[time-nuts] Re: Crystal sweet-spot (was: Best frequency to start for GHz synth ?)

[Bob kb8tq]

Hi

Back it the “old days” ( so 1960’s in this case ) glass packages 
were very commonly used for precision crystals. They were available 
in large diameters ( think of transmitting sized vacuum tubes). This allowed
use of larger diameter blanks than what fit in today’s much smaller packages. 

The result was that things like 2.5 MHz fifth overtone parts could be made. 
Cute things like silk thread supports for the blank were not uncommon. Yields 
simply due to the “thread tweaking” process often ran in the 10 to 15% range 
(as in 8 or 9 out of ten failed …) . 

Since there was no way to re-do the process once the part was under 
vacuum ( and no way to test it before that) this was indeed black magic. 
Occasionally somebody would do a batch and 25% would work. They
would then talk about that event for at least the next 20 years ….

One would *not* want to go back and do it the “good old way”. 

Bob

> On Apr 5, 2021, at 4:02 PM, Attila Kinali <attila at kinali.ch> wrote:
> 
> On Wed, 31 Mar 2021 16:44:40 +0000
> "Poul-Henning Kamp" <phk at phk.freebsd.dk> wrote:
> 
>> It used to be that 5MHz was the "hot spot" for crystals on the 
>> parameters we care about as time-nuts.
> 
> Depends on what kind of time-nut you are ;-)
> 
> If you are going for high frequencies beyond 1GHz, then a mesa
> type high frequency BAW is the best you can do (given you don't
> want to use frequency comb to divide down a cryogenic silicon
> cavity).
> 
> As Bob wrote, for low-frequency, high stability applications, 
> the lower the frequency of the crystal the better. Or rather,
> the thicker the crystal the better. I.e. you want to use an
> as low frequency crystal with an as high as possible overtone.
> Unfortunately, to make full use of the properties of low
> frequency crystals, you need to scale the diameter with the
> frequency. Otherwise, the energy loss due to the edges of the
> crytal will limit the Q.
> 
> For historical reasons, 3rd overtone 5MHz turned out to be the
> lowest that could be done economically with the avaible tools
> and methods and still fit the size constraints.
> 
> Today we could probaly go lower, but the market demands for
> large crystal units is shrinking steadily and, as Bob wrote
> a few times in the past, nobody has the tooling to do so.
> 
> 			Attila Kinali
> -- 
> The driving force behind research is the question: "Why?"
> There are things we don't understand and things we always 
> wonder about. And that's why we do research.
> 		-- Kobayashi Makoto
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