[time-nuts] Bye-Bye Crystals
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Wed Mar 15 22:11:31 UTC 2017
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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