[time-nuts] Bye-Bye Crystals

Gilles Clement clemgill at club-internet.fr
Wed Mar 15 15:36:07 EDT 2017


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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