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Crystal oscillators can be seen all over PCB. While processor is in use, there must be a crystal oscillator. Even if there is no external crystal oscillator, there is a crystal oscillator inside chip.

1. Overview of crystal oscillator

Crystal oscillator usually refers to a crystal oscillator. A crystal oscillator refers to a thin slice (called a plate) cut from a quartz crystal at a certain azimuthal angle, and a quartz crystal resonator is referred to as a quartz crystal or crystal or crystal oscillator for short.

A quartz element that adds a microcircuit inside case to form an oscillatory circuit is called a crystal oscillator. Its products are usually packaged in metal cases as well as glass, ceramic or plastic cases.

2. The principle of operation of a crystal oscillator

A crystal oscillator is a resonant device created using piezoelectric effect of a quartz crystal. Its basic design is roughly as follows: cut a thin slice from a quartz crystal at a certain azimuthal angle and place it on two corresponding surfaces. Plating a silver layer as electrodes, soldering a lead wire to each electrode to connect to pins, and adding a case body forms a quartz resonator called a quartz crystal or crystal, a crystal oscillator for short. Its products are usually packaged in metal cases as well as glass, ceramic or plastic cases.

If an electric field is applied to two electrodes of a quartz crystal, plate will be mechanically deformed. Conversely, if mechanical pressure is applied to both sides of plate, an electric field will be generated in corresponding direction of plate. This physical phenomenon is called piezoelectric effect.

If an alternating voltage is applied to two poles of microcircuit, microcircuit will generate mechanical vibration, and at same time, mechanical vibration of microcircuit will generate an alternating electric field.

Under normal conditions, amplitude of mechanical vibrations of plate and amplitude of alternating electric field are very small, but when frequency of applied alternating voltage reaches a certain value, amplitude increases significantly, which is much larger than amplitude at other frequencies. Many refer to this phenomenon as piezoelectric resonance, which is very similar to LC circuit resonance phenomenon. Its resonant frequency is related to cutting method, geometry and size of plate.

When crystal is not vibrating, it can be thought of as a flat capacitor called electrostatic capacitance C. Its size depends on geometric size of chip and area of ​​the electrode, usually from a few picofarads to tens of picofarads. When crystal oscillates, inertia of mechanical vibrations can be equivalent to inductance L.

As a rule, value of L is from tens of millions to several hundreds of millions. The elasticity of chip can be equivalent to capacitance C, and C value is very small, usually only 0.0002-0.1 picofarads. Friction loss during chip vibration is equivalent to R and is about 100 ohms.

Since equivalent inductance of circuit is very large, and C and R are small, quality factor Q of circuit is very large and can reach 1000-10000. In addition, resonant frequency of plate itself is mainly only related to cutting method, geometric shape and size of plate, and it can be made accurately, so oscillation circuit composed of quartz resonators can ensure high frequency stability.

Computers have a timing circuit. Although word "watch" is commonly used to refer to these devices, they are not actually watches in usual sense. It is more correct to call them timers.

A computer's timer is usually a precision machined quartz crystal that oscillates at a frequency within its voltage range, depending on how crystal itself is cut and how much voltage it is subjected to. Two registers are associated with each crystal: a counter and a holding register.

Each oscillation of quartz crystal decreases counter by one. When counter reaches zero, an interrupt is generated and counter is reset to initial value from holding register. This approach allows you to program timer to generate interrupts 60 times per second (or whatever frequency you want). Each interrupt is called a clock.

A crystal oscillator can be electrically equivalent to a two-terminal network in which a capacitor and a resistor are connected in parallel, and then capacitor is connected in series. This network has two resonance points in electrical engineering, and lower frequency is divided into series resonance in frequency, higher frequency is parallel resonance.

Due to characteristics of crystal itself, distance between two frequencies is quite close. In this extremely narrow frequency range, crystal oscillator is equivalent to an inductor, so as long as two ends of crystal oscillator are connected in parallel with a suitable capacitor, it forms a parallel resonant circuit.

This parallel resonant circuit can be added to a negative feedback circuit to form a sinusoidal oscillation circuit. Since a crystal oscillator is equivalent to an inductance, frequency range is very narrow, so even if parameters of other components change a lot, the frequency of this oscillator will not change, there will not be big changes.

The crystal oscillator has an important parameter, that is, value of load capacitance, and nominal resonant frequency of crystal oscillator can be obtained by choosing a parallel capacitor equal to value of load capacitance.

The general circuit of crystal oscillator is connected to crystal oscillator at both ends of inverting amplifier, and then two capacitors are connected to two ends of crystal oscillator respectively, and other end of each capacitor is connected to ground. , The capacitance value in series must be equal to load capacitance. Please note that pins of conventional ICs have an equivalent input capacitance, which cannot be ignored.

The load capacitance of a conventional crystal oscillator is 15 pico or 12.5 pico. If we consider equivalent input capacitance of output of component, then best choice would be two 22 picocapacitors that make up crystal oscillator circuit.