What are the characteristics and specifications of crystal oscillators?

Crystal oscillators use piezoelectric quartz resonators providing excellent frequency stability (1-100 ppm typical, 0.1 ppm for TCXO). Key specifications include: Frequency tolerance (initial accuracy), Temperature stability (variation over temperature range), Aging (long-term drift, typically 1-5 ppm/year), Phase noise (affects signal purity), and Load capacitance (must match circuit for accurate frequency). Operating modes include fundamental and overtone. Pierce, Colpitts, and Butler configurations are common crystal oscillator circuits.