLL) circuit, designed to achieve speeds applicable to mixed-signal RF operations. Our analysis shows that the sensitivity of a PLL system is strongly dependent on which of its modules is subjected to ionizing radiation. Computer simulations of single-event transients on the phase-frequency detector module and the voltage-controlled oscillator module indicate that their radiation responses have a negligible impact on the PLL normal operations. More importantly, our findings identify the sensitivity of the charge pump module as the dominant contributor to the radiation vulnerability of the PLL system. The charge pump incorporates a design element that is favorable to achieving high-speed operations, but simultaneously introduces a circuit configuration which can be easily perturbed by the impact of a heavy-ion. Hardening by design techniques that could be used to mitigate this issue are also discussed.