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
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