Why must a very stable reference oscillator be used as part of a phase-locked loop (PLL) frequency synthesizer?

A phase-locked loop (PLL) synthesizer works by continuously comparing the phase of an adjustable voltage-controlled oscillator (VCO) to a highly stable reference oscillator. If the reference oscillator itself has phase variations (jitter), the PLL will attempt to track them, multiplying those errors into the final output frequency. This resulting error manifests as phase noise, which broadens the transmitted signal and severely degrades the selectivity of a receiver. Therefore, using a high-quality, stable reference oscillator (usually a temperature-compensated crystal oscillator) is absolutely essential to keep the synthesizer's output clean. The distractors focus on amplitude variations or harmonic distortion. Because a PLL is specifically a phase-tracking system, it usually utilizes limiters and is largely immune to amplitude variations, but it remains extremely sensitive to any phase instabilities in the reference signal.