[time-nuts] Bye-Bye Crystals
Bob Camp
kb8tq at n1k.org
Wed Mar 15 22:51:25 UTC 2017
Hi
Ok, I missed that.
Unless there is also a trimmer cap, the EFC will be >> 0.01Hz if it needs to be
on frequency for any rational amount of time. If the crystals are the typical old fundamentals,
they may age 5 to 10 ppm / year when heated to OCXO temperatures. That’s +/- 25 to +/-
50 Hz just for the first year.
Bob
> On Mar 15, 2017, at 6:11 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
>
> Bob
>
> He stated 0.01Hz EFC tuning range was adequate.
> Gave no spec as to how close to nominal frequency is required though.
>
> Bruce
>> On 16 March 2017 at 10:53 Bob Camp <kb8tq at n1k.org> wrote:
>>
>>
>> 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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>>>>>
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