Crystal oscillator knowledge

In the electronic circuit, if the IC is compared to the brain, then the crystal is the heart.

Oscillators with high precision and high stability are widely used in various types of oscillating circuits such as color TVS, computers, remote controls, and communication systems for frequency generators, clock signals for data processing equipment and reference signals for specific systems.

First, the basic principle of quartz crystal oscillator

1, the structure of quartz crystal oscillator

Quartz crystal oscillator is a resonant device made of the piezoelectric effect of quartz crystal (silicon dioxide crystal), its basic composition is roughly: From a quartz crystal according to a certain azimuth cut thin slice (referred to as wafer, it can be square, rectangular or circular, etc.), in its two corresponding surfaces coated with a silver layer as an electrode, each electrode welded a lead wire connected to the pin, coupled with the packaging shell constitutes a quartz crystal resonator, referred to as quartz crystal or crystal, crystal oscillator. Its products are generally encapsulated in a metal shell with a glass shell, ceramic or plastic package.

2. Piezoelectric effect

If an electric field is applied to the two electrodes of a quartz crystal, the chip will be mechanically deformed. Conversely, if mechanical pressure is applied to both sides of the wafer, an electric field will be generated in the corresponding direction of the wafer, a physical phenomenon called the piezoelectric effect. If an alternating voltage is applied to the two poles of the wafer, the wafer will produce mechanical vibration, and the mechanical vibration of the wafer will produce an alternating electric field. Under normal circumstances, the amplitude of the mechanical vibration of the wafer and the amplitude of the alternating electric field are very small, but when the frequency of the external alternating voltage is a specific value, the amplitude is significantly increased, much larger than the amplitude at other frequencies, this phenomenon is called piezoelectric resonance, which is very similar to the resonance phenomenon of the LC circuit. Its resonant frequency is related to the cutting method, geometry and size of the chip.

3. Symbol and equivalent circuit

When the crystal does not vibrate, it can be regarded as a flat plate capacitor called electrostatic capacitor C, its size is related to the geometric size of the chip and the electrode area, generally about several PF to dozens of PF. When the crystal oscillates, the inertia of the mechanical vibration can be equivalent to the inductance L. Generally, the value of L ranges from tens of mH to hundreds of mH. The elasticity of the chip can be equivalent to the capacitance C, and the value of C is very small, generally only 0.0002 ~ 0.1pF. The loss caused by friction during wafer vibration is equivalent to R, and its value is about 100Ω. Because the equivalent inductance of the chip is large, and C is small, and R is small, the quality factor Q of the loop is large, up to 1000 ~ 10000. In addition, the resonant frequency of the wafer itself is basically only related to the cutting method, geometry, and size of the wafer, and can be done accurately, so the oscillation circuit composed of quartz resonators can obtain high frequency stability.

Although the crystal oscillator is a small device, its failure will cause the shutdown of the entire system. For example, when investigators analyzed the 1972 Fremont, California, train crash, they found that the cause was a faulty crystal oscillator on a control panel. Therefore, small crystal oscillator is a big article, and the role and significance of crystal oscillator can not be underestimated.