晶振理论知识.doc

技术术语01：NO signal output from the crystal ?1-1.Please measure the signal output by two terminals of the crystal using Oscilloscope or Frequency Counter. If there is nosignal output, please follow step 1-1 to step 1-4 to execute the examination. If there is signal output from out- terminal ofthe crystal ( Xout), but no signal output from the in-terminal (Xin), please check the crystal following step1-5 to step 1-6.1-2.Please uninstall the crystal and test its frequency and load capacitance to see whether they vibrate and meet yourspecifications using a professional testing machine. You can also send it to your supplier to have them test it for you.1-3.If any of the following situations happen, the crystal doesnt vibrate, its load capacitance doesnt match your specification,or there is a huge gap between current frequency and your targeted frequency, please send the crystal to your supplier toconduct Quality Analysis. If the frequency and load capacitance meet your specifications, we will need to conductEquivalent Circuit Test.1-4.Equivalent Circuit Test1-4-1.Generally, the oscillation circuit of Microprocessor derives from Colpitts circuit showing below:Picture 1Cd and Cg are external load capacitances, which have been built in the chip set. (Please refer to the Specifications of thechip set)Rf is the feedback resistance with200K1M. Its built in the chip set generally.Rd is the Limit Resistor with 4701K. This resistance is not necessary for common circuit but only for circuits havinghigh power supply.1-4-2.A stable oscillation circuit requires a negative resistance and its value should be at least five times of the crystalresistance. It can be written as |-R| 5 Rr.For example, to acquire a stable oscillation circuit, the value of negative resistance of the IC must be under 200 whenthe value of the crystal resistance is 40.1-4-3.“Negative resistance” is the yardstick to evaluate the quality of an oscillation circuit. Under some circumstances such asaging, thermal change, voltage change, and etc., the circuit might not oscillate if the value of “ Q” is low. Thus, its veryimportant to measure the negative resistance (-R )following the instructions below:(1) Connect the resistance (R) with the crystal in series(2) Adjust the value of R from the start point to the stop point of the oscillation.(3) Measure the value of R during oscillating.(4) You will be able to obtain the value of negative resistance, |R| = R + Rr, and Rr = crystal resistance.P.S. the stray capacitance of the connected circuit might affect measured values.1-4-4.If the parameters of the crystal are normal but its not working steadily within the oscillation circuit, we will have to findout whether the resistance value of the IC is too low to drive the circuit. If thats the case, we have three methods toimprove such situation:Lower the value of external capacitance(Cd and Cg), and adopt other crystal with lower load capacitance (CL).Adopt a crystal with lower resistance (Rr).Use the design of unequal values of Cd and Cg. We can increase the load capacitance of Cd (Xout) and decrease theload capacitance of Cg(Xin) to raise the output of waveform amplitude from Xin which will be used in its back-endcircuit.1-5.When there is signal output from Xout but not Xin, it represents the case that the power consumption of the rear -electrode Backend Circuit is extremely huge. We can add a buffer between the output of the circuit and its rear electrodeto drive the back-end Circuit.1-6.Except the method of 1-5 mentioned above, you can also follow the three methods in step 1-4-4. Pleasecontact the field application engineers of crystal or IC manufacturers for further assistance, if your problem cant besolved.02. System is not functioning because of no adequate output waveform amplitude from the crystal ?2-1.Please measure the signals from the two terminals of the crystal using Oscilloscope or Frequency Counter, if the Frequencyis not within the specification and its output waveform amplitude is not adequate (for example, over+/- 200ppm), pleasefollow step 2-3 to step 2-5.2-2.The formula for Capacitances versus Frequency is as following: FL = FR * ( 1 + C1 / 2 * ( C0 + CL) ) whereThe curve represents the variation of capacitance changes versus variation of frequency changes ( Frequency pullability):If the frequency measured by Frequency Counter is higher than the targeted frequency, we should increase the value ofcapacitances (CL, or Cd & Cg ) to lower the frequency to the targeted frequency, vice versa.Please check whether the waveform amplitude is improved or not after we adjust the frequency. If its improved, thatindicates the case that the original design of the circuit is not tuned to the best resonant point for the crystal. The crystalshould function normally after the resonant point is adjusted.2-3.If the waveform amplitude is not improved even the frequency is pretty much close to the targeted frequency, we canimprove it using three methods below:Method 1: Lower the value of external capacitance (Cd, and Cg) , and adopt crystal with lower loadapacitance (CL).Method 2: Adopt the crystal with lower resistance(Rr).Method 3: Use the design of unequal values of Cd and Cg.We can increase the load capacitance of Cd (Xout) and decrease the load capacitance of Cg(Xin) to raise the output ofwaveform amplitude from Xin which will be used in its back-end Circuit.We suggest that you use above methods to save costs and assure safety2-4.Please use the Frequency Counter to measuring the crystal to ensure that the adjusted frequency still meets originalspecification after the waveform amplitude has been improved. If the frequency doesnt meet the specification, pleaseadopt a crystal with suitable CL value according to your targeted frequency.2-5.Please adopt a crystal with lower CL if the frequency is much higher than the targeted frequency, vice versa.03. System is not functioning due to high deviation of output frequency ?3-1.We can improve the problem that deviation of frequency output is over the limit by following methods:Adjust the values of external capacitance, Cd & Cg.If the frequency measured by Frequency Counter is higher than targeted frequency, we should increase the externalcapacitance, CL ( or the values of Cd & Cg ), to lower the frequency to our targeted frequency, vice versa.Adopt a crystal with different value of capacitance(CL).Adopt a crystal with lower capacitance if the frequency is much higher than the targeted frequency, vice versa.3-2.Please check whether the waveform amplitude is normal or not using Oscilloscope, after the correct capacitance is adoptedand the frequency is adjusted to target. Under the situation that the waveform amplitude is shrunk due to adding externalcapacitances, please use method 2 to adjust the frequency ( lower external capacitances and adopt a crystal with lowercapacitance).石英晶体基础理论石英晶体特性石英是一种压电材料，被发觉是自然界所产生的透明结晶物 ，它所展现的是非常稳定的压电效应，从化学与机械上的特性在早期的电子实验引起了极大的关注。石英的主要成份为二氧化硅，虽然地壳表面有14%是由SiO2二氧化硅所组成的，但相对可使用部份是较稀少的，是因为需要的纯度不足以及其缺陷与瑕疵。晶体元件具备着组成一极高稳定性频率产生电路的能力。其振荡模式及角度方位如表1，它支配着晶体的特性，如等效电路参数、频率、温度特性、负载电容特性及频率老化等。从使用的观点而言，这些特性是极重要的，将叙述如下。厚度切变振荡模式已广泛的被使用，其谐振频率是由晶片厚度决定，像AT，BT切角；调谐音叉振荡模式则由晶片长度来决定频率之高低。表1所示不同的振荡模式决定于切割角度、频率范围、厚度或长度及频率与曲率常数之间的关系。 石英晶体的谐振频率通常是由组合振荡模式之振动片尺寸来决定的。振荡晶体的等效电路有益于说明支配晶体性能的基本概念。如图2所示，晶体等效电路所表示的如下：L1：动态电感（机械振荡）C1：动态电容（机械弹力） R1：串联电阻（能量损耗） Co(静电容) Co=Ct-CL（静电容为平行于电极之间再加上两引线间之杂散电容）负载电容CL是跨接在振荡电路上集晶体座与分布电容之总合,这是决定晶体在振荡电路上的条件因素,在晶体使用范围其功能被视为感抗,也就是说振荡电路所呈现的是一个负电阻-R及一个电容CL串接在电路上,这电容称为“负载电容”。负载电容与频率的关系是非线性的。当电容量小时频率变化量大，但当电容量加大时，其频率变化量就变小。在电路上，若负载电容量减小而使振荡频率有很大的宽限，这时频率稳定性将受到极大的影响，即便是微量的改变。振荡电路之等效回路晶体在振荡电路上被视为一感抗：如图4所表示的，为要改善振荡电路的起振条件，最佳方法就是增加振荡电路的负电阻值-R，如果振荡电路的负电阻值不足，那将会使晶体谐振电阻过大，导致晶体起振条件差，因此振荡电路在设计时需要使负电阻值大于谐振电阻的5到10倍。Fig.4:振荡电路之等效回路AT切频率温度特性温度系数是晶体谐振频率在温度变化时的稳定性或偏差量振荡模式、轴向与晶棒平面的关系、晶片的尺寸及谐波都是决定温度系数的因素。 AT切割所表现三次方程式的频率对温度特性曲线，是目前最通用的，它显示最优质之宽温范围下的频率稳定性。晶片的切割角度是在正常的工作温度范围内被要求频率的容许误差。事实上，晶片在切割、抛光的连续加工过程中，都会因加工的精度导致切割的散乱。石英晶体振荡器1、 频率准确度：在规定条件下，晶振输出频率相对于标称频率的允许偏离值。常用其相对值表示。2、 频率稳定度：2.1时域表征 在规定条件下，晶振内部元件由于老化而引起的输出频率随时间的漂移。通常用某一时间间隔内的老化频差的相对值来量度（如日、月或年老化率等）。 日稳定度（或称日波动）：指晶振的输出频率在24小时内