What determines frequency of a crystal oscillator?

We only know that crystal oscillator is a frequency component, and there may be very few people who divide crystal oscillator into a fundamental crystal oscillator and an overtone crystal oscillator. So, what is a fundamental frequency crystal oscillator, what is an overtone crystal oscillator and what is difference between using them in a circuit.

The vibration of a quartz oscillator is like a spring: vibration frequency of crystal is related to area, thickness, cut orientation, etc. of quartz crystal. Longer ones shake slower, thicker ones shake slower, and softer ones shake slower. But it's too short, too thin, and too hard to shake.

A crystal oscillator is a mechanical vibration that has characteristics of mechanical vibration: shape, geometric size, quality, etc., which determine the vibration frequency.

Crystal oscillators are usually made of quartz or ceramic materials and have an internal chip, and frequency of crystal oscillator depends on thickness of chip. First of all, from point of view of manufacturing process, wafer size and wafer thickness are closely related to frequency of crystal oscillator. Generally speaking, higher frequency of crystal oscillator, thinner quartz wafer. necessary. For example, wafer thickness required for a 40 MHz quartz crystal is 41.75 microns, which is still achievable, but for a 100 MHz quartz crystal, required wafer thickness is 16.7 microns. Even if thickness can be achieved, losses are very high and chips will break if dropped slightly after finished product has been made. Therefore, as a rule, in high-frequency crystals, it is necessary to use technology of third overtone, fifth overtone and seventh overtone. For example, for a quartz with a fundamental frequency of 20 MHz, you can get a quartz of 100 MHz after five overtones.

Generally speaking, base frequency crystal oscillator is mostly below 40 MHz, and overtone crystal oscillator is above 40 MHz. Therefore, it is not difficult for us to understand why frequency of many active crystal oscillators is basically high and cost is relatively high. The cost of active crystal oscillators lies not only in thinner internal chip, but also in addition of an oscillator.

Then it is not necessary to use a fundamental crystal oscillator and an overtone crystal oscillator. There must be a difference in use of two. For example, a fundamental frequency crystal can only work when an appropriate capacitor is connected, while an overtone crystal oscillator needs an inductor and a capacitor to work together to vibrate overtone frequency, otherwise it can only vibrate at fundamental frequency .

An introduction to overtone crystal oscillators: crystal oscillators are made of quartz plates, and size and thickness of quartz plates corresponding to different frequencies of crystal oscillators are different. Generally speaking, higher frequency of quartz crystal, higher frequency of crystal oscillator, more crystal oscillators are required generators, thinner plate. For example, wafer thickness required for a 40 MHz quartz crystal is 41.75 microns, which is still achievable, but for a 100 MHz quartz crystal, required wafer thickness is 16.7 microns. Even if thickness can be achieved, losses are very high and chips will break if dropped slightly after finished product has been made. Therefore, as a rule, in high-frequency crystals, it is necessary to use technology of third overtone, fifth overtone and seventh overtone.

For example, for a crystal with a fundamental frequency of 20 MHz, you can get a crystal of 100 MHz after five overtones. Generally speaking, below 40MHz it is mainly fundamental crystal oscillator, and above 40MHz it is fundamental crystal oscillator.overtones.

There is a definite difference between two in terms of usage. For example, a fundamental crystal can only be operated by connecting an appropriate capacitor, while an overtone crystal requires a combination of an inductor and a capacitor to vibrate overtone frequency. , It can only vibrate at fundamental frequency.