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Although the frequency stability of a typical TCXO is more than an order of magnitude worse than a precision OCXO, their acceleration sensitivities are often similar. This means that their dynamic stability and phase noise will also be similar since the vibratory performance is determined by the Γ characteristic and the applied acceleration level and is largely independent of the static phase noise and frequency stability. Unless Γ and/or a are extremely small, any random vibration will cause the noise floor of the oscillator to rise. The phase noise degradation that occurs in a 10-MHz TCXO with Γ of 2.5 ppb/g is shown in Fig. 7

Manage Quartz Crystals Under High Vibration, Fig. 7

If it is determined that the acceleration sensitivity of an oscillator must be improved to meet system specifications, there are several approaches that can be considered. If the highest vibration levels come primarily along one axis, it may be possible to mount the oscillator so that the Γ vector is perpendicular to the axis of vibration o that the vibration is applied along the plane of “zero” sensitivity. In some instances, a 10:1 improvement could be realized with this approach. In most real-world applications, however, there is significant vibration present in all axes, so this approach may be of limited benefit.

Special crystal design techniques can be used to reduce the acceleration sensitivity to some degree. Precision SC-cut resonators that employ unique stress relieved mounting structures are available that exhibit acceleration sensitivities as low as 0.2 ppb/g. Other crystals, such as certain rectangular strip-type crystals, can be manufactured with special mountings that achieve sensitivities as low as 0.1 ppb/g with some yield.

Mechanical vibration isolators can be effective in some applications by reducing the vibration energy that is conducted to the oscillator. Isolators act as lowpass filters: attenuating the vibration energy above their natural resonant frequency, but passing all of the energy below said frequency. This frequency is dependent upon the stiffness of the isolator and the weight of the supported assembly. With a small, lightweight assembly such as an oscillator, it is difficult to achieve effective attenuation below a few hundred Hz. And depending upon the “Q” of the structure, the vibration energy is actually amplified near the natural frequency. Care must be taken to ensure sufficient “sway room” to allow for movement of the assembly, especially if excited near the resonant frequency.

Other considerations with mechanical isolators include directional dependence, changes in their stiffness over temperature, additional weight to lower the resonant frequency, and the space required to mount the assembly. A properly designed isolation system can be quite effective in reducing high-frequency vibration energy. However, a system which is not properly designed can actually make performance worse under some conditions.

Even after many years of work, there has been little progress in producing resonators with sensitivities below the low parts in 1010/g level. In order to achieve better performance, some sort of compensation must be used. There are two basic types of compensation: active and passive. Active techniques employ an acceleration-sensitive component such as an accelerometer to sense the applied vibration or acceleration. That signal is then processed with the proper amplification and phase reversal and fed back to the oscillator to cancel the effects of the vibration on the resonator.

This method can be quite effective, producing oscillators with Γ to low parts in 1012/g at lower vibration frequencies. But as the vibration frequencies increase beyond about several hundred Hz, it becomes very difficult to maintain the amplitude and phase linearity required to sustain those levels for cancellation. And adding the required circuitry significantly increases the size and cost of the oscillator.

Manage Quartz Crystals Under High Vibration, Fig. 8

Passive compensation methods employ multiple crystals which are carefully aligned to that their Γ vectors are aligned anti-parallel (pointing in exactly opposite directions). The acceleration-induced positive frequency shift of one resonator is therefore cancelled by a corresponding negative shift from the other resonator. The crystals are connected either in series or in parallel in the oscillator circuit and function as a single composite resonator.

This method has been known for more than 30 years,6 but has historically been difficult to implement due to lack of consistency in the Γ vectors of conventional crystals. Both the magnitude and the direction of Γ must be matched closely if significant cancellation is to occur. Recent advances in small AT-cut strip crystals designed for rugged applications have achieved better consistency in the Γ vector, enabling practical designs that achieve sensitivities in the low parts in 1011/g.7

Passively compensated TCXOs are now available that provide ultralow acceleration sensitivity in all axes. The performance of a passively compensated TCXO is shown in Fig. 8. Even under the same moderate random vibration, as shown in Fig. 6, the vibration induced noise is still below the quiescent phase noise of the oscillator since |Γ| is a very low 0.02 ppb/g or 2 × 10-11/g. Oscillators of this type are being produced to frequencies greater than 100 MHz.8 For applications where operation in the presence of significant vibration is required, this type of passively compensated crystal oscillator may provide the best combination of critical performance for the cost.

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