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首頁新聞中心 IQD幾皮法會(huì)破壞你的計(jì)時(shí)準(zhǔn)確性LFXTAL052799CUTT

IQD幾皮法會(huì)破壞你的計(jì)時(shí)準(zhǔn)確性LFXTAL052799CUTT

來源：http://www.techzozo.com 作者：金洛鑫電子 2024年03月08
IQD Frequency Products - Picofarads

IQD幾皮法會(huì)破壞你的計(jì)時(shí)準(zhǔn)確性LFXTAL052799CUTT
 
石英晶體因其精確的頻率而廣泛用于電子應(yīng)用中。對(duì)高精度時(shí)鐘頻率的需求正在增加，特別是由于物聯(lián)網(wǎng)、5G和工業(yè)4.0。然而，在設(shè)計(jì)過程中的幾個(gè)錯(cuò)誤就可能破壞石英晶體的整體頻率精度。
 
負(fù)載電容對(duì)石英晶振頻率和我的應(yīng)用有什么影響？
 
根據(jù)應(yīng)用中所需的時(shí)序精度，這可能非常重要。電路的電容決定了晶體的諧振頻率。這意味著如果電路中的電容發(fā)生變化，晶體的頻率也會(huì)發(fā)生變化。在生產(chǎn)過程中，晶體被校準(zhǔn)到其特定負(fù)載電容。由此可見，如果電路中的負(fù)載電容合適，晶體將在其容差范圍內(nèi)振蕩。然而，負(fù)載電容的每次變化都會(huì)改變頻率。這意味著在最壞的情況下，晶體將在其容差之外振蕩。
 
確定晶體頻率將偏移多少的參數(shù)是微調(diào)靈敏度。這表示如果負(fù)載電容變化1pF，頻率將變化多少ppm。調(diào)整值可在1ppm/pF至100ppm/pF之間變化，石英水晶振子具體取決于各種因素，如空白的尺寸、電極的尺寸和形狀、頻率以及負(fù)載電容。一方面，根據(jù)經(jīng)驗(yàn)法則，封裝尺寸越大，調(diào)整靈敏度越高。因此，1.6x1.2mm晶體的切邊將小于7.0x5.0mm貼片晶振晶體的切邊。另一方面，你需要記住負(fù)載電容越低，調(diào)整就越高。因此，如果一個(gè)晶體的負(fù)載電容為10pF，另一個(gè)晶體的負(fù)載電容為20pF，則10pF晶體的調(diào)整會(huì)更高。在下圖（圖1）中，您可以看到調(diào)整是如何隨負(fù)載電容變化的。
 
How a few Pico Farads can ruin your timing accuracy
圖1石英晶體的調(diào)整靈敏度取決于負(fù)載電容
 
因此，如果電路中的負(fù)載電容與石英晶振的額定負(fù)載不同，頻率就會(huì)偏移。此外，根據(jù)額定負(fù)載電容以及電路中負(fù)載與額定負(fù)載之間的差異，頻率變化可能會(huì)很大。在最壞的情況下，頻率會(huì)超出規(guī)格，可能無法在您的應(yīng)用中工作。這就是為什么在設(shè)計(jì)過程中更詳細(xì)地觀察負(fù)載電容如此重要的原因。

IQD幾皮法會(huì)破壞你的計(jì)時(shí)準(zhǔn)確性LFXTAL052799CUTT
 
How a few picoFarads can ruin your timing accuracy
Quartz crystals are widely used in electronic applications because of their accurate frequency. The demand for high precision clock frequencies is increasing, particularly owing to IoT, 5G and Industry 4.0. However, just a few mistakes during the design-in process can ruin the entire frequency accuracy of the quartz crystal.
 
What influence does load capacitance have on the frequency of the crystal and to my application?
Depending on the required timing accuracy in your application, this can be very important. The capacitance of the circuit defines the resonant frequency of the crystal. This means if the capacitance in the circuit changes, the frequency of the crystal changes as well. During the production process, the crystal is calibrated to its specific load capacitance. From this, it follows that with the right load capacitance in the circuit the crystal will oscillate within its tolerance. However, every change to the load capacitance will shift the frequency. This means that in the worst case scenario the crystal will oscillate outside its tolerance.
 
The parameter that identifies how much the frequency of the crystal will shift is the trim sensitivity. This shows by how many ppm the frequency will change, if the load capacitance is changed by 1 pF. The value of the trim can vary from 1 ppm/pF up to 100 ppm/pF and it depends on various factors, such as the size of the blank, the size and shape of the electrode, the frequency, as well as the load capacitance. On one hand, you can say as a rule of thumb, that the bigger the package size the bigger the trim sensitivity. Therefore, the trim of a 1.6 x 1.2 mm crystal will be smaller than the trim of a 7.0 x 5.0 mm crystal. On the other hand, you need to keep in mind that the lower the load capacitance, the higher the trim will be. Consequently, if you have a crystal with 10 pF load capacitance and a crystal with 20 pF load capacitance, the trim of the 10 pF crystal will be higher. In the picture below (Figure 1), you can see how the trim varies with the load capacitance.
 
Figure 1 Trim sensitivity of a quartz crystal depending on the load capacitance
 
Therefore, if the load capacitance in the circuit differs from the specified load of the crystal, the frequency will shift. In addition, depending on the specified load capacitance and the difference between the load in the circuit and the specified load, the frequency change can be dramatic. In the worst-case scenario, the frequency will be outside of the specification and it may not work in your application. This is why it is so important to look at the load capacitance in more detail during your design process.

進(jìn)口貼片晶振 型號(hào) 頻率 負(fù)載電容 頻率公差 頻率穩(wěn)定性
LFXTAL081314REEL IQXC-240 37.4MHz 5.00pF ±7ppm ±15ppm
LFXTAL081316REEL IQXC-240 38.4MHz 5.00pF ±7ppm ±15ppm
LFXTAL072539REEL IQXC-26 24.0MHz 8.00pF ±15ppm ±15ppm
LFXTAL078691CUTT IQXC-26 24.0MHz 20.00pF ±10ppm ±30ppm
LFXTAL081608CUTT IQXC-26 24.0MHz 8.00pF ±10ppm ±20ppm
LFXTAL081608REEL IQXC-26 24.0MHz 8.00pF ±10ppm ±20ppm
LFXTAL081609CUTT IQXC-26 25.0MHz 8.00pF ±10ppm ±20ppm
LFXTAL083890CUTT IQXC-26 25.0MHz 20.00pF ±10ppm ±20ppm
LFXTAL052799CUTT IQXC-26 26.0MHz 10.00pF ±10ppm ±30ppm
LFXTAL066742CUTT IQXC-26 26.0MHz 8.00pF ±15ppm ±15ppm
LFXTAL066742REEL IQXC-26 26.0MHz 8.00pF ±15ppm ±15ppm
LFXTAL069296CUTT IQXC-26 26.0MHz 8.00pF ±15ppm ±20ppm
LFXTAL070729CUTT IQXC-26 26.0MHz 12.00pF ±10ppm ±15ppm
LFXTAL081614CUTT IQXC-26 26.0MHz 8.00pF ±10ppm ±20ppm
LFXTAL081614REEL IQXC-26 26.0MHz 8.00pF ±10ppm ±20ppm
LFXTAL072541CUTT IQXC-26 27.0MHz 8.00pF ±15ppm ±15ppm
LFXTAL072541REEL 進(jìn)口晶振 27.0MHz 8.00pF ±15ppm ±15ppm
LFXTAL081610CUTT IQXC-26 27.0MHz 8.00pF ±10ppm ±20ppm
LFXTAL072302CUTT IQXC-26 30.0MHz 8.00pF ±15ppm ±15ppm
LFXTAL081612CUTT IQXC-26 30.0MHz 8.00pF ±10ppm ±20ppm
LFXTAL056523CUTT IQXC-26 32.0MHz 8.00pF ±20ppm ±20ppm
LFXTAL056523REEL IQXC-26 32.0MHz 8.00pF ±20ppm ±20ppm
LFXTAL069526CUTT IQXC-26 32.0MHz 8.00pF ±15ppm ±15ppm
LFXTAL069526REEL IQXC-26 32.0MHz 8.00pF ±15ppm ±15ppm
LFXTAL079322CUTT IQXC-26 32.0MHz 12.00pF ±10ppm ±20ppm
LFXTAL079322REEL IQXC-26 32.0MHz 12.00pF ±10ppm ±20ppm
LFXTAL081613CUTT IQXC-26 32.0MHz 8.00pF ±10ppm ±20ppm
LFXTAL083149REEL IQXC-26 32.0MHz 6.00pF ±10ppm ±20ppm
LFXTAL072304CUTT IQXC-26 37.4MHz 8.00pF ±15ppm ±15ppm
LFXTAL081616CUTT IQXC-26 37.4MHz 8.00pF ±10ppm ±20ppm

 

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