[time-nuts] Super stable BVA Quartz resonators... BVA??

[Bruce Griffiths]

Bernd T-Online wrote:
> Hal Murray wrote:
>   > Why is the top domed?  I assume flat would be easier to manufacture.
>   
>> Why is it not symmetrical?  If the top is domed, why not the bottom too?
>>     
> For a plano-parallel qaurtz resonator the diameter must be at least 60 
> times larger than the thickness, otherwise the vibration amplitude would 
> be too high at the edges, and the suspension would damp teh vibration 
> hence the Q would be lowered. For a 10 MHz 3rd overtone SC-cut you would 
> need a crystal of 40 to 50 mm diameter!
> By making a convex contour, the vibration is trapped in the center of 
> the plate, and therefore the Q keeps high (and other parameters improve 
> also).
> Bi-convex contours are more difficult to manufacture, as it is required 
> that the symmetry axis of the upper and lower contour must coincide. 
> Also other parameters become worse. For the BVA the manufacturability 
> would also be much worse as it already is.
>
> Regards
>
> Bernd
> DK1AG
>   
Bernd

Whilst traditional optical edging techniques can easily remove any wedge
when both surfaces are convex, maintaining alignment of the "lens" axis
with respect to the crystal axes is another matter, at least when using
traditional surfacing techniques, with computer controlled surfacing
techniques even this can be done.

With a planoconvex "lens" bias polishing or an equivalent technique can
be used to adjust the inclination of the plano surface with respect to
the crystal axes and this alignment is maintained during edging leaving
only axial thickness adjustments to be made. The etching process used to
remove cracks and defects after mechanical polishing is anisotropic
which may introduce further complications in maintaining alignment and
shape.

Bruce