[time-nuts] Another "atomic" clock question

Sun Mar 2 11:21:08 EST 2014

Hi

Trimble is one of many companies in the GPSDO business. A GPSDO is an oscillator that is locked up to the GPS signal. In many cases the oscillator is a pretty good OCXO. Trimble made a lot of these for the cell tower people. They now show up pretty regularly on the surplus market.

A GPSDO from just about anybody will give you a very good local reference. They cost $100 to $20,000 depending on where you get them and how hard you shop. I would suggest one at the lower end of that range for what you are trying to do :).  A number of them have sold recently on eBay for about $130. 

Most GPSDO’s will put out 10 MHz. Plug that into your counter / synthesizer / radio / service monitor and it’s as accurate as the GPSDO. Doing a frequency check is now as simple as running your test gear. No added steps no messing around. 

Bob

On Mar 2, 2014, at 11:03 AM, Bob Albert <bob91343 at yahoo.com> wrote:

> Yes Nigel, it's a waste of time but so are computer games and going to Disneyland and such.  We do it because we get pleasure, and nobody can criticize that.
> 
> I am a bit confused over your mention of Trimble units.  I'm not familiar with them or what they are supposed to do.  I better do some homework.
> 
> I know if I use an X-Y 'scope with two reasonably clean signals I can adjust one for a stable pattern and so, depending on how long it holds still, know how close the two frequencies are.  I can get one signal from my counter time base, but where do I get the standard signal?
> 
> Bob
> 
> 
> On , 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
> 
> 
> 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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> 
> 
> 
> 