This is a 12Mhz Crystal Oscillator HC49S The crystal oscillator is used in microprocessor clock, network card, various consumer electronics, etc.12 MHz Crystal Oscillator provides clock pulses of 12 MHz frequency, Used in USB 1.0 and 2.0 systems (with an accuracy of 500 ppm) as the reference clock for the full-speed PHY rate of 12 Mbit/s, or multiplied up using a PLL to clock high-speed PHYs at 480 Mbit/s; common clock for Intel 8051 microprocessors also used in CAN bus systems. Standard clock frequency for MCS-51 MCU's at 1MIPS. Crystal Oscillator is an Electronics Oscillator circuit which uses the mechanical resonance of a vibrating crystal of piezoelectric material to generate an electrical signal with an accurate frequency. It also has automatic amplitude control and frequency drift is also very low due to change in temperature. Crystal Oscillators are only suitable for high-frequency application. 

Specification :-