Crystal oscillators are available in single-end logic output like CMOS and differential logic output like LVPECL, LVDS and HCSL. Compared with single-end signals, differential signals have the advantages of good noise immunity, lower EMI, and lower power consumption for high-speed data transmission systems. LVPECL stands for “Low-Voltage Positive Emitter-Coupled Logic” and LVDS stands for “Low-Voltage Differential Signaling”, and HCSL stands for “High Speed Current Steering Logic”. Specific termination circuits are required for LVPECL, LVDS, and HCSL Logic. See chart shown here for details.