How the crystal oscillator works

The author:admin Number of visits: Date:2020-12-28 13:47

Crystal oscillator is generally called crystal resonator, is a kind of electromechanical device, is a small electrical loss of quartz crystal by precision cutting and grinding, plating electrode welding lead made.This crystal has a very important property, if you give it an electric current, it will produce mechanical oscillations, and if you give it a mechanical force, it will produce electricity, and this property is called electromechanical effect.They have a very important characteristic, the frequency of oscillation is closely related to their shape, material, cutting direction, etc.Because the chemical properties of quartz crystals are very stable, the thermal expansion coefficient is very small, and the oscillation frequency is also very stable. Because the control of the geometric size can be very precise, the resonance frequency is also very accurate.

According to the electromechanical effect of quartz crystal, we can equate it to an electromagnetic oscillation circuit, that is, a resonant circuit.Their electromechanical effect is mechanic-electric-mechanic-electric....The resonant circuit composed of inductor and capacitor is the continuous conversion of electric field to magnetic field.The application in the circuit is actually to regard it as a high Q value of the electromagnetic resonance circuit.Because the loss of quartz crystal is very small, that is, the Q value is very high, when used as an oscillator, it can produce very stable oscillations, and when used as a filter, it can obtain very stable and steep bandpass or bandstop curves.

Crystal oscillator is the abbreviation of quartz oscillator, English name Crystal, it is the most important component in the clock circuit, its role is to provide the reference frequency to the graphics card, network card, motherboard and other parts of the reference frequency, it is just like a ruler, the unstable working frequency will cause the relevant equipment operating frequency is unstable, naturally prone to problems.Due to the continuous improvement of the manufacturing process, now the crystal frequency deviation, temperature stability, aging rate, tightness and other important technical indicators are very good, is not prone to failure, but still can pay attention to the quality of the crystal oscillator in the selection.

What role does crystal oscillator play in application

The clock source of the microcontroller can be divided into two categories: the clock source based on mechanical resonant devices, such as crystal oscillator and ceramic resonant channel;RC(resistor, capacitor) oscillator.One is the Pierce oscillator configuration, suitable for crystal and ceramic resonator channels.The other is a simple discrete RC oscillator.

Oscillators based on crystal oscillators and ceramic resonant channels usually provide very high initial accuracy and low temperature coefficients.RC oscillators can start quickly and cost less, but are generally less accurate over the entire range of temperature and operating supply voltage, varying between 5% and 50% of the nominal output frequency.

However, its performance is affected by the environmental conditions and the selection of circuit components.The selection of components and the layout of the circuit board of the oscillator should be taken seriously.When in use, the ceramic resonant groove circuit and the corresponding load capacitance must be optimized for a specific logic series.Crystal oscillators with high Q values are not sensitive to the selection of amplifiers, but can easily cause frequency drift (or even damage) when overdriven.The environmental factors affecting the work of the oscillator are: electromagnetic interference (EMI), mechanical vibration and impact, humidity and temperature.These factors can increase the output frequency variation, increase instability, and in some cases, cause the oscillator to stop.

Most of these problems can be avoided by using an oscillator module.These modules come with an oscillator, provide a low impedance square wave output, and can be guaranteed to operate under certain conditions.The two most commonly used types are crystal modules and integrated RC oscillators (silicon oscillators).The crystal oscillator module provides the same precision as the discrete crystal oscillator.The accuracy of silicon oscillator is higher than that of vertical RC oscillator, and in most cases it can provide the same accuracy as ceramic resonant channel.

Power consumption also needs to be considered when choosing an oscillator.The power consumption of the discrete oscillator is mainly determined by the supply current of the feedback amplifier and the capacitance value inside the circuit.The power consumption of CMOS amplifier is proportional to the operating frequency, which can be expressed as the value of power dissipation capacitance.For example, the HC04 inverter gate circuit has a power dissipation capacitance of 90pF.When working under 4MHz, 5V power supply, it is equivalent to 1.8mA power supply current.Combined with a 20pF crystal load capacitor, the entire supply current is 2.2mA.Ceramic resonant channels generally have larger load capacitance and correspondingly require more current.In contrast, crystal oscillator modules typically require a power supply current of 10mA to 60mA.Depending on the type and function of a silicon oscillator, the supply current can range from a few microamperes for low-frequency (fixed) devices to a few milliamperes for programmable devices.A low-power silicon oscillator, such as the MAX7375, operates at 4MHz with less than 2mA of current.