[time-nuts] Another "atomic" clock question

Bob Albert bob91343 at yahoo.com
Sun Mar 2 01:48:51 EST 2014


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.


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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Chris Albertson
Redondo Beach, California

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