The core function of quartz crystal oscillator is a high-precision frequency source, which is widely used in wireless data transmission and timing. It is often known as the "heart" of electronic products. There are many types of crystal oscillators. Let's talk about the classification of crystal oscillators. 1、 Package by crystal oscillator Quartz crystal oscillators can be divided into direct insertion (DIP) and chip mounting (SMD) according to packaging or installation methods. SMD crystal oscillators are characterized by small size and easy mounting, and are mainly used in electronic products with relatively small space, such as clocks, tablets, microcomputers, etc. SMD is used in smart phones, wireless Bluetooth, tablet computers and other digital products. 2、 According to crystal oscillator type According to the type of crystal oscillator, it can be divided into passive crystal oscillator (crystal resonator) and active crystal oscillator (crystal oscillator). 1） Passive crystal oscillator 1. Ordinary passive resonator (low frequency KHz/high frequency Mhz): KHz is mainly used for clock signals of mobile terminals, consumer electronics, small electronic products, clocks, industrial automatic control and other applications. MHz is mainly used as the reference frequency signal for mobile terminals, network devices, automotive electronics, household electronics, consumer electronics, smart home, smart wear, smart medical and other new applications. 2. Passive crystal oscillator with built-in thermistor (TSX): mainly used in intelligent terminals, navigation and positioning applications. （2） Active crystal oscillator Active crystal oscillator can also be called crystal oscillator, that is, passive crystal oscillator+IC: 1. General crystal oscillator (SPXO): mainly used in communication equipment, network equipment, mobile TV, DVD, Blu ray player, video monitoring, audio equipment, data and image processing and other related equipment. 2. Temperature compensated crystal oscillator (TCXO): a quartz crystal oscillator that reduces the change of oscillation frequency caused by ambient temperature change through an additional temperature compensation circuit. It is mainly used in 5G small base stations, intelligent terminals, the Internet of Things, navigation, WiFi, intelligent medicine, etc. 3. Voltage controlled crystal oscillator (VCXO): a quartz crystal oscillator with variable or adjustable oscillation frequency by applying external control voltage. It is mainly used for data encoding and transmission of communication equipment, switch, network equipment, mobile TV, DVD, Blu ray player, video monitoring, audio equipment, debugging and demodulation, frequency synthesizer, etc. 4. Constant temperature crystal oscillator (OCXO): use a constant temperature bath to keep the temperature of the crystal oscillator or quartz crystal oscillator constant, and reduce the change of the oscillator output frequency caused by the ambient temperature change to the minimum crystal oscillator. It is mainly used in 5G mobile communication synchronization, base station, aerospace, navigation, power, traffic control, instruments and meters with high requirements on frequency and temperature. 3、 According to crystal frequency 1. Low frequency KHz: mainly 32.768khz, such as YT310S, YT-38 and YT-26 of Yangxing YXC. It is mainly used to provide clock RTC signal and standard frequency source of clock signal for electronic circuit. 2. High frequency: 0-50MHz: mainly used in mobile communication, GPS positioning, automatic control system, audio-visual equipment, etc. 3. High fundamental frequency: above 50MHz: higher frequency brings better communication effect, which is mostly used in 5G, WiFi and other scenarios.