We only know that the crystal oscillator is a frequency component, and there may be few people who have come to realize that crystals divide into two types, one is fundamental and the other is overtone., So what are the fundamental crystals, what is the overtone crystals, and what is the difference between the two in the circuit.
The crystals are generally composed of quartz materials or ceramic materials with inner wafer, and the frequency of the crystal vibration depends on the thickness of the wafer. .First of all, in the production process, the wafer size and the thickness of the wafer are closely related to the frequency of the crystal oscillator. Generally speaking, the higher the frequency of the quartz crystal, the thinner the wafer is needed .For example, the thickness of the wafer required for a 40MHZ quartz crystal is 41.75 microns, which can be done in such a thickness, but the quartz crystal of 100MHZ is 16.7 microns in thickness .Even if the thickness can be achieved but the loss is very high, the finished product will be crushed by a single drop of the wafer. .So the high frequency crystals are to be achieved by 3rd overtone, 5th overtone, and seven overtone. .For example, the base frequency is 20MHZ of crystals, and after five overtones, you get a 100MHZ crystal. Generally speaking, when the frequency drops below the 40MHZ , it is the fundamental crystal, and when frequency is above 40 MHZ, it is the overtone crystals. Therefore, it is easy to understand, why a lot of basic active crystal vibration frequency is high frequency, and the cost is relatively expensive, active the cost of the crystals in addition to the internal chip is thinner, and itself by a oscillation.
So there's something different during the usage of fundamental crystals and overtone crystals .Be sure there is a difference between the two kinds of crystals, when we use the fundamental crystals, they can work by the insertion of the appropriate capacitor, but when we use the overtone crystals, the overtone frequency can be output by the support of inductance and capacitance, otherwise only the fundamental frequency can be output.