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High-frequency oscillators developed in CMOS technologies for space applications are vulnerable to single-event transient (SET) effects caused by ionizing radiation strikes. We have designed a phase-locked loop (PLL) with a quadrature phase current-controlled oscillator (QCCO) to overcome this limitation. The feedback mechanism in the QCCO helps in improving the SET tolerance, while parallel coupling achieves wide tuning range. The oscillator is irradiated with laser pulses at a certain pulse repetition frequency, which results in spurious tones in the oscillator output spectrum. The strengths of these spurs are used for evaluating the SET tolerance of the oscillator. Under identical conditions, experiments show that the QCCO has an average of 15-dB lower spur level compared to a conventional single-phase $LC$ voltage-controlled oscillator (SVCO) and an SVCO-based PLL loses lock at a certain laser pulse energy, while the QCCO-based PLL remains in the locked state. These results confirm that the QCCO significantly improves the reliability of high-frequency PLLs in radiation environments. [ABSTRACT FROM AUTHOR]