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Crystal oscillator - it is the core of digital circuits


Crystal oscillator overview:

 

The crystal oscillator we often say is actually obtained by cutting the artificially cultivated crystal through a cutting device. The artificial culture of the crystal is mainly related to the growth environment. At present, the crystal quality of Russia is better.

 

The reason why the crystal oscillator is the core of the digital circuit is that all digital circuits need a stable working clock signal. The most common one is to use a crystal oscillator. It can be said that the crystal oscillator can be seen as long as there is a digital circuit.

 

We often say that there are two types of crystal oscillators, one that needs to add a driver circuit to generate a frequency signal. Such crystal oscillators are called crystal resonators, such as the common 49S package, the SMD3225 5032 in two-legged package, and a small four-pin SMD package. A kind of crystal oscillator can be generated without adding a driving circuit, and only needs to add a voltage signal. The crystal oscillator is basically a 4-pin package and includes a power pin, a ground pin, a frequency output pin, and the like.

 

The main parameters of the crystal:

 

Here we mainly focus on crystal resonators. The main parameters of the general crystal oscillator are core frequency, operating temperature, accuracy value, equivalent series impedance, matching capacitor, package form, and so on.

 

The core frequency of the vibration, the choice of the general core frequency depends on the requirements of the frequency requirement component. For example, the clock chip needs a crystal oscillator of 32.768KHz, and the MCU is generally a range, basically from 4M to tens of meters.

 

The operating temperature of the crystal oscillator, the reason why the working temperature is taken out separately, is mainly because the crystal oscillator is a physical device, the working temperature is proportional to the price, the higher the working temperature requirement, the higher the price, so the crystal oscillator needs to be considered when considering the work. temperature.

 

The precision value of the crystal oscillator is generally 0.5 ppm, ±5 ppm, ±10 ppm, ±20 ppm, ±50 ppm, and the like. Among them, 0,5ppm domestically can only be achieved through digital compensation, and foreign countries have achieved high precision in 3225 or even 2016. The choice of accuracy generally refers to the accuracy requirements of the frequency requirement device. For example, high-precision clock chips are generally within ±5ppm, and common applications are chosen to be around ±20ppm.

 

The equivalent series impedance of the crystal oscillator. This parameter is mainly related to the driving capability, that is to say, it is related to the driving current. If the equivalent resistance is small, the required drive current is small. The ability to adapt to external drive circuits is higher.

 

The matching capacitance of the crystal oscillator can change the core frequency of the crystal oscillator by changing the parameter value of the matching capacitor. That is to say, the precision of the crystal can be fine-tuned by adjusting the matching capacitance of the crystal oscillator. This is also the main method for making high-precision temperature-compensated crystal oscillators in China.

 

The crystal package type, the current crystal package form is diverse, the user needs to choose according to their actual situation, mainly based on the board space, processing methods, cost and so on.

 

Common considerations for crystal oscillators:

 

In general, the crystal is the core of a system. The quality of the crystal is directly related to the stability of the entire system. The main points to note are the following.

 

There are two aspects related to the processing technology. One is over-high temperature reflow soldering. Since the crystal oscillator is a physical device, the high temperature may have a certain influence on the frequency of the crystal oscillator during reflow soldering, which deviates from the core frequency. Special attention is required when using a K-level crystal. One is the ultrasonic cleaning in the cleaning process. This is mainly because the ultrasonic frequency may cause the resonance of the crystal oscillator if it falls on the working frequency of the crystal oscillator, causing the wafer inside the crystal oscillator to be broken and defective.

 

Generally, the application needs to pay attention to the crystal oscillator to work in a stable state. Many crystal oscillator failures occur when the crystal oscillator is operated for a long time or is under-driven. This can be analyzed by looking at the output pin waveform of the crystal oscillator. Over-driving may cause the crystal oscillator to not reach the normal service life. Under-actuation may cause the anti-interference ability of the crystal oscillator to be weakened, and the system often loses the clock without any reason.

 

The anti-interference design of the crystal oscillator, because the crystal oscillator is a small signal device, is easily subject to external interference, which causes problems in the system clock. This block is mainly handled from two aspects, one is to pay attention to the processing of the crystal clock signal on the layout, and the common one is the package processing. One is the processing of other frequency devices on the board, which requires isolation between different frequencies.

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