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

[Bob kb8tq]

Hi

A “modern” 5 MHz 5th overtone AT cut resonator *should* be able to 
hit a Q of 5 million. Indeed one has to do everything right to hit that 
number. More or less it is the design that “defines” the Q of a bar of 
quartz. (Yes, there’s more to it …..). 

Since that part uses a blank that is about 15 mm in diameter you *could*
scale from there. This is never going to be the whole story some things 
get messy. 

Freq	 MHz		diameter mm

5 				15
2.5				30
1				75
0.5				150
0.1				750

The obvious point here being that a 3/4 M diameter resonator blank 
just isn’t going to work. Things like the Essen Ring are a way to get 
around some of this. Yes, there are trade off’s. That’s what keeps 
crystal design guru’s employed.

Bob

> On Apr 5, 2021, at 5:40 PM, Alan Melia via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Bob somewhere buried in my collection of interesting bits I have a rather battered demo sample of what the British GPO Research crystal labs refered to as an Essen Ring. It is indeed around 3 inches in outer diameter and almost an inch thick and wide. this would have been cut from natural quartz. I suspect this specimen is a 'failed' sample. The ring is suspended in a couple of silk threads.I also have what I think is a higher frequency ring which is mouned in a 1.5inch diameter evacuated glass holder.
> I was told that if the large ring was tapped gently with a pencil it would 'ring' for 5 minutes (that there may be some exageration  there) I believe Qs of 5E^6 were mentioned. More reliable info would be found in the reports :-))  This was around 60 years ago.
> 
> Alan
> G3NYK
> 
> ----- Original Message ----- From: "Bob kb8tq" <kb8tq at n1k.org>
> To: "Discussion of precise time and frequency measurement" <time-nuts at lists.febo.com>
> Sent: Monday, April 05, 2021 9:43 PM
> Subject: [time-nuts] Re: Crystal sweet-spot (was: Best frequency to start for GHz synth ?)
> 
> 
>> 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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