"Stable" time references

Something like this?

Simple divider to get 1 PPS

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:

(much cheaper second hand)

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.

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Regards, 
Martin Brown

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

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.

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.

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

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:-

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.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward" 
speff@interlog.com             Info for manufacturers: http://www.trexon.com 
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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.

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.

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

Cheers

--
Syd

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

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

[snipped URL due to line length]

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)

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)

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... :-(

So

ObDisclosure: I don't even know if that thing even works; just indicating that it exists.

"Requires shipping" is a logistical/planning/information gathering failure.

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Les Cargill

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.

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John Devereux

The manufacturers of femtocells have this kind of problem. It might be worth your researching their solutions.

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.

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.

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.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

GPS!

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators