Principles and Characteristics of Quartz Crystal Resonators

Quartz crystal resonator is an oscillator with high precision and good stability, which is widely used in radar, navigation, telemetry, radio, instrumentation and other fields, as well as frequency generators in communication systems, clock signals of data processing equipment and specific system reference signal.

The basic principle of quartz crystal resonator:

1. The structure of the quartz crystal resonator

Quartz crystal resonator is a resonant device made of piezoelectric effect of time-responsive crystal (silicon dioxide crystal). Its basic structure is roughly as follows: a slice is cut from the crystal at a certain azimuth angle (referred to as the slice can be a square, a rectangle or a circle, etc.), and a silver layer is coated on its corresponding two surfaces as electrodes, Solder a lead to each electrode to connect to the pin, and add an encapsulation case to form a quartz crystal resonator, abbreviated as a time crystal or. Its products are generally packaged in metal casings, but also in glass casings, ceramics or plastics.

2. Piezoelectric effect

If an electric field is applied to the two electrodes of the crystal at the same time, the wafer of the quartz crystal resonator will be mechanically deformed. Conversely, if mechanical pressure is applied to both sides of the wafer, an electric field is created in the corresponding direction of the wafer, a physical phenomenon known as the piezoelectric effect. If an alternating voltage is applied to the two poles of the wafer, the wafer will generate mechanical vibration, and the mechanical vibration of the wafer will generate an alternating electric field. In general, 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 applied alternating voltage is a certain value, the amplitude increases significantly, which is much larger than other frequencies. This phenomenon is called piezoelectric resonance and is very similar to the resonance phenomenon of LC circuits. Its resonant frequency is related to cutting method, geometry and wafer size.

3. Symbols and Equivalent Circuits

When the crystal of the quartz crystal resonator does not vibrate, it can be regarded as a flat capacitance, which is called electrostatic capacitance c, and its size is related to the geometric size of the wafer and the electrode area, generally ranging from a few PF to several tens of PF. When the crystal oscillates, the inertia of mechanical vibration can be equivalent to the inductance l, and the general value of l is tens of mH to hundreds of mH. The elasticity of the wafer 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 when the wafer vibrates is equivalent to R, and its value is about 100 . Because the equivalent inductance of the chip is very large, C is very small, and R is also very small, so the quality factor Q of the loop is very large, which can reach 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, which can be fabricated. Therefore, generally speaking, the oscillator circuit composed of quartz crystal resonators can obtain higher frequency stability.