An important term to understand is frequency stability. Unlike a crystal unit where frequency tolerance is just the frequency deviation at 25°C, for a crystal oscillator the frequency stability is the overall frequency deviation which includes deviation at 25°C, over the temperature range, reflow drift, supply voltage and output load. Aging may or may not be included depending on the type of oscillator. The common outputs are single-ended like CMOS and differential outputs like LVDS, LVPECL and HCSL. Differential outputs are a newer technology and has many advantages including fast bit rates, low power consumption and good noise performance.