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Total Frequency Control News ....

Quartz crystals for OCXO in telecomms applications - May 2013

Applications for precision oscillators in telecomms demand the finest specifications that must include long term and short term accuracy, jitter and phase noise. (oven controlled precision oscillators) provide the very best and reliable specifications for telecomms.

Quartz crystals have excellent temperature stability in their own right, but their physical properties still change with respect to temperature and that causes a corresponding change in their resonant frequency. These changes are dependent on the orientation of the crystal when "cut".

The above diagram shows the precise cut orientation to provide an SC (stress compensated) quartz blank. This is one of the double rotation “cuts” popular with OCXO designs. In practice, to get the required stability at higher temperatures, most SC cut OCXO designs employ ‘modified’ SC cut crystals. These are crystals that have one of the cut angles modified in order to provide a shallow inflection point at a higher temperature than can be achieved with a ‘true’ SC cut.
SC cut crystals are the most commonly quartz resonators used in precision OCXO and have their temperature stabilised by the oven in order to minimise errors due to the temperature coefficient of the quartz material.
The design process starts with the required stability and frequency.  This will then dictate the crystal design parameters, such as: does it have to be a fundamental or an overtone crystal?  What temperature does the OCXO oven have to operate at in order to achieve the final design parameters?  What actual size must the crystal be to meet the physical demands of the final design? All of this will require a considerable amount of detail to be specified for the crystal.  Once this is understood a crystal specification can then be generated.

The design of the oven required to hold the crystal at its most stable point will depend on the size of the crystal and the final size of the oscillator package, as this will set the offset required in order to meet the final temperature range of theOCXO.

Careful design of the oscillator utilised within the OCXO will ensure that the crystal design parameters are not degraded more than necessary, often with particular care given to certain areas of the design that are application specific. If the OCXO is intended for a timing requirement, maybe as a master reference in some network application, then care may be taken to only lightly drive the crystal so that the long-term stability of the OCXO
Here there will be an emphasis on the pulling range of the oscillator, which is generally in contradiction to the phase noise requirement. If the final application is in ruggedized portable devices, then the requirement may have to focus on g sensitivity, which will dictate not only the oscillator design, but will have a heavy influence on the crystal design options.

A further enhancement may include a precision OCXO locked to GPS signals and the inherent Caesium precision of this data. Such oscillators provide a precision holdover when GPS lock is lost and regain the time and frequency lock when the signal is regained.
As with most precision oscillator applications there is often a compromise between the ideal and the practical and detailed consideration of all parameters both of the crystal and the oscillator is crucial to optimise the oscillator specification.

RFX Ltd manufacture bespoke precision oscillators using SC cut crystals and are also able to qualify their designs to military specifications and for extreme shock and vibration requirements.

Please send any enquiries to sales@tfc.co.uk


 

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