Quartz crystal characteristics

Quartz is a piezoelectric material, which is found to be a transparent crystal produced in nature. It exhibits 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 surface of the earth's crust is composed of Sio2 silica, the relatively usable part is relatively rare because of insufficient purity required and its defects and flaws.

The crystal element has the ability to form an extremely stable frequency generating circuit. Its oscillation mode and angle orientation are shown in Table 1. It dominates the characteristics of the crystal, such as equivalent circuit parameters, frequency, temperature characteristics, load capacitance characteristics, and frequency aging. From the viewpoint of use, these characteristics are extremely important and will be described as follows.

·Oscillation mode

The thickness-shear oscillation mode has been widely used. Its resonance frequency is determined by the thickness of the wafer, like AT and BT cut corners; the tuning fork oscillation mode is determined by the length of the wafer. The different oscillation modes shown in Table 1 are determined by the relationship between the cutting angle, frequency range, thickness or length, and frequency and curvature constant.

·Equivalent Circuit

The resonant frequency of a quartz crystal is usually determined by the size of the vibrating plate of the combined oscillation mode. The equivalent circuit of the oscillating crystal is useful for explaining the basic concepts that govern the performance of the crystal.

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

L1: Dynamic inductance (mechanical oscillation)

C1: Dynamic capacitance (mechanical elasticity)

R1: Series resistance (energy loss)

Co: (Electrostatic capacitance) Co=Ct-CL (Electrostatic capacitance is parallel to the electrodes plus the stray capacitance between the two leads)

·Load capacitance characteristics

The load capacitance CL is the sum of the crystal base and the distributed capacitance connected across the oscillation circuit. This is the conditional factor that determines the crystal on the oscillation circuit. Its function is regarded as inductive reactance in the range of crystal use, which means that the oscillation circuit is What is shown is a negative resistance -R and a capacitor CL connected in series on the circuit, this capacitor is called "load capacitance". The relationship between load capacitance and frequency is non-linear. When the electric capacity is small, the frequency change is large, but when the electric capacity increases, the frequency change becomes small. In the circuit, if the load capacitance is reduced and the oscillation frequency has a large margin, then the frequency stability will be greatly affected, even if it is 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 oscillation condition of the oscillation circuit, the best way is to increase the negative resistance value -R of the oscillation circuit. If the negative resistance value of the oscillation circuit is insufficient, it will cause the crystal The resonance resistance is too large, resulting in poor crystal starting conditions, so the oscillator circuit needs to be designed with a negative resistance value greater than 5 to 10 times the resonance resistance.

·AT cut frequency temperature characteristics

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

 The frequency-to-temperature characteristic curve of the cubic equation displayed by AT cutting is currently the most common, and it shows the best quality of frequency stability under 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 processing of wafer cutting and polishing, the cutting will be scattered due to the processing accuracy.