The sensitivity of total-ionizing-dose (TID) response on a single-event hardened phase-locked loop (PLL) fabricated in a 130-nm silicon-on-insulator process is investigated. Based on the study of device parameter degradation under radiation exposure, the changes of dc and RF performance of the PLL have been presented. TID experiments on the PLL show that the power-down current increases by 1.5 times, and the tuning range drops by 11.5%. At 600 MHz and up to 500 krad(Si), the integrated phase noise increases by 29.2%, and the reference spur increases by 3.4 dB. Finally, the mechanism underlying impact of TID on the phase noise and reference spur has been discussed comprehensively, which combined measured observations with circuit modeling and simulations.