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"Stable" time references

You need a TCXO or OCXO as part of a GPSDO. A GPSDO works by tweaking the oscillator up or down, depending on a phase or frequency measurement performed anywhere between 1 sec and several minute intervals. In effect, it's a very slow phase or frequency locked loop. That means that the noise, jitter, spurs, and junk produced by the oscillator will appear directly on the GPSDO output signal. If you start with a dirty oscillator, you end up with a dirty output, no matter how elaborate your GPSDO design. Try running some test equipment with an external reference signal that has a mess of 60 Hz FM noise on it, and see how far you get. Think of the FM noise as producing "uncertainty" in any measurement.

I don't understand what you mean by "external" TCXO. You can put the TXCO anywhere, but if it's part of the GPSDO loop, it has to operate as part of the system.

Ultimately, most frequency references either talk to a time service, or have a built in atomic reference available. Those references are transferred to your TCXO and OCXO which in turn produces your output. You could probably build something based on the transit of the moons of Jupiter, but I don't think you have the R&D budget.

Drivel: Speaking of a noisy reference, I had an AC power ground loop problem as parts of the building were on different AC power grounds. I could produce a substantial spark when connecting coax grounds that ran between buildings. There was also an RF leak that superimposed a local AM radio station onto the 10 MHz reference signal. Since I was doing the bulk of the complaining, I was soon sentenced to fix the problem (late at night, so I wouldn't disrupt production). The company distributed the 10 MHz reference signal over a fairly large area, through hundreds of feet of variable quality coax, to assorted test equipment, some of with had the external reference shield at case ground. In order to fix this, I added home made isolation xformers and filters at each station. I also built several distribution amplifiers so that some idiot (i.e. me) would not shut down the entire company by accidentally shorting the reference signal to ground. I also made a feeble attempt to properly terminate the cables at 50 ohms, but that was futile due to the large number of T connectors in the system. I almost forgot to mention replacing about 50+ badly soldered BNC connectors with crimp BNC.

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Reply to
Jeff Liebermann
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The use of the word "stable" in both metrology and animal husbandry is not a coincidence. While the equine meaning of "stable droppings" is quite obvious, the same term was also used to describe crappy measurements caused by unstable references in a calibration lab about

40 years ago. 
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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

I compared my ancient HP 5328A with the

010 high stability time base option to a Sulzer Laboratories frequency standard that I rescued from the recycling bin.

The two agreed within something like 30 parts per billion over a week's time.

The 5328A is nice because it has a 10mhz bnc on the back and standby mode that keeps the crystal warm. Plus you have a counter timer.

D> Hi,

Reply to
Jim Stewart

Does this relate to time-of-day time? If you have internet connection, NTP (US naval observatory network time protocol) does a very good job, even compensates for transmission delays.

Milliseconds over a one-year period makes it a solution with 10**-9 accuracy. It's commonly used in a mode where you only update once a day, but that's something you can program around.

Many crystal clocks (not all) can get 10**-7 precision over short periods (but even an ovenized crystal clock isn't as good as the atomic time standards).

Reply to
whit3rd

You can buy a GPS time reference module from Trimble. About $40 or $50 from a hobby shop in England. I've made up two of these systems for our shop. The module has a 10MHz and a 1 PPS output. You can use these outputs to train a master clock as they do have a bunch of jitter.

Reply to
qrk

As someone else mentioned, the stabilized RTC chips from Maxim/Dallas can be pretty good. I've used them for years in moored oceanographic instruments that can spend a year out of touch with the world----including mains power and GPS signals. We often get instruments back whose clocks are within

30 seconds of each other after a year under water. That's a difference of about 1PPM. These moored instruments do have the advantage that the temperature probably doesn't change more than 5 deg C. over the interval and the instruments are within about 5 deg. C. of each other.

Another option might be a receiver for the 60KHZ time broadcasts if the system is going to a place where those can be received. The receivers must be fairly low power and cheap----there's one built in to my $35 Casio wristwatch.

Mark Borgerson

Reply to
Mark Borgerson

In the words of Ford Prefect "Time is an illusion; Lunchtime doubly so" ;-)

Following the actual rate of time would rather depend on how the anomilies in the gravity field affect its apparent passing and the viewpoint you are observing its passing from.

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Reply to
Paul E Bennett

I just thought the choice of the word "ensuring" was irresistible. ;-)

Reply to
krw

All I've seen were 10 KHz, not 10 MHz out.

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Reply to
Michael A. Terrell

Ok, Which horse do you use for a NIST?

Example: Years ago, a friend of mine saw this nice 2 HP bench grinder, made in CHINA of course. Now since this deal was irresistible, he purchased this grinder to replace a 0.5 HP grinder he had with a large stone to sharpen things like axes, knifes etc..

Upon setting it up and trying it out, he found out quickly that just about anything would slow it down to a crawl at a point to which it would trigger the centrifugal switch.. After playing around, he came to the final conclusion that the horses in CHINA must be a lot smaller than those in the US!

Hence, which horse do you use for a NIST calibration ?

Jamie

Reply to
Maynard A. Philbrook Jr.

Note the 10 Mhz outputs:

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

"10MHz" has been referenced several times in this thread. Of course, any frequency can be a "reference" (to the stated accuracy *of* that reference). But, is "10MHz" a "standard" -- even if informally so -- in much the same way that 1PPS is a defacto "standard" output from a GPS Rx?

I.e., if I design an input that expects a digital signal and, if I see "more than several" transitions in a given "nominal, uncalibrated second", *assume* it is a 10MHz reference? And, if I see *some* transitions but not "enough" in that same second, assume it is 1PPS? Otherwise, assume there is no reference connected?

Does this cover the majority of cases? Or, do you need to be able to *specify* that you have a 23.5KHz "reference" attached? (remember, I'm not just looking for a "frequency" but a "standard")

Thx,

--don

Reply to
Don Y

I think I can guarantee the 5C range -- though I'll be up around room temperature (whereas I suspect you were closer to 0C?) Not sure if that lessens or exacerbates the problem...

But, that seems lie a "design *in*" solution vs. a "bolt on"

*addition*. I.e., I assume you need a bus (even if it's a "one wire") to talk to the device -- you don't just "listen".

Accuracy? Do you have any way of knowing how often your watch *loses* access to that reference? (it's actually more than I was asking for as I don't really care what "time" it is -- though I can obviously work with that)

Thx,

--don

Reply to
Don Y

It's to drive the basic timebase in the system. ToD ends up being one aspect of that -- but that's not an essential function.

I'm trying to avoid unnecessary reliance on external references THAT THE USER CAN'T CONTROL as they represent potential attack vectors.

Yes, though that doesn't ensure that the device remained that closely synchronized throughout the entire period -- just that it was able to get to that precision *at* the end of the year.

Yes. What I want is a "cheap" solution for the masses that others can "upgrade" (without my direct involvement) to fit their needs.

Reply to
Don Y

On Sun, 08 Dec 2013 02:46:29 -0700, Don Y Gave us:

Yes, 10MHz is a timing standard for any systems of devices which are not all from the same maker, yet all need to operate on the same RTC or time bases.

Satellite baseband stations, networking gateways. Shipboard navigation systems, Targeting, etc.

Anything where inter-device timing slews can cause major problems with being able to rely on the information such a system produces.

Reply to
DecadentLinuxUserNumeroUno

I do use an I2C bus (2 wires plus ground) to communicate with the DS3232 as I set and read the clock and calendar. However, you could simplifiy things by just using either the 8192Hz clock output or the 1Hz interrupt output from the DS3232. In some systems, I use the 1Hz output as an interrupt input in a sofware phase locked loop to correct the clock on a microprocessor with a less-stable crystal oscillator.

My watch checks that reference about once a day. Sometimes it misses a day if it spends the night in a place with too much surrounding metal. I guess the maximum error your would expect would be the difference between your system clock and the time standard over the maximum time you might expect the system to miss the 60KHz signal.

Mark

Reply to
Mark Borgerson

WWVB and other time broadcasts are fairly inconvenient sources of accurate time. There are two big problems: unknown propagation delay and slow modulation rate. Propagation delay could be as large as ~tens of milliseconds; you have to know your location to correct. Slow modulation makes difficult to find exact location of time marks and relate them to carrier period; it could take hours of averaging. There is no problem if the accuracy of 1s is sufficient. Getting to the accuracy of 1us isn't trivial.

Vladimir Vassilevsky DSP and Mixed Signal Designs

formatting link

Reply to
Vladimir Vassilevsky

Is the Loran-C navigation system at 100 kHz still working in the USA ?

It should solve the location and propagation delay issues, however, you would need some means of telling when (ToD) the transmission code sequence for each transmitter started.

Reply to
upsidedown

Unfortunately not. Shut down in 2010. Stupid, stupid, stupid. But that's a rant for another day.

Reply to
Robert Wessel

I'd need someone to put it in a box and agree to sell it as a product. Hard to see much of a market for it other than as a 1PPS frequency reference (in that configuration).

I'll keep it in mind and see if I can cajole my buddy to supporting it as well...

Oh, so there is a *time* at which it goes looking for the signal? (I assumed it was monitoring it constantly and just freewheeling when it lost signal)

Yes. Watches also have the benefit of reasonably constant temperature operation (at least while they are being worn).

I've got an "atomic clock" here. Maybe I should open it up and see what's inside.

Again, I'd need to find a COTS "module" that just gave me some convenient signal that I could "listen to" instead of "interact with". This approach has the downside that if it can't lock onto the signal *when* it's needed, it's ineffective (whereas the Dallas device would be pushing out "valid signal" regardless).

Thx!

--don

Reply to
Don Y

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