"Stable" time references

Hi,

I need a relatively stable timebase against which to calibrate my own *local* timebase.

Assume *my* environment is reasonably well controlled (temp, etc.).

Also, assume I can make measurements over VERY long periods of time (24/7/365 duty).

Ideally, I would like the choice of reference to be something that user could trade cost/convenience/performance.

To that end, I've got a few basic ideas:

- local XTAL (too easy to "pull", no real advantage over TCXO)

- local TCXO (overkill -- hard to make this "optional")

- external PPS input from GPS (requires view of sky, expensive)

- network time service (hole in firewall, latency issues)

- LFC (low accuracy but dirt cheap, LONG integration periods)

To be clear, I am calibrating *time* -- beyond that is my own concern.

Any other ideas I should entertain?

Thx,

--don

Reply to
Don Y
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Something like this?

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Simple divider to get 1 PPS

Reply to
Kennedy

Atomic clock. Hydrogen maser.

You don't say how precise you need it or over what sort(s) of timescale so it is impossible to give a meaningful answer. Either of these reference standard clocks would be good enough to find the white light fringe again on separately recorded signals on an VLBI system.

You can buy rubidium frequency standards or modules for example:

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(much cheaper second hand)
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These days they almost all VLBI uses H-masers but there were systems in the 1980's using Rb atomic clocks synchronised to MSF Rugby or DCF77.

--
Regards, 
Martin Brown
Reply to
Martin Brown

Start with a XTAL oscillator with a constant load; whatever "pulling" there is, that would tend to make that constant, eh?

Reply to
Robert Baer

Hi, don:-

Can you quantify "relatively stable"?

You can get used Rubidium 10MHz (sine wave) timebases for about $150. They're a little bulky, draw significant juice at sometimes inconvenient voltages, and the sine out probably isn't what you want. But they're stable in the ppb range. Life is supposed to be 5-10 years, so the used ones are going to die some day not too far away.

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I'm using these little guys (below) now.. overkill for my applications, but they're only US $15 in quantity 1, they use < 20mW, and are only 5x7mm.

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If you don't mind OCXO power consumption (a few watts at start-up), these guys are in the intermediate $55-ish range in singles, and 20ppb

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Again, used ACROss are available (mostly from China).

Of course you can discipline the oscillator with NTP etc. to reduce the 1/f noise. The cheaper option above- 200ppb-ish stability, is going to drift five or ten seconds per year after calibration, and with their 4.6ppm absolute accuracy spec, two or three minutes per year before calibration.

Here's an open-source Ham project for a GPS-disciplined oscillator:-

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And, if you happen to have $1K+ to blow, here's a component-size atomic clock that fits into a relatively compact package and draws relatively little current.

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Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

By LFC, do you mean picking up radio time broadcasts? There are others RF broadcasts beyond the nominal "atomic clock" radio broadcasts. HDTV broadcasts all have a time signal, if any FM radio stations in your area support RDS (most should), that has a time signal, the digital radio broadcasts all(?) do (although those are not very popular in the US).

Other than that, the only thing that comes to mind is picking the phases off the mains (that won't get you an actual time setting, but it'll keep your clock synchronized). While there's been some talk of relaxing the standards, over longer periods (~one day) the grids are adjusted to run at exactly a net 50 or 60Hz. While there is short term drift (at high loads the generators tend to drag a bit), the operators then bump the frequency up (or down, if necessary) to compensate once enough error has accumulated (usually no more than about 10 seconds, absolute). The change is usually on the order of

+/-.01 or .02Hz. You do need a lot of filtering to read that though. Many actual line powered clocks depend on that for their timekeeping.

Given your comments one other possibility is probably out of budget, but Chip Scale Atomic Clocks can make the problem mostly go away - set it and forget it. Several small devices are available, unfortunately they're not cheap. One from Symmetricom is about $1500. ~ppb drift per month. Lower power consumption than a TCXO too (with several orders of magnitude better performance). Some of the competitors appear to be cheaper.

Reply to
Robert Wessel

I have a Rubidium 10 MHz IIRC frequecy standard from ebay. Works very well. Maybe 65 $ ? something like that. You will need a power supply too, I use a Toshiba laptop supply for it.

Reply to
Jan Panteltje

Then you will need a sonic screwdriver at the very least.

Cheers

--
Syd
Reply to
Syd Rumpo

Hi, I am not quite sure what kind of application or timebase and accuracy/resolution you are looking for. Considering that you believe a TCXO to be overkill and you want to keep correct time over a year maybe an RTC chip would work for you for instance the DS3234 RTC with integrated MEMS oscilator from Maxim Dallas it has a worst case drift

+-3.5ppm in the temperature range -40 to +85C. I had one installed outdoors for a year in temperatures going from -25 to +25 over the year. The 1 year drift was less than 1 minute. You could do fairly well with this one at your controlled temp, if you are looking for 1 second resolution at least.

Best Regards, Rune

Reply to
Rune

Why is a TCXO overkill, or even a VCTCXO, when it will do the job and is quite an affordable solution

Reply to
Mike Perkins

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Yes, apparently a TCXO.

The problem with a TCXO is EVERYONE pays for that accuracy even if they don't want it.

I.e., it's not like you can "add" the TCXO after-the-fact to an existing design (at some price adder) easily. By contrast, a GPS PPS signal can just be *optionally* connected to a spare input -- if there's no signal there (in 10 seconds), then the accuracy associated with the "GPS option" just isn't available.

(yes, I am aware that you can do differential stuffing, have different product/model numbers, etc. -- that's not what I'm looking for.)

I'm not worried about getting a particular frequency/pulse rate out of it. Rather, trying to ensure that "X units of time" is, in fact, "X units of time" (to some cost related level of performance)

Reply to
Don Y

It's overkill in the context of the XTAL option presented immediately before it. TCXO has to go in at time of manufacture. So, everyone pays for it -- even if they don't want or *need* it (or, you start dealing with different versions of a product... one with TCXO, one without).

XTAL is dirt cheap -- "everyone" can afford one (but the accuracy sucks -- esp without tweaks)

Reply to
Don Y

Sorry, "Line Frequency Clock" (AC Mains). But, the radio approach is one I hadn't thought of!

Thanks, I'll look into it. I've not been keen on relying on an external reference simply because it presents another attack vector (why expose something if you don't *have* to?)

Yes -- my LFC reference.

To some extent, I can deal with *really* long sampling intervals (to get finer precision/accuracy).

However, there *has* been talk of easing this (in US -- I think Europe expects EXACTLY 50Hz*60sec*60min*24hr cycles in a day!). I am not sure what the benefits would be (why dick with something this fundamental if there isn't some *big* payoff?) but am leary that all these alternative power sources (e.g., solar) coming on line may cause something like this to be sacrificed on the altar of expediency...

Yes. I've looked at some power line monitors and rarely see any "events" outside the 0.05Hz band.

Ow, ow, ow... I'm really trying to be cost-conscious, here. Getting as much as I can for as few dollars -- even if it requires more "smarts"/planning.

THE LFC option is one I have typically used in the past. Let it act as a reference over a *long* integration period for the finer grained local clock. Takes a while to settle but for things running 24/7 that's not an issue (as long as your "nominal" is pretty close and you open up the loop a bit to start with).

What I *don't* want is the "ceramic resonator" sort of "it kinda works" approach. Yet, don't want to count electrons flying around a nucleus... :-(

Reply to
Don Y

Is it possible to get the effect of a TCXO by thermally characterizing a normal XO? Then the "value added" would be making the tempco measurements and setting the correction factors, for those that pay. You would still need to measure temperature but perhaps you do that already or in any case it can be done very cheaply if needed.

--

John Devereux
Reply to
John Devereux

I may have taken a wrong turn in interpreting your requirement, so apologies if that is the case.

A GPS-derived solution is not all that expensive, and it doesn't have to be complicated. What they can provide is a reference frequency and a serial output providing time with minimal latency and jitter. A "view of the sky" isn't always a necessity for GPS reception.

Many such systems (as developed way back by Brooks Shera, documented along the way in dozens of other incarnations, and improved somewhat in the article linked by Spehro) use a 1pps output from a GPS receiver. Far better short-term acquisition is achieved if a GPSr with a 10kHz output is used, and the disciplined crystal oscillator itself can be relatively primitive in that case, dependent on your perceived need. We developed a disciplined 10MHz frequency reference oscillator for in-house use, and the oscillator was a crystal/74HC04 combination!! Later we discovered that James Miller had documented his very similar system:

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where he used a Rakon OCXO, and his current product:

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probably still does.

The only difficulty in DIY is with sourcing a GPSr with the 10kHz output. The Conexant/Rockwell Jupiters appear only occasionally on fleabay. If you go the route of purchasing the Miller unit, that problem has been solved for you.

Usual caveat - I have no pecuniary interest in Miller's product, but I do have respect for the solution he offers.

Reply to
pedro

On a sunny day (Tue, 03 Dec 2013 06:22:25 -0700) it happened Don Y wrote in :

That will not work in many places (reception conditions, I have tried it), and is exactly why I bought the Rubidium unit. Just add a 10 MHz input that the user can PLL the crystal oscllator to if they want.

Reply to
Jan Panteltje

Is the 60Hz AC line still "correct" once a day? What about the broadcast time stamp they use for the cheap "atomic clocks" we hang on the wall. (I can't recall the name.) Just look up on the web and get the time.

Try asking at time-nuts.

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George H.

Reply to
George Herold

That's, in a sense, what I am trying to do "at runtime" -- *if* I have access to some other reference. I want every device to be built the same way -- no "separate processing" for different accuracies, etc.

What I want is user to be able to "add" something to the system after-the-sale (i.e., I don't even want to be in the business of *making* that "something") that magically unlocks extra capabilities (that were present all along just not usable because the system didn't have this "external reference" to consult).

E.g., the example, elsewhere, that I posted re: monitoring a digital input for a PPS signal and, if present (and *appears* to be "about 1Hz") using it *as* the "1 Hz reference". OTOH, if it is *absent*, the associated software disables itself simply because there's nothing it can work off of!

Imagine a timepiece in your home. You can buy a really *good* timepiece that keeps really accurate time. Or, settle for "whatever is cheapest". *OR*, hire someone to keep reseting them on a daily basis!

I.e., folks who don't care about accuracy buy whatever is cheapest (basic system). The "really good" option isn't available (cuz I don't want to be in that business of making different models, etc.). So, those who want better hire someone to keep their clocks up to date -- an external GPS, LFC, . And, no one else "pays extra" (software is free :> ) for their needs.

Reply to
Don Y

It is held in theory so that the long term average is right so that any older synchronous motor based mains kit keeps good time. However, it runs slow during the day and fast at night which used to annoy astronomers back in the days when they used mains synchronous motors for sidereal drives to track the rotation of the Earth.

The usual way is to control a local oscillator by phase locking it to one or other broadcast reference time signal. The wall clocks use a standard watch crystal and lock it to the reference second either analogue with a tunable capacitor or by tweaking the divisor slightly.

--
Regards, 
Martin Brown
Reply to
Martin Brown

How about a Rubidium oscillator from ebay? We got an old one and it's a couple parts-per-billion off from a caesium standard.

Fun: trigger a scope from one, and look at the rising edge of the other at, say,

10 ns/div. It looks like an internal trigger. Come back an hour later, and it's moved a little.
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Reply to
John Larkin

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