A little background on the acronyms. Historically, in the US, military requirements have been the main impetus for research on crystal oscillators. The US Army sponsored most of the research, especially in the early days. According to folklore, the abbreviation XO instead of CO came about because, in the military, CO is the abbreviation for "commanding officer" and because "crystal" sounds a little like "xtal". The rest of this module will focus on SPXO and VCXO, although Epson offers all the other high-end oscillator as well. An SPXO is a simple type of oscillator that consists of a crystal unit and an oscillation circuit, with no voltage control or temperature compensation. A VCXO is similar to the SPXO, but has a voltage control function to vary the output frequency. For example, a VCXO used in a data application is used to lock onto incoming data which maybe at a slightly different clock frequency from the system, thus synchronizing with system timing to avoid missing or dropping data packets.