"Stable" time references

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

Reply to
krw
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Ah, I'm not a fan of "doing things over" :> If a client starts talking about a "model 2", I bow out of the transaction: "No bid". Why do a model 1 if you're only going to then do a model 2? Do the model 2 and be done with it! (if you want to cripple it, then plan on that from the outset -- don't try to "enhance" it later!)

"Yeah, well, we've got this *horse*, see? And we're planning on adding a *hump* in the next version... we're tentatively thinking of calling it a *camel* so customers can differentiate between the two models..."

Hump envy?? :>

Reply to
Don Y

"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

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

Reply to
Robert Wessel

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

Reply to
Robert Wessel

.....

The first question must be: What sort of accuracy and stability do you need ?. Also, possibly phase noise performance ?.

I've used a variety of frequency standards in the lab over the years, ranging from ex mil ovenised oscillators to compact rubidium. Currently using one of the ex telco HP gps disciplined smart clock units, (Z3816) which I bought from Ebay US around 7 years ago, It has a 1pps, 10MHz and other telco related frequency outputs, which drive all the signal generators, counters and other test gear, as well as an hf receiver. Doing that means that all the test gear agrees to within 1Hz or better and the accuracy approaches Caesium. Have only in the past few weeks finished a crystal filter / distribution amplifer to improve the performance and it all just sits there in the rack quietely doing it's job.

If you are interested in time and frequency in general, suggest you have a look at the archives and perhaps subscribe to the "time nuts" list over at febo.com. There's also a volt nuts list as well. Good stuff, both of them...

Chris

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Reply to
tridac

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

The whole point is to let the user buy/add whatever *his* needs dictate. Some may want good short term accuracy. Others long term. Still others settle for out-of-the-box performance levels.

I don't want to be in the business of supporting all those options. Rather, would just like to add a *hook* that will accommodate a range of *likely* devices.

Yes, already suggested upthread.

Thx,

--don

Reply to
Don Y

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,

All you need is a decent shortwave radio with a decent antenna that can pick up 5000 Hz, 10,000, 15,000, 20,000 Hz. Depending on the time of year; different frequencies will travel to you home local. The atomic clock in Bolder Colorado transmits the time on these frequencies, one second ticks. You will have to compensate for your distance away from Boulder Colorado of course to get a precise time. There are also some digital clocks that pick up 60 Khz which is a sync frequency also for the atomic clock in Boulder Colorado.

Shaun

Reply to
Shaun

No.

There are still environmental factors that affect the signal. But, some of these can be mitigated depending on usage patterns.

I'm not interested in time-of-day. The LORAN beacons would be most effective as frequency references. I.e., using the GRI as a reference. Or, time between pulses. Or, counting 100KHz cycles within the modulated envelope.

[But, I've never looked into the accuracy of these aspects of the signal -- usually more involved with finding the correct zero-crossing to track, noting envelope polarity, TD's, blink codes, ECD, etc. You know, the "good stuff" :> ]
Reply to
Don Y

For simple time transfer applications, the only thing you need is a common signal source, this might be a local analog TV transmitter, some Loran-C type LF transmitter or a pulsar in the sky.

In addition to this, you need some means to communicate the difference from the reference pulse from the reference source (atomic clock) to your local clock.

Without exaggerating too much, you could use a local homing pigeon nest, transport one pigeon to the atomic clock site, attach the time difference to the pigeon, let the pigeon return to the nest and read out the time difference. There might be some more cost effective ways of doing this :-)

Reply to
upsidedown

Don,

I tend to take a systems approach to this sort of thing, in that where there's a fresh requirement, the solution should ideally integrate with other kit in the lab and be generally useful. ie: a permanent install that becomes part of the infrastructure, usable with nothing more than a power up.

If there's the opportunity, buy better than currently needed as well, on the basis that even if the accuracy, whatever, is not needed now, it most likely will be in the future...

Chris

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Reply to
tridac

Hf time signals are inconvenient, but surprisingly accurate, even allowing for path fading and doppler shift. Comparing the output from the gps standard here with hf 10MHz time signal usually shows a propagation effect error of less than a cycle every 10 seconds and often much better, which is good enough for many frequency reference applications. If you want absolute time to ns, then the only cost effective way is via a gps receiver.

But, absolute time is not the same thing as absolute frequency. The latter is usually all that's needed for electronics lab work. Things that depend on absolute time, servers and desktop machines, can use internet ntp servers. If you use ntp with 2 or more remote servers, you can get accuracies of better than milliseconds, which is good enough for most computer applications and it's free...

Chris

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Reply to
tridac

Most test equipment that can use an external reference is designed to take a 10Mhz input, though 5MHz was a more common years ago and some used 1MHz.

The choice of 5MHz for high quality standards in the old days was due to the optimisation of crystal characteristics and later HP test equipment internal oven references still used a 5MHz crystal, doubled to 10.

FYI, google "sultzer double oven standard", for an early example from the 60's, which is still hard to match even now...

Chris

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Reply to
tridac

Exactly. Though you can even fudge the "frequency" requirement (in that you needn't require it to be fixed, etc. -- just *known*)

Reply to
Don Y

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