Understanding crystal oscillator:Principle of crystal oscillator & Passive crystal oscillator & active crystal oscillator

The crystal oscillator is like the heart of the single-chip microcomputer. If there is no heart beating, the single-chip microcomputer cannot work. The larger the crystal oscillator value, the faster the single-chip microcomputer runs. Sometimes it is not as fast as possible. For electronic circuits, sufficient speed is the best. The faster the easier it is to be interfered, the worse the reliability!

Crystal oscillator, the full name is quartz crystal oscillator, is a high-precision and high-stability oscillator. Through a certain external circuit, a sine wave with stable frequency and peak value can be generated.

When the single-chip microcomputer is running, it needs a pulse signal as the trigger signal to execute the instruction. It can be simply imagined as: the single-chip microcomputer executes one or more instructions when it receives a pulse.

Electrically, it can be equivalent to a two-terminal network of a capacitor and a resistor in parallel and a capacitor in series. In electrical engineering, this network has two resonance points. The lower frequency is series resonance, and the higher is the frequency. The frequency is parallel resonance.

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

This parallel resonant circuit can be added to a negative feedback circuit to form a sine wave oscillation circuit. Since the crystal oscillator is equivalent to an inductance in a very narrow frequency range, even if the parameters of other components vary greatly, the frequency of this oscillator will not be great change.

The crystal oscillator circuit is connected to the crystal oscillator at both ends of an inverting amplifier, and then two capacitors are connected to both ends of the crystal oscillator, and the other capacitor is grounded. The capacitance of the two capacitors in series is equal to the load capacitance. The specific circuit is shown in the figure below.

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

The general crystal oscillator circuit is connected to the crystal oscillator at both ends of an inverting amplifier (note that the amplifier is not an inverter), and then two capacitors are connected to both ends of the crystal oscillator, and the other end of each capacitor is connected Ground, the capacitance value of these two capacitors in series should be equal to the load capacitance. Please note that the pins of general ICs have equivalent input capacitance, which cannot be ignored.

The load capacitance of the general crystal oscillator is 15p or 12.5p. If the equivalent input capacitance of the component pin is considered, two 22p capacitors forming the crystal oscillator circuit is a better choice. Load capacitance + equivalent input capacitance = 22pF.

Passive crystal oscillator & active crystal oscillator

Crystal oscillator is a component that provides a frequency reference for the circuit. It is usually divided into two categories: active crystal oscillator and passive crystal oscillator.

Passive crystal oscillators require an oscillator inside the chip, and the signal voltage of the crystal oscillator depends on the starting circuit, allowing different voltages. However, passive crystal oscillators usually have poor signal quality and accuracy and require precise matching of peripheral circuits (inductance, capacitance, resistance) Etc.), if you need to replace the crystal oscillator, you must also replace the peripheral circuit.

The active crystal oscillator does not need the internal oscillator of the chip, can provide a high-precision frequency reference, and the signal quality is better than that of the passive crystal oscillator.

Principle of crystal oscillator

The reason why a quartz wafer can be used as an oscillator circuit (resonance) is based on its piezoelectric effect. It is known from physics that if an electric field is applied between the two plates of the wafer, the crystal will be mechanically deformed; When mechanical force is applied, an electric field will be generated in the corresponding direction. This phenomenon is called piezoelectric effect.

If an alternating voltage is applied between the plates, it will produce mechanical deformation vibration, and at the same time, the mechanical deformation vibration will produce an alternating electric field. Generally speaking, the amplitude of this mechanical vibration is relatively small, and its vibration frequency is very stable.

But when the frequency of the applied alternating voltage is equal to the natural frequency of the wafer (determined by the size of the wafer), the amplitude of the mechanical vibration will increase sharply. This phenomenon is called piezoelectric resonance, so quartz crystal is also called quartz crystal resonator . It is characterized by high frequency stability.

Both quartz crystal oscillators and quartz crystal resonators are electronic devices that provide stable circuit frequencies.

The quartz crystal oscillator uses the piezoelectric effect of the quartz crystal to vibrate, while the quartz crystal resonator uses the quartz crystal and the built-in IC to work together.

The oscillator is directly used in the circuit, and the resonator generally needs to provide a 3.3V voltage to maintain operation. The oscillator has one more important technical parameter than the resonator: resonance resistance (RR). The resonator has no resistance requirement. The size of RR directly affects the performance of the circuit, and is also an important parameter for competition among various businesses.

Some beginners may feel strange about the frequency of the crystal oscillator. Crystals such as 12M and 24M are better understood. Choosing a crystal oscillator such as 11.0592MHZ gives people a strange feeling. This problem is more troublesome to explain. If the beginner is practicing the serial port When programming, you will understand this. This kind of crystal oscillator is mainly used to design the baud rate of the serial port or other asynchronous communication conveniently and accurately.