We only know that the crystal is a frequency component, and for the crystal has a sub-frequency crystal and overtone crystal people may be few and far between. So what is the fundamental frequency crystal, what is the overtone crystal, and what is the difference between the use of the two in the circuit?
Crystal vibration is like a spring, the vibration frequency of the crystal and the quartz crystal area, thickness, cutting orientation, etc..
The longer it is, the slower it shakes, the thicker it is, and the softer it is, the slower it shakes.
However, too short, too thin, too hard can not shake.
Crystal oscillator is mechanical vibration, with the characteristics of mechanical vibration: shape, geometry, mass, etc., determine the frequency of vibration.
Crystal oscillator is generally made of quartz or ceramic material plus the combination of the internal wafer, and the frequency of the crystal oscillator depends on the thickness of the wafer impact. First of all, in terms of production process, the size of the wafer and the thickness of the wafer are closely related to the frequency of the crystal oscillator. For example, a 40MHz quartz crystal requires a wafer thickness of 41.75 microns, which is quite achievable, but a 100MHz quartz crystal requires a wafer thickness of 16.7 microns.
Even though the thickness is achievable, the loss is very high, and the wafer will be broken when it is dropped gently after the finished product is made. Therefore, it is necessary to use three times overtone, five times overtone, and seven times overtone technology to achieve high frequency crystals. For example, for a crystal with a fundamental frequency of 20MHz, a crystal of 100MHz can be obtained after five overtones. Generally speaking, in terms of experience, below 40MHz is basically the fundamental frequency crystal, while above 40MHz, it is the overtone crystal.
Therefore, it is easy to understand why many active crystal frequency is basically considered high frequency, and the cost is relatively expensive, the cost of the active crystal in addition to the internal chip is thinner, and then there is an oscillation piece added to itself.
So, what is the difference in the use of fundamental frequency crystal and overtone crystal? There is definitely a difference in the use of the two, for example, the base frequency crystal, only need to access the appropriate capacitance to work: in the MCU crystal on each side of a pair of ground capacitor reason. The overtone crystal, on the other hand, requires inductors and capacitors to be used together in order to vibrate the overtone frequency, otherwise it can only vibrate the fundamental frequency.
Overtone crystal introduction: quartz crystal is made of quartz wafer, and different frequencies of quartz crystal corresponding to the size, thickness of the quartz wafer is not the same, in general, the higher the frequency of the quartz crystal, the thinner the quartz wafer needed. For example, a 40MHz quartz crystal requires a wafer thickness of 41.75 microns, which is quite achievable, but a 100MHz quartz crystal requires a wafer thickness of 16.7 microns. Even though the thickness is achievable, the loss is very high and the wafer will be broken after a slight drop when the finished product is made. Therefore, it is necessary to use three times overtone, five times overtone, and seven times overtone technology to achieve high frequency crystals.
For example, for a crystal with a fundamental frequency of 20MHz, a crystal of 100MHz can be obtained after five overtones. Generally speaking, from experience, below 40MHz is basically the fundamental frequency crystal, while above 40MHz is the overtone crystal.
There is also a certain difference in the usage of the two, for example, the crystal of the fundamental frequency only needs to access the appropriate capacitance to work, while the overtone crystal needs inductance and capacitance to work together to vibrate the overtone frequency, otherwise it can only vibrate the fundamental frequency.
