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
Relative to what? How about a clue as to your accuracy and stability requirements? The appropriate device could be anything from a free running oscillator to a GPSDO.
I also don't like the idea of what sounds like a transfer standard. With a synthesizer, you can drive whatever clock your driving directly from your timebase.
One of my early GPSDO contrivances would take about an hour to recover from every time I opened the outside shop door. That was eventually fixed by double insulating the OCXO. I also had plenty of fun trying to keep the 5 and 10 MHz outputs from leaking out of the box, and trashing my WWV reception.
How long? With all due respect, you're being far too vague here. Different technologies offer different short and long term accuracies.
That's always the case. More accuracy and stability always costs more, usually on a non-linear scale. At the high end, a small increase in accuracy and stability can cost outrageous amounts in construction costs. However, the worst cost of accuracy and stability is your time in calibrating the reference OCXO. I generally have to tinker with the OCXO for several days before it settles down. Note the differences between accuracy and stability. Also note that the Allen variance goes out to 10^4 seconds (about 7 days) and that the next order of magnitude in accuracy requires 10 times that or 70 days.
Who's time? There are subtle differences between UTC, astronomical, navigational, space, and political clocks. Einstein's time dilation is involved. Not the table at the top of the page: Looks like about 16 seconds difference between UTC and GPS time.
A mechanical wind up clock would work if you don't bother to specify your accuracy and stability requirements.
I can't tell what you're trying to accomplish, but if all you want is accurate time, and not accurate frequency, then all you need is an accurate clock. The 1pps from most GPS receivers will drive almost any kind of clock with good accuracy (as long as you don't have any missing pulses). If you don't mind the clock wandering around a bit during the day, a WWVH 60 KHz clock might suffice.
On the other foot, if all you want is a stable frequency, you can get close with an OCXO or possibly a TCXO. An OCXO is probably best, especially if you add extra insulation. Start at 10 MHz and divide down to 5, 2.5, and 1 MHz depending on what you need. I have such a system distributing 5 and 10 MHz to all my test equipment that will accept an external reference. On a good day in the ionosphere, I get about 0.01 ppm accuracy against WWV.
Incidentally, I have a pile of about 200 Novatel/CMC Allstar 12 GPS receiver boards if you want to play. They're old (2001), not the best, and don't have a frequency output, but are good enough for 1pps and for building ham-radio APRS trackers. The catch is that you'll have to build an interface board to make it useful:
-- 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
That's a lot of work.
How about putting a normal VCXO in an analogue PLL with an XOR phase detector--it'll servo right at half scale unless you put in an external reference. A little 4-pin connector and a baby daughtercard would let you sell the reference as an add-on.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Yes. It's commonly done in land mobile radios that require high stability but cannot tolerate the power drain of an oven. Also used for stabilizing oscillators over very wide temperature ranges.
Officially, it's a DTCXO (digital temp compensated crystal osc)[1]. The crystal and oscillator circuitry is characterized over the desired temperature range, and the required frequency compensation is saved in a lookup table. It's usually controlled by a PIC. Here's an example:
0.3 to 1 ppm stability over various temperature ranges (depending on model). Here's another: with varying specs.The big problem is crystal ageing, which can sometime be as large as the temperature drift. I've forgotten everything I had learned about accelerated crystal aging, but suffice to say that it's closer to magic than science.
[1] More oscillator acronyms:-- 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
If you are making a time base (not just a frequency standard), how are you going to handle leap seconds ?
Several years ago, I interviewed at a company that does time stuff. Xtals and atomic clocks. I had this idea of cutting the ovenized X-tals for oper ation near 25C (rather than the 70C) and then using TEC's to keep the tempe rature stable, but near room temp. I was told that aging was bigger probl em. Basically due to atoms moving onto or off of the xtal.. the mass loadi ng changes the frequency. There may be other aging effects too.
George H.
NIST offers a variety of broadcast time references.
I have two Symetricom references. Just look for them used. Mine were $30 a pop.
The cellular companies dumped the crystal references for Rb year ago for long operating time without GPS discipline. The old crystal based references are everywhere, though in the US most were probably crushed. On ebay they seem to all be out of China. Mine were new old stock from "I can't say", but these time references are everywhere in high tech areas. Data centers, research labs, company wide clock references in the better companies, etc.
In the old days, the HP labs in Santa Clara had a cesium clock in the lobby. Supposedly it was the reference for the facility, but who knows. It could have been a busted cesium clock that some engineer just hacked to display time from another source.
Get some caesium and make your own clock ref!
Jamie
Cesium is really nasty stuff.
And making a Cs beam tube is not for chickens.
Ah, so what you require is a flexible model for PRODUCTION, not a one-off for your lab/workshop/test-bench.
Maybe that should have been made clearer at the outset.
digital
So what you are approaching is a second optional product they can purchase, your time keeper so to speak. Then you have multiple variations (GPS, CDMA, RF, etc). Your main product should supply power to this addon (you dont want another wall wart or such) and it has to take the pps input. You would be wise to have power limiting on the supply side and need to handle all the ESD and EMI issues with the connector and.. and ...
By the time you get done with all that you might as well just put the TCXO in and be done with it. Cost wise it could quickly be close to the same.
-- Chisolm Republic of Texas
Bounce an ultrasonic signal off the pile of horse poop and see how long it takes to come back. Shorter times = bigger pile. Then you just need an NIST-traceable horse to calibrate against. And possibly to not care about RoHS.
7 years, wow!
For a good time, call +1 303 494 4774. Use a landline for lower latency.
I hear you can get cell phone modules that allow your device to either make a phone call or send texts over the cell networks. I don't know if these modules can tell you what time the cell network thinks it is. You might need a SIM card to make this work.
Matt Roberds
Hi, Don:-
Ah, well... if you pick a common standard frequency like 10MHz, and a standard footprint like 5 x 7mm, and a standard voltage like 3.3V you can populate either a $15 TCXO or a $1.00 oscillator on the same PCB.
If you want field options, you could also easily make a 4-pin plug-in module that would contain just the TCXO (or even an OCXO) and switch over the clock source (use the 4th pin to ground the tristate output selection line on the on-board XO). That way the added "cost" per non-TCXO unit is just a 3-cent 4-pin header eg. SWR201-NRTN-S04-SA-WH.
One thing I've done is to have an oscillator input over an SMA or whatever and detect the signal & switch over automatically to the external source if it's present. That takes a few more parts, but allows the simplest possible connector and is harder (never say impossible!) for the user to screw up.
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
No! I don't want to be in that business! What I want is a means by which *they* can "enhance" what I've made TO FIT THEIR NEEDS. I.e., the guy they hire (in the paragraph above) has no affiliation with me -- I've just provided a means by which "he" can adjust the clocks.
[clocks are a flawed analogy because they imply actual time-of-day is significant. I'm really only interested in ensuring that time *passes* at the actual rate]
No. Instead, I'm looking at just an "input" for this reference (DC coupled wrt ground, transformer coupled, optically coupled, ). I expect *it* to provide whatever power it needs.
TCXO (or *any* "fixed design choice") means accuracy is fixed and can't change after-the-fact as the user's needs change. Annoying (to the user) to realize you have to buy a different version as your needs increase. Or, have *your* device retrofitted to accommodate something this "trivial".
Yes. It is the "long term" accuracy that I exploit "on the cheap" cuz I'm running 24/7/365 (count very long intervals so you are effectively frequency locking a high frequency "clock" -- MHZ -- to a subharmonic -- uHz -- derived from the AC mains)
Good point!
Yes. But they rely on that signal (when present) having the "final accuracy" desired. The LFC approach relies on the *average* mains frequency having the desired accuracy -- and expects to be able to watch it over very long periods of time.
For years I've been making "gift timepieces" (the gift aspect being some unique -- literally "one of a kind" -- and atypical way in which I "display" the time) that watch the AC mains to discipline the MCU's XTAL. Cheap solution and works well as most "(wall) clocks" sit in reasonably well controlled environments.
Exactly! That's the whole point! Let the *user* decide what level of accuracy he wants/needs. And, whether he is interested in short term and/or long term goals. Why should he be "stuck" with a design decision that I've made "for the masses"?
Your "HiFi" lets you decide is you want to buy a crappy cassette deck or a high end DAT or a CD, etc. Why should the manufacturer
*pick* one of these options and lock you into it -- forever? (Ah, because the manufacturer wants to sell you another model next year...)Exactly! A frequent technique I use is to monitor the AC mains to achieve long term accuracies -- but requiring day(s) to lock to the "average line frequency". If you're running 24/7/365, this isn't an issue -- except in your "initial accuracy".
"Time" in the sense of "time passing" -- not in the sense of "the (current) time is:"
Yes, it would! Provided there was a means of "observing" it!
I've got a drawer full of different GPS devices -- which is where the idea for an "external reference" originated. What I'm looking for is other options that I may not have considered. Esp if they affect the sort of "interface" I would have to implement/support
E.g., using an *external* TCXO would mean I would have to be able to capture a very high frequency signal. Very different approach than talking to a time server over the network or listening to a PPS signal.
If I had one, I wouldn't be wasting my time on matters as *mundane* as these! :>
Means two different "models" have to be maintained. Or, the user has to ship the product back for an "upgrade"
"Grandma" is probably not going to be comfortable making that sort of change. Or, will be calling for "Support" when she's bent a pin (and doesn't yet know it).
This is the approach I think I am going to settle on. Let the user connect having and :) figure out how to use that information. (i.e., don't *assume* it's 1Hz -- or, perhaps *look* at it and if it *seems* like ~1PPS, then make that assumption to save the user having to *tell* me it is 1.0000Hz and not 29.97Hz, etc.)
If it doesn't, what are you prepared to do about it?
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