[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. 
>   
That problem was solved over a century ago in optical lens manufacture.
If the two surfaces are spherical, then such decentering is equivalent
to adding a wedge, which is easily removed by optical centering and
edging techniques.
> Also other parameters become worse. For the BVA the manufacturability 
> would also be much worse as it already is.
>
> Regards
>
> Bernd
> DK1AG
>   
Bruce