[time-nuts] Bye-Bye Crystals

Bob Camp kb8tq at n1k.org
Wed Mar 15 17:53:14 EDT 2017


Hi

By most modern definitions, “high stability” starts around 1x10^-12 (1 ppt)  at a tau of 1 second to 
10 seconds. There are $20 eBay OCXO’s that run at that level.  With a fundamental crystal you 
aren’t going to get to that point. 

How much EFC range are you after? 

How good a CNC setup do you have?

What kind of temperature test setup do you have? 

Simply put, the design approach is a “test over temperature / collect data -> optimize” loop. 
Without good frequency vs temperature data, you are flying totally blind. Even on a production
design, this is how it’s done. The parts you fiddle are likely to be odd shaped chunks of metal 
that fit here or there. 

Bob

> On Mar 15, 2017, at 3:36 PM, Gilles Clement <clemgill at club-internet.fr> wrote:
> 
> Hi, 
> I have a bunch of 5.184Mhz crystals. Large metallic tanks: HC33U case
> Maybe not OCXO grade, but I build a simple oscillator with a 4060 chip
> placed in a double oven, and reached 10E-9 short term stability up to 10sec tau.
> Not bad, so wondering if I can get better with a more advanced design. 
> Gilles. 
> 
> 
>> Le 15 mars 2017 à 12:45, Bob Camp <kb8tq at n1k.org> a écrit :
>> 
>> Hi
>> 
>> Where do you plan on getting an OCXO grade crystal at an odd frequency like 
>> that? Much of the performance of a good OCXO is in the crystal. Doing a proper 
>> design on one is a lot of work. You *might* think that having a design for 5.000000
>> MHz would give you a good design for 5.000050 MHz. I have empirical evidence that
>> this isn’t the case. Many years later, I’m still utterly amazed that this is the way things
>> work in the crystal business ….( = it’s not just a design issue, it’s also a business decision) 
>> 
>> More or less the crystal needs to be:
>> 
>> 1) Cut specifically to have a turn at a temperature that makes sense for your application.
>> 2) A “large blank” design (for it’s frequency)
>> 3) In a cold weld package (most of the normal crystals are resistance weld)
>> 4) Run through a high vacuum / high temperature process
>> 5) Be plated with gold rather than something like silver or aluminum (unless it’s at VHF).
>> 6) Have a motional capacitance that makes sense for your EFC range ( normally = minimize)
>> 7) Preferably be an SC or modified SC cut. 
>> 
>> This is for a high stability part. The list does keep going on for a while, but that should 
>> give you a pretty good idea. 
>> 
>> Bob
>> 
>>> On Mar 15, 2017, at 3:11 AM, Gilles Clement <clemgill at club-internet.fr> wrote:
>>> 
>>> Hi, 
>>> So what is the « best » design for DIY a high stability OCVXO ? 
>>> I am looking after one, needed for an exotic frequency : 5184kHZ 
>>> Thx, 
>>> Gilles. 
>>> 
>>> 
>>>> Le 14 mars 2017 à 18:02, Richard (Rick) Karlquist <richard at karlquist.com> a écrit :
>>>> 
>>>> 
>>>> 
>>>> On 3/14/2017 4:03 AM, Bruce Griffiths wrote:
>>>> 
>>>>> 
>>>>> Looking at oscillator circuits like the HP10811A will give some idea of some of the additional complexity required for a overtone operation. Dissecting a few ocxos may also be helpful. Some start with a 10MHz crystal and a Colpitts sustaining stage and use a 74HC74 or similar to  divide the 10Mhz by 2 and drive the output pin. Even when a sinewave output is required often a CMOS inverter drives the output pin via an LC filter.
>>>>> 
>>>>> Bruce
>>>>> 
>>>> 
>>>> I don't agree here.  The 10811 is not a good tutorial for general oscillator design.  Because it is SC cut, it has a complicated
>>>> mode suppression network across the base emitter junction to
>>>> suppress mode B as well as the fundamental.
>>>> 
>>>> The E1983A oscillator uses the same crystal (in a low profile
>>>> package).  You can read my paper about it and see that I
>>>> used a very simple bridged tee oscillator circuit.  That is
>>>> all you need to select the right overtone and mode.
>>>> 
>>>> This is the same circuit that I used at Zeta Labs 40 years
>>>> ago to design hundreds of custom VCXO's, up to the 9th
>>>> overtone.  It simply worked every time, unlike various other
>>>> designs that were in use at Zeta.
>>>> 
>>>> Around 1985, I got a consulting gig at Equatorial Communications
>>>> to redesign their 5th overtone VCXO.  Only about half of the
>>>> crystals would work in their circuit.  They had thousands
>>>> of "reject" crystals.  I just used my old Zeta circuit and
>>>> all the crystals started working again.
>>>> 
>>>> Equatorial owned the 10 meter dish that you used to see on
>>>> your right going south on 237 just before passing over
>>>> Central Expressway in Mountain View.
>>>> 
>>>> Rick N6RK
>>>> 
>>>> 
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