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.