# [time-nuts] Another "atomic" clock question

Bob Camp lists at rtty.us
Sun Mar 2 11:13:08 EST 2014

```Hi

On Mar 2, 2014, at 10:58 AM, Bob Albert <bob91343 at yahoo.com> wrote:

> I'm not ready to delve into temperature measurement.  But I thought conventional wisdom is that most crystals are AT cut and an attempt at zero average coefficient is made, causing a nonlinear characteristic.  But perhaps over a limited range it's linear.  The problem of course is calibration.
>
> Again, how does one calibrate those 3 MHz ovenized units?
>
> Bob
>

Depending on their age and intended use, the crystal in an OCXO could be an AT, a BT, an X-cut, or an SC. In all cases the adjustment process is similar. The oven temperature is adjusted for minimum (or maximum) frequency to put it on the “turn” of the crystal. Then the unit is put in a test chamber and run over temperature. Based on the data, the oven temp is adjusted to zero out the temperature effects of the whole circuit. If absolute best temperature performance is not required, the temp run step may be left out.

>
> On Sunday, March 2, 2014 7:41 AM, Bob Camp <lists at rtty.us> wrote:
> Hi
>
> Assuming you are after a reference at 10 ppb accuracy:
>
> 10 ppb would be a 10 second beat note on WWV at 10 MHz. (I *hope* I got the decimal point right that time).
>
> Fire up your radio and start listening to the various frequencies. You want a time when it’s crystal clear with absolutely no fade. Yes you will wait a while to do that. Pad down your reference and do a good zero beat. Observe it for at least 10 minutes. Come back another day and check it again.
>
> You may / may not actually have 10 ppb doing this, but you will be pretty close. It assumes you have a radio, antennas, time, and a way to zero beat at more than one frequency. If you are stuck at 10 MHz it will take more time ….
>
> ———
>
> A GPSDO will run you far less than the cost of all the gear you already have for the WWV zero beat. It also will not involve a few weeks of your time checking for a good set of band conditions. Finally it will give you a reference that is at least 10X better than your target. If you intend to *set* stuff to 10 ppb then the reference needs to be 1 ppb….
>
> The other assumption above is that your existing reference is stable to much better than 10 ppb. If it’s not, then you need both a reference and a way to calibrate it. The GPSDO would give you both, since it’s got a 10 MHz OCXO built into it.
>
> Bob
>
>
> On Mar 2, 2014, at 1:48 AM, Bob Albert <bob91343 at yahoo.com> wrote:
>
> > Chris,
> >
> > Okay you want numbers.  Well, I think 10 ppb or thereabouts should do it.  Somewhere there is a discontinuity in accuracy plotted against cost and I don't want to cross that barrier just yet.  If I can get 1 ppb without a big increase in cost, I'll take that.
> >
> > My need for this is nonexistent.  I am only interested in doing it for the fun of seeing all zeros on the counter and having it give me that repeatedly.  The pleasure of knowing I am as close as the equipment is capable is what I seek.
> >
> > I'm sure many time nuts feel the same.  I am not interested in offering a calibration service or tracking spacecraft or measuring the diameter of the moon.  How do I get accurate frequency from GPS?
> >
> >
> > I have the same fetish regarding components, resistors and capacitors and inductors.  I have lots of good meters but am always looking for a better one.  I am trying to get six useful digits of voltage and resistance measurement and eventually want to do it with current as well.  Not so sure about temperature, mass, and force.
> >
> >
> > Once I get where I want to be, I'll probably go into basket weaving.
> >
> > Bob
> >
> >
> >
> >
> > On Sunday, March 2, 2014 5:46 AM, Chris Albertson <albertson.chris at gmail.com> wrote:
> >
> > On Sat, Mar 1, 2014 at 8:05 PM, Bob Albert <bob91343 at yahoo.com> wrote:
> >> Paul, as I said I just want to know how close my crystals are and be able to adjust them as well as they can be.
> >
> > Don't say "as well as can be" that can get expensive and time
> > consuming.  You need to use numbers.  Say "and be able to adjust them
> > at the 1E-8 level."
> > Then you will get advice to just use WWV.  But what if you need
> > 10,000 times better?  Then use GPS  After that it starts getting
> > harder but you still are not up to "as well as they can be."
> >
> > I admit to a few years ago using a  50 cent TTL can oscillator as my
> > "lab standard"  The part was salvage from some junk and was good to
> > about 5 digits accuracy.  It worked actually better than I needed.  My
> > RF signal generator was from the 1960's with a hand turned dial to
> > adjust the frequency.  The TTL can let me calibrate the dial.
> >
> >
> >
> >>
> >>
> >> I probably will never go rubidium (note that I qualified that) but still somewhere one has to decide where to set the frequency.
> >>
> >> I did WWV at 20 MHz for a beat of somewhat slower than one per second.  I know the phase changes but probably not much in a few minutes, as the path length doesn't vary very quickly.  And I don't need phase lock to them anyway.  In the old days they had 25 MHz and even 30 MHz for a slight improvement in settability if not stability.
> >>
> >> Bob
> >>
> >>
> >>
> >>
> >> On Saturday, March 1, 2014 7:38 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
> >>
> >>
> >>> I am trying to understand how this is done.  Should I ever get a rubidium
> >>> standard, I'd want to check its calibration, and that's not a trivial
> >>> exercise.
> >>
> >> If you assume your rubidium is stable, then it's pretty easy to check and/or
> >> calibrate.
> >>
> >> The trick is that you need someplace to stand.  A PC running ntp is good long
> >> term.  There is a tradeoff between good and long.  Good is ambiguous, but
> >> both how-good is your PC clock and how good/accurate a measurement do you
> >> want are appropriate.
> >>
> >> Probably the simplest way is to get one of tvb's preprogrammed PICs.
> >>  http://www.leapsecond.com/pic/picdiv.htm
> >>  http://www.leapsecond.com/pic/picpet.htm
> >>
> >> One approach is to use a picDIV to make a PPS and then monitor that.
> >>
> >> If you have Linux, you can feed the PPS to a serial port.  My hack for
> >> counting 60Hz will work fine at 1 Hz.
> >>  http://www.megapathdsl.net/~hmurray/time-nuts/60Hz/60Hz.py
> >>
> >> Another approach is to use a picPET and connect a modem control signal from
> >> the monitoring PC to the Event input on the picPET.  Then the data collection
> >> program grabs the time, flaps a modem control signal, grabs the time again,
> >> then grabs the text from the picPET and logs everything.
> >>
> >>
> >>
> >> --
> >> These are my opinions.  I hate spam.
> >> _______________________________________________
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> >> and follow the instructions there.
> >
> >
> >
> > --
> >
> > Chris Albertson
> > Redondo Beach, California
>
> > _______________________________________________
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> > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> > and follow the instructions there.
>
>

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