Basic Introduction

Crystal oscillator refers to a quartz crystal from a quartz crystal according to a certain azimuthal angle cut off thin slices (referred to as wafers), quartz crystal resonator, referred to as quartz crystals or crystals, crystal vibration; and in the package inside the addition of the IC composition of the oscillator circuit of the crystal element is called a crystal oscillator. Its products are generally packaged in metal shells, but also used in glass shells, ceramics or plastic packages.

Principle of operation

Quartz crystal oscillator is a high-precision and high-stability oscillator, is widely used in color TV, computers, remote control and other types of oscillator circuits, as well as communication systems used in frequency generators, data processing equipment to generate clock signals and provide reference signals for specific systems.

Quartz crystal oscillator is the use of quartz crystals (silicon dioxide crystals) piezoelectric effect made of a resonant device, its basic composition is roughly: from a quartz crystal according to a certain azimuthal angle cut down the sheet (referred to as the wafer, which can be a square, rectangular or round, etc.), in its two corresponding surfaces are coated with silver as the electrodes, each electrode is welded to the pin on each one of the lead, and then add the The package case constitutes a quartz crystal resonator, referred to as quartz crystal or crystal, crystal oscillator. The product is usually encapsulated in a metal case, but there are also glass, ceramic or plastic encapsulation.If an electric field is applied to the two electrodes of a quartz crystal, the wafer 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 known as the piezoelectric effect. If an alternating voltage is applied to the poles of the wafer, the wafer will generate mechanical vibration, and at the same time 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 is very small, but when the frequency of the applied alternating voltage is a specific value, the amplitude increases significantly, much larger than the amplitude of other frequencies, this phenomenon is called the piezoelectric resonance, which is very similar to the resonance phenomenon of the LC circuit. Its resonant frequency is related to the wafer's cutting method, geometry, size, and so on.

When the crystal does not vibrate, it can be regarded as a flat capacitor called electrostatic capacitance C, the size of which is related to the geometric size of the wafer, the electrode area, generally about a few PF to dozens of PF. when the crystal oscillation, the inertia of the mechanical vibration can be equated with the inductance L. Generally, the value of L is tens of mHF, and the value of L is tens of mHF. Generally, the value of L is tens of mH to hundreds of mH, and the elasticity of the wafer can be equated with the capacitance C. The value of C is very small, generally only 0.0002~0.1pF, and the loss due to friction when the wafer vibrates can be equated with R, whose value is about 100 Ω. Since the equivalent inductance of the wafer is very large, while C is very small, and R is also small, the quality factor of the circuit is very large, which is up to 1,000~10,000, and together with the resonant frequency of the crystal itself, the inductance of the wafer is very large. Since the resonant frequency of the wafer itself is basically only related to the wafer's cut, geometry, and size, and can be made precise, oscillator circuits composed of quartz resonators can be used to obtain a high degree of frequency stability.

Computers have a timing circuit, although the general use of the word "clock" to indicate that these devices, but they are not actually the usual meaning of the clock, they are called a timer (timer) may be more appropriate. A computer's timer is usually a precision-machined quartz crystal, which oscillates within its tension limits at a frequency that depends on how the crystal itself is cut and how much tension it is under. There are two registers associated with each quartz crystal, a counter and a holdingregister. Each oscillation of the quartz crystal decrements the counter by 1. When the counter decreases to 0, an interrupt is generated and the counter is reloaded from the holding register with the initial value. This method makes it possible to program a timer to generate 60 interrupts per second (or at any other desired frequency). Each interrupt is called a clocktick.

Crystal can be equated electrically into a capacitor and a resistor in parallel and then a capacitor in series two-terminal network, electrical engineering on this network has two resonance points to the frequency of the higher and lower points of which the lower frequency for the series resonance, the higher frequency for the parallel resonance. Due to the characteristics of the crystal itself resulting in the distance between the two frequencies is quite close, in this very narrow frequency range, the crystal is equivalent to an inductor, so as long as the crystal on the two ends of the parallel connection of the appropriate capacitance it will form a parallel resonant circuit. This parallel resonant circuit added to a negative feedback circuit can constitute a sine wave oscillator circuit, because the crystal equivalent to the frequency range of the inductor is very narrow, so even if the other components of the parameter changes are very large, the frequency of this oscillator will not have a great change. The crystal oscillator has an important parameter, that is, the value of the load capacitance, select and load capacitance value equal to the value of the parallel capacitance, you can get the crystal oscillator nominal resonant frequency. General crystal oscillation circuit is in an inverting amplifier (note that the amplifier is not an inverter) access to both ends of the crystal, and then there are two capacitors were received at both ends of the crystal, the other end of each capacitor and then connected to the ground, the two capacitors in series with the value of the capacity of the value should be equal to the load capacitance, please note that the general IC pins have an equivalent input capacitance, which can not be ignored. General load capacitance of the crystal for 15p or 12.5p, if you consider the equivalent input capacitance of the component pins, then two 22p capacitors constitute the oscillation circuit of the crystal is a better choice.