Basic characteristics, frequency stabilization conditions, and circuit design of quartz crystal

Crystal oscillator

Quartz Oscillator 3225 20M OSC

Quartz crystal oscillator is a high-precision, high-stability oscillator. It is a resonant component made up of the piezoelectric effect of quartz crystal to replace the LC frequency selective circuit with frequency stability up to 10 - 9 - 10 - 11 . It is used in oscillation circuits of various electronic products, such as frequency generators in communication systems. Function: Generate clock signal for data processing equipment, provide system oscillation pulse, stabilize frequency, select frequency. This article will briefly introduce the basic characteristics of the quartz crystal oscillator, the frequency stability conditions, and the circuit design.

First, the basic characteristics of crystal materials

The quartz crystal oscillator is a resonant device made by utilizing the piezoelectric effect of a quartz crystal (crystal of silicon dioxide), and its basic constitution is roughly: cutting a thin sheet from a quartz crystal at a certain azimuth angle, in its two A corresponding layer is coated with a silver layer as an electrode, and each lead is soldered to a pin on each electrode, and a packaged outer casing is formed to form a quartz crystal resonator, which is simply referred to as a quartz crystal or a crystal or a crystal oscillator. Its products are typically packaged in a metal case and are also available in glass, ceramic or plastic.

Quartz crystal resonator structure

The quartz crystal is deformed under pressure and is also polarized. The polarization is proportional to the pressure. This phenomenon is called the "positive piezoelectric effect". Conversely, under the action of the electric field, the crystal is deformed, and its deformation is proportional to the electric field strength. This phenomenon is called "inverse piezoelectric effect." ".

(a) Piezoelectric effect (b) Positive piezoelectric effect (c) Inverse piezoelectric effect.

Among the more than twenty types of crystals with piezoelectric effects, quartz crystals are one of the most satisfactory materials in wireless communication equipment. It has high mechanical strength, stable physicochemical properties, low internal loss, etc. Devices made of it are widely used in frequency control and frequency selection circuits. Quartz sheets have different orientations, and their piezoelectric, elastic and strength properties are different. The performance of resonators made with them is also different. Several useful cutting methods have been discovered.

Second, the stability condition of the quartz resonator

The frequency stability of a quartz crystal oscillator is related to three aspects: load capacitance, excitation level, and frequency temperature characteristics.

1, load capacitance. Quartz resonators generally function as an inductive component in the oscillating circuit, and other components of the circuit can be equivalent to an equivalent effect of load capacitance and quartz resonator and frequency stability.

All the effective capacitances seen from the two legs of the quartz crystal oscillator element toward the oscillating circuit are the load capacitances that the oscillating circuit applies to the quartz crystal oscillator. The load capacitance together with the quartz crystal determines the operating frequency of the circuit.