Feature Frequency Control


The effects of vibration on crystal oscillators


Crystal oscillators provide a stable and accurate frequency source for many products and applications. However, phase noise and spectral purity have become increasingly more important to design engineers in many applications so it is important to consider the effects of vibration. Andy Treble at Euroquartz, in collaboration with Greenray Industries Inc., explains


O


ne aspect that is often over- looked during the initial design stages – especially if phase noise is a critical consideration – is degradation in spectral output and phase noise due to effects of vibration. It has been found that even moderate levels of vibration can significantly introduce an unwanted low noise signal. Therefore, in applications such as


mobile platforms or vibrating


machinery, vibration induced phase noise can often be greater than all other noise sources.


The acceleration sensitivity of com- ponents is often a limiting factor in such applications, and the quartz crys- tal has proved to be the component most susceptible. It has been shown that the sensitivity of quartz is vecto- rial in nature and is heavily dependent on the direction of the applied force. Although there have been many improvements in quartz crystal tech- nology the basic acceleration sensitiv- ity remains the same. The acceleration sensitivity is known as ‘g-sensitivity’ and in well designed oscillators is mainly due to the crystal element itself. The stresses imparted into the crystal will result in small changes in the series resonant frequency of the oscil- lator. The magnitude of these


The magnitude of Γ is linear for applied forces greater than 50gs while


the frequency response of Γ will be relatively


flat through vibration


frequencies of several kHz. However, mechanical resonances in the crystal blank and mounting structures may cause peaking depending on the type of crystal used.


Oscillator characterisation The acceleration


sensitivity of an


oscillator intended for use in a high vibration environment is typically measured by applying a known vibra- tion level to the oscillator and moni- toring the output spectrum responses. If a single frequency sinewave vibra- tion is applied, a discrete spur will be


frequency changes will depend on the instantaneous amount of the accelera- tion as well as the direction in which it is applied. Maximum frequency shift will occur when the acceleration is applied in a parallel direction to the vector. The frequency during accelera- tion can be written as the product of two vectors:


observed and it will be offset from the carrier by the vibration frequency. By measuring the spur level relative to the power of the carrier, the amount of deviation may be calculated by FM modulation theory.


It is necessary to measure the responses along three mutually per- pendicular axes which are usually selected to be normal to the oscillator package faces. Once the magnitude and angular orientation of the vector for acceleration sensitivity have been determined, it is possible to predict the oscillator performance under vibratory conditions.


The application environment Naturally, it would be easy if the oscil- lator experienced a vibration at a single frequency as the sideband levels can be calculated if the applied ‘g’ is known. Unfortunately in most applica- tions the vibration energy will be spread across a frequency band pro- ducing a random vibration profile. This profile will be entirely applica- tion specific with most of the energy typically ranging from 5 or 10Hz up to 10-20kHz.


Andy Treble is Sales Director at Euroquartz


Bandwidth vector signal analyser released T


ektronix has introduced its latest MDO4000B series of mixed domain oscilloscopes that features enhanced spectrum analyser performance. When used with the company’s SignalVu-PC, they offer wide bandwidth vector signal analysis capability with advanced support for WLAN 802.11 a/b/g/j/n/p/ac testing. The oscilloscopes feature an integrated spectrum analyser enabling capture of time-correlated analogue, digital and RF signals for a complete system view of a user’s device. They combine the functionality of the company’s mixed signal oscilloscopes (MSO) with a spectrum analyser in a single instrument and feature significantly enhanced performance compared to previous generations. Tektronix www.tektronix.com Enter 218


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The source of vibration and the mechanical mounting of components will cause peaks in response due to energy concentrations. Unlike sine vibration which produces a discrete spur, this random vibration will increase the phase noise floor of the oscillator across the frequency band of the vibration. In order to ensure that an oscillator will meet the performance required in the final application it is imperative that the manufacturer understands the details of the application, such as the expected vibration levels and frequen- cies, plus the required performance under these conditions.


In conclusion, it is not possible to eliminate all of the effects of vibration on a crystal oscillator, but by under- standing the system requirements, selecting the best technology and properly specifying the oscillators, it is possible to achieve the best match and deliver acceptable performance for the application. Euroquartz www.euroquartz.co.uk


Enter 209 DECEMBER/JANUARY 2014 Electronics


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