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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Dec 10 20:40:56 UTC 2007


Chuck Harris wrote:
> Bruce Griffiths wrote:
>   
>> Bernd T-Online wrote:
>>     
>
>   
>>> 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
>>     
>
> So Bruce, what source did you read that led you to change your original
> answer from --it's easy--, to --it's difficult--?  (reference your quoted
> text below:
>
>   
No reference (unless you want references to optical manufacturing
techniques), its obvious when you stop to think about it, the various
crystal cuts (AT, BT, SC etc) require a specific crystallographic
orientation, changing the orientation changes the characteristics.
When I realised that Bernd hadn't explicitly stated the requirement to
maintain crystallographic alignment when contouring the 2 surfaces the
real cause of the difficulty of manufacture became obvious.

Introducing wedge when generating the spherical (other contours are much
more difficult to produce unless one uses computer controlled equipment
together with optical shape measurement) surfaces is geometrically
equivalent to changing the crystallographic orientation of the blank.
This doesnt arise when imparting a spherical contour to only one
surface, as the plane surface defines the crystallographic orientation
before and after removing any wedge using optical centering/edging
techniques.

The original statement merely indicated that well known techniques
provide a solution to his actual statement of the problem of centering
the 2 surfaces.
>  > 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.
>
>
> -Chuck Harris
>
>   





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