Basic knowledge of crystal oscillator

Crystal oscillator generally refers to a crystal oscillator. A crystal oscillator refers to a thin slice (referred to as a chip) cut from a quartz crystal at a certain azimuth angle, and a quartz crystal resonator, also known as a quartz crystal or crystal oscillator. The crystal component that adds an IC inside its package to form an oscillation circuit is called a crystal oscillator. Their products are generally packaged in metal shells, but can also be packaged in glass shells, ceramics, or plastics.

The working principle of crystal oscillator

Quartz crystal oscillator is a resonant device made by utilizing the piezoelectric effect of quartz crystal. Its basic structure is roughly as follows: a thin sheet is cut from a quartz crystal at a certain azimuth angle, and silver layers are coated on its two corresponding surfaces as electrodes. A lead wire is soldered to each electrode and connected to the pin, and the quartz crystal resonator is formed by packaging the outer shell. If an electric field is applied to the two electrodes of a quartz crystal, the chip will undergo mechanical deformation. On the contrary, if mechanical pressure is applied on both sides of the chip, an electric field will be generated in the corresponding direction of the chip, and this physical phenomenon is called the piezoelectric effect.

If an alternating voltage is applied to the two poles of the chip, the chip will generate mechanical vibration, and at the same time, the mechanical vibration of the chip will generate an alternating electric field. In general, the amplitude of mechanical vibration of chips and the amplitude of alternating electric fields are very small, but when the frequency of the applied alternating voltage is a specific value, the amplitude increases significantly, much larger than the amplitude at other frequencies. This phenomenon is called piezoelectric resonance, which is very similar to the resonance phenomenon of LC circuits. Its resonant frequency is related to the cutting method, geometric shape, size, etc. of the chip.

When the crystal does not vibrate, it can be regarded as a flat capacitor called electrostatic capacitance C. Its size is related to the geometric dimensions of the chip and the electrode area, generally ranging from a few picofarads to tens of picofarads. When the crystal oscillates, the inertia of mechanical vibration can be equivalent to inductance L. Generally, the value of L ranges from tens of henries to hundreds of henries. The elasticity of the chip can be equivalent to the capacitance C, which is very small, usually only 0.0002-0.1 picofarads. The loss caused by friction during chip vibration is equivalent to R, which is approximately 100 ohms.

How does a crystal oscillator generate clock signals?

A crystal resonator is made by processing quartz or ceramic crystal pieces and coating them with electrodes. Quartz crystals or ceramic crystal pieces have a special characteristic - piezoelectric effect, which means that when we apply voltage to the crystal, the lattice in the crystal will undergo strong internal stress and deformation under the action of electric field force. Under the action of electric field, the internal stress and deformation of the crystal will change, resulting in mechanical vibration. When the electric field disappears, the deformation of the crystal will also disappear. When we apply a stable alternating signal to the crystal oscillator, it will produce stable mechanical vibration. The symbol and equivalent circuit of the crystal oscillator are shown in the following figure.

When the frequency of the alternating signal applied at both ends of the crystal oscillator is equal to the resonant frequency of the crystal oscillator, the reactance of the crystal oscillator is 0 and exhibits resistance characteristics. The resonant frequency is also the resonant frequency of the C1, L1, R1 series branch in the equivalent circuit of the crystal oscillator, so it is also known as the series resonant frequency. The anti resonant frequency of a crystal oscillator refers to the resonant frequency of the entire equivalent circuit, also known as the parallel resonant frequency. When the frequency of the input signal approaches the parallel resonance frequency, the reactance of the crystal oscillator tends to infinity. The region between Fa and Fs is commonly referred to as the "parallel resonance region" and is also the normal operating area of the crystal oscillator. In this region, the crystal oscillator exhibits inductive characteristics, resulting in a phase shift equivalent to 180 °. So in practical circuit design, who will provide stable alternating signals? It's just a microcontroller (chip). The specific reference circuit is shown in the following figure. The crystal oscillator circuit of the Pierce oscillator circuit is usually composed of an inverter and feedback resistor inside the microcontroller, as well as two external capacitors. With the chip providing a mutation signal crystal oscillator, a frequency stable oscillation signal can be generated.

Classification of crystal oscillators

There are generally two types of crystal oscillators: active crystal oscillators and passive crystal oscillators. Active crystal oscillator, also known as crystal oscillator or oscillator. Passive crystal oscillator is sometimes referred to as passive crystal, crystal, or crystal resonator. Simply put, an active crystal oscillator can output oscillation signals by supplying power on its own; Passive crystals require additional circuits to oscillate.

How to choose a crystal oscillator?

In general, we need to choose the type of crystal oscillator based on our application, such as whether it is active or passive. If it is passive, attention should be paid to the following parameters:

1) Nominal frequency: The frequency can be found on the outer shell of the crystal oscillator, and different crystal oscillators correspond to different frequencies. The choice of crystal oscillator frequency depends on the requirements of the system.

2) Accuracy: refers to the maximum allowable deviation of the frequency of the crystal oscillator relative to the nominal value, generally expressed in ppm, which is one millionth. The smaller the value, the higher the accuracy.

3) ESR equivalent series resistance: The internal resistance of a crystal oscillator is related to its power consumption. If the ESR is too high, the power consumption will increase and it may even fail to start oscillating. If the ESR is smaller, the cost will also increase, so the choice can be made according to the actual situation.