Why is the 32.768K crystal oscillator the most frequently used frequency point for electronic products? 32.768K crystal oscillator is a clock crystal oscillator specially used for time meter system. The 32.768K crystal oscillator will be used for products requiring timing function and time display.
32.768KHZ tuning fork crystal
32.768K crystal oscillator provides a clock cycle for the product so that the product can be used normally. 32.768K crystal oscillator has excellent stability, high temperature resistance, impact resistance and harsh environment resistance. There are mainly two types of cut commonly used for crystal oscillators, one is AT cut and the other is BT cut.
The crystal frequency temperature curve of tuning fork 32.768KHZ is a quadratic parabola. As the operating temperature deviates from the normal temperature by 25 ℃, the temperature drift also increases. The temperature drift reaches nearly 50ppm at – 10 ℃~+60 ℃. If the industrial grade – 40 ℃~+85 ℃ is used for calculation, the temperature drift reaches 151ppm. It is difficult to adapt to the industrial grade operating temperature range of electronic products, and it is also difficult to conduct temperature compensation. Therefore, there are few temperature compensated crystal oscillators for 32.768KHZ in the market, And the price is extremely expensive. For the general consumer electronics industry, if the industrial grade – 40 ℃~+85 ℃ is required, and the temperature frequency difference is controlled within ± 30ppm, the common tuning fork type 32.768KHZ crystal cannot meet the requirements. However, if the chip cutting type can be changed to AT cutting type, it will not be a problem to control the industrial grade temperature frequency difference within ± 30ppm. Now let’s know how to realize the 32.768KHZ bell ringing.
The frequency temperature curve of AT cut crystal is a cubic curve, showing a lying “S” curve. With the change of temperature, the temperature drift shows a “S” shaped track change. There are two “inflection points” at roughly – 10 ℃ and+60 ℃, that is, the temperature drift will turn back in the opposite direction. Therefore, as long as the chip chamfer is controlled within a certain tolerance range, it is not difficult to ensure that the temperature drift of the two inflection points does not exceed ± 30ppm at – 40 ℃~85 ℃.
However, AT cut crystal is only for quartz crystal with MHZ frequency. How to convert it to 32.768KHZ frequency? Clock vibration 32.768KHZ can be realized by frequency division. If AT is used to cut 16.777216MHZ crystal, the desired 32.768KHZ frequency can be obtained through 512 frequency division. It is not difficult to realize frequency division of the clock oscillator, which is integrated in the oscillation IC. Therefore, the 32.768KHZ clock oscillator realized by using the AT switching MHZ frequency division has a great improvement in the frequency and temperature characteristics. Without temperature compensation, it is possible to maintain the temperature frequency difference at ± 30ppm or even ± 20ppm at – 40 ℃~85 ℃.
32.768KHZ bell
As mentioned above, the key to the high accuracy and stability of the clock oscillator is to reduce the tedious process of crystal load matching, and to use a professional oscillation IC, which improves the stability of the product. 32.768KHZ clock oscillator adopts AT cut MHZ chip through frequency division, which greatly improves the temperature frequency difference characteristics of the product. However, it has to be pointed out that the power consumption of 32.768KHZ made by MHZ frequency division is slightly higher than that of KHZ as the most oscillating source. Generally, the working input current is less than 0.5mA (3V), and the static consumption current is less than 10uA. From the perspective of actual test, the power consumption is relatively small. Therefore, for products with specific temperature frequency difference requirements for 32.768KHZ tuning fork crystal frequency, consider 32.768KHZ bell vibration.
Application case of crystal oscillator: inventory of epidemic prevention bracelet: introduction to selection of various active crystal oscillators
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