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| Drive Level (DLD) (激励功率依赖性) |
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The amplitude of mechanical vibration of the quartz resonator increases proportionally to the amplitude of the applied current. The power dissipated in the resonance resistance is given by Pc=1 2 q R 1 . High drive levels lead to the destruction of the resonator or the vaporisation of the evaporated electrodes, The upper limit for drive level is approximately 10 mV.
As the reactive power oscillating between L 1 and C 1 is represented by Q c =Q x P c , for P c =1 Mw and with a Q of 100.000, Q c is equal to 100Watts, The oscillation amplitude can be exceeted with relatively low level of drive P c , thus resulting in the crystal frequency moving upwards.
This frequency dependence on drive level is more pronounced with increasing overtone order. Figure 9 shows typical effects but exact prediction of the effect is not possible as it is influenced by all the elements of crystal design and operation.
Mechanical blank parameters, mounting arrangements and so on. Is it can be seen that the drive level must be specified carefully, if there is to be good correlation between the frequency of the crystal at the end of its production and in the end use equipment.
Today with semiconductor oscillator circuits a drive level of approximately 0.1 Mw appears normal, where this parameter is most specified, our production will use 0.1 Mw.
A well performing crystal should start to oscillate easily and its frequency should be virtrally independent of the variation of drive level from a starting level of about 1 nW. In todays semiconductor circuits with very low power consumption the crystal has to work well also at very low drive levels.
In fig.10 we show the effect of crystals with and without the problem of frequency dependence on drive level.
Crystal that have badly adhering electrodes or on which the surface of the resonator is not fine enough exhibit the curved effect. At low drive level they have higher resistance.
This effect is called the drive level dependence (DLD). Usually production tests of DLD are performed between 1 and 10 microwatts and then at 1Mw and again at a low load. The relative change in resistance is then used as the test criterion. Needless to say, making more measurements intermediate level increases production costs considerably.
Using suitable test oscillators permits fast of the DLD limit value, but only in the form of a Go/No-go test. IEC Draft 248 covers measurement of the drive level dependence of the resonance impedance in accordance with (DIN) IEC444-6.
Oscillation build-up problems can very largely be eliminated by optimizing the oscillator circuit by providing a sufficient feedback reserve and a "hard" switch-on pulse.
石英振荡器的机械振动的振幅会随着电流的振幅成正比例地上升. 功率与响应阻抗的关系为Pc=12qR1, 高激励功率会导致共振的破坏或蒸镀电极的蒸发,最高允许的功率不应超过10mV.
由于L1和C1电抗性的功率振荡,存在Qc=Q x Pc. 若Pc=1mV, Q=100.000, Qc则相当于100W. 由于低的Pc功率会导致振荡幅度的超过,最终导致晶体的频率上移.
随着晶体泛音次数的增加, 对于激励功率的依赖性更加显著.上图显示了典型的结果, 但是精确的预期结果还是要受到包括晶体设计和加工,机械性晶片参数,电极大小,点胶情况等的影响.
可以看出, 激励功率必须被谨慎地确定,以使晶体在生产中和使用中保持良好的关系.
当今,一个半导体振荡回路的激励功率一般为0.1mV,故在生产晶体时也一般按0.1mV进行.
一个品质良好的晶体可以容易地起振,其频率在自1nW逐步增加时均能保持稳定.现在, 晶体两端的功率很低的半导体回路也可以在很低的功率的情况下工作良好.
上图显示了一个对激励功率有或无依赖性的晶体的工作曲线的比较.
晶体存在蒸镀电极不良,晶片表面洁净度不足, 都会存在如图所示的在低功率时出现高阻抗的情况, 这一影响称为激励功率依赖性(DLD). 通常生产中测试DLD是用1~10mV测试后再用1mV测试, 发生的阻抗变化可作为测试的标准. 很显然, 在增加测试内容会相当大的提高晶体生产的成本.
利用适当的测试仪器可以很快地进行DLD极限值的测定,但是只能进行合格/不合格的测试.IEC草案248覆盖了根据(DIV)IEC444-6制定的激励功率的依赖性的测量方法.
提供具有充分的反馈和良好脉冲的最优化的振荡回路,可以极大的消除振荡的内部问题. | |
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