[time-nuts] Bye-Bye Crystals
kb8tq at n1k.org
Wed Mar 15 07:45:55 EDT 2017
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.
> On Mar 15, 2017, at 3:11 AM, Gilles Clement <clemgill at club-internet.fr> wrote:
> So what is the « best » design for DIY a high stability OCVXO ?
> I am looking after one, needed for an exotic frequency : 5184kHZ
>> 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.
>> 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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