Quartz crystal characteristics

Quartz is a kind of piezoelectric material. It is found to be a transparent crystal produced in nature. It shows a very stable piezoelectric effect. Its chemical and mechanical properties have attracted great attention in early electronic experiments.

The main component of quartz is silica. Although 14% of the earth's crust surface is composed of SiO2 silica, the relatively usable part is rare because of the lack of purity and its defects and defects.

The crystal element has the ability to form a very stable frequency generation circuit. The oscillation mode and angular orientation are shown in Table 1. It controls the characteristics of the crystal, such as equivalent circuit parameters, frequency, temperature characteristics, load capacitance characteristics and frequency aging. These features are extremely important from the point of view of use and will be described below.

·Oscillation mode

The thickness shear oscillation mode has been widely used, and its resonant frequency is determined by the wafer thickness, such as at, BT corner cutting; The tuning fork oscillation mode is determined by the chip length. The different oscillation modes shown in Table 1 depend on the cutting angle, frequency range, thickness or length, and the relationship between frequency and curvature constant.

·Equivalent circuit

The resonant frequency of quartz crystal is usually determined by the size of the vibrating plate of the combined oscillation mode. The equivalent circuit of oscillating crystal is helpful to explain the basic concepts that govern the performance of crystal.

As shown below, the crystal equivalent circuit is represented as follows:

L1: dynamic inductance (mechanical oscillation)

C1: dynamic capacitance (mechanical elasticity)

R1: series resistance (energy loss)

Co: (static capacitance) Co = CT Cl (static capacitance is the stray capacitance parallel to the electrodes and between the two leads)

·Load capacitance characteristics

The load capacitance CL is the sum of the crystal base and the distributed capacitance across the oscillation circuit, which is the conditional factor determining the crystal in the oscillation circuit. In the scope of crystal use, its function is regarded as inductive reactance, that is, the oscillation circuit presents a negative resistance-r and a capacitance CL connected in series on the circuit, which is called "load capacitance". The relationship between load capacitance and frequency is nonlinear. When the capacitance is small, the frequency change is large, but when the capacitance increases, the frequency change becomes small. In the circuit, if the load capacitance decreases and the oscillation frequency has a great margin, the frequency stability will be greatly affected, even a small change.

·Equivalent circuit of oscillation circuit

The crystal is regarded as an inductive reactance in the oscillation circuit: in order to improve the starting condition of the oscillation circuit, the best method is to increase the negative resistance value - r of the oscillation circuit. If the negative resistance value of the oscillation circuit is insufficient, the crystal resonance resistance will be too large, resulting in poor starting condition of the crystal. Therefore, the negative resistance value of the oscillation circuit needs to be greater than 5 to 10 times of the resonance resistance in the design.

·At cut frequency temperature characteristic

Temperature coefficient is the stability or deviation of crystal resonant frequency when temperature changes. The oscillation mode, the relationship between the axial direction and the wafer plane, the wafer size and harmonics are all the factors that determine the temperature coefficient.

The frequency versus temperature characteristic curve of the cubic equation expressed by at cutting is the most common at present. It shows the best frequency stability in a wide temperature range. The cutting angle of the wafer is the allowable error of the required frequency within the normal operating temperature range. In fact, in the continuous machining process of cutting and polishing, the wafer will be scattered due to the machining accuracy.