Discuss the principles and challenges of designing low-jitter clock sources for high-speed digital systems, including voltage-controlled oscillators (VCOs), phase-locked loops (PLLs), and crystal oscillators.

Low-jitter clock sources are critical for maintaining timing accuracy and signal integrity in high-speed digital systems. Challenges include minimizing phase noise, jitter, and frequency drift. Techniques like PLL-based clock generation, VCO design optimization, and external clock conditioning are used to achieve low-jitter clock sources suitable for high-speed applications.