BVA Crystal Oscillators

BVA is a manufacturing technique for crystals used in precision reference oscillators with improved aging characteristics. BVA stands for either “Besson Vieillissement Amélioré”, from the name of it's inventor Raymond Besson, or simply “Boîtier à Vieillissement Amélioré”, from one of the few manufacturers using this technology, Oscilloquartz http://www.oscilloquartz.com

Here is a short explanation of the features and design challenges brought by this technology, from the TimeNutsMailingList:

For an explanation of how the BVA works, please see the picture below which shows schematically its internal construction. (The explaining text is in German, as it is from my periodically given crystal seminar.)

The key points which yield the BVA's improved aging are:

• the whole resonator package is made from quartz, consisting of the resonating quartz plate in the middle section and the two mounting & sealing plates on top and bottom - you may call it Hamburger style

• the resonating plate is held through quartz bridges rather than metal suspensions, thus reducing mounting stress.

• the BVA is “electrodeless”, as the electrodes are evaporated on the inner side of the upper and lower mounting plates. Therefore no stress between the quartz surface and the metal electrode.

• Therefore the energizing electrical field is applied through an airgap, which represents two small load capacitors in series with the resonator, thus making the resonator electrically “stiffer” and less sensitive to circuit influences.

On the other side you may imagine the main problems associated with such a construction:

• The difficulty of manufacturing the convex and concave shaped parts with the required precision, so that the curvature yields a constant and very small airgap.

• To realize the fine adjustment to the final frequency, because the unit cannot be tuned in the conventional way, i.e. by plating some metal on the electrode.

• The frequency accuracy to which the resonator has to manufactured, because the resonator frequency cannot be pulled with the external circuit elements by more than some hundred ppb.

It may be interesting to note, that there was a company “BVA Industries”, which wanted to generate their income solely from making BVA - which failed. Maybe because the costs could not be covered by the revenues from the rather limited market.

Bernd DK1AG