The standard reference on this kind of project is "A GPS based Frequency Standard", Brooks Shera, QST July 1998, pp 37-44. I think the article is available on the net somewhere, and if you Google you will find many many examples of implementations.
Most variations start with a GPS with PPS output, a OCXO with electronic tweaking (e.g. HP 10811), and a microprocessor with accompanying circuitry to measure the phase difference and tweak the oscillator. For even better stability rubidium standards are available in the low k$ range.
Z3801A's were commonly available surplus in the $300 range a few years ago. They aren't as common now but I thijnk you can find them for just a little more. Certainly there are non-Z3801A units coming off of cellphone sites regularly.
That article desribes a GPS-conditioned VCXO using the 1pps output from a GPS receiver ("GPSR)". This requires long conditioning times, in the order of hours, before real long-term performance is achieved.
There are numerous GPSR that provide a 10kHz output which facilitates a far shorter settling time in a hardware-only device, resulting in a much simpler configuration.
I'm sure it is, as it often is for others including me. I originally built a
10MHz TV-derived frequency reference, but was never entirely happy with that. With the impending demise of analog TV here, I looked at various alternatives. Eventually I modified the TV-derived scheme to use the GPSR 10kHz output. and built my own 10MHz GPS-derived frequency reference.
I'm not aware of any GPSR that have a 10MHz output, but there are numerous projects very similar to mine (which I only discovered after reinventing the wheel). One by James Miller G3RUH is fully documented at:
There are other similar OCXO and rubidium clock devices out there on the surplus market. If you look at
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the Z3801A-related stuff at
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and check out the time-nuts mailing list (@febo.com) then you will see what's out there for the picking.
The surplus-timing-source market isn't quite as plentiful as in the year or two after the telecom crash, but there is at least a trickle.
Your 10^-8 second stability over 8 hours actually does require some clarification: Do you need 10ns stability after 8 hours of unlock? At that level OCXO stuff definitely isn't good enough and you'll have to go to rubidium or cesium, and while rubidium stuff does show up at the low k$ level new and less than that new, lucking across a cesium reference for that little money will take extremely good fortune.
If you only meant 1 part out of 10^8, that would be 36 microseconds over 8 hours, and that is easily done with an OCXO.
10ns precision over any number of hours is going to be pushing the edge with a GPS-locked device, because these typically do not provide that sort of precision at the PPS output. Typical $30-$50 range GPS modules only guarantee under a microsecond of jitter (and typically achieve
100ns) on their PPS output. There are ones that go to the under-100ns jitter guaranteed level but I don't know of any that promise to the
10ns level. (Some may deliver circa 10ns level under kid-glove conditions although it's not guaranteed.)
If you need absolute time at the 10ns level from GPS then you need a truly excellent survey, even then I think it's pushing the edge.
references. For my purposes 10 MHz is preferable output.
I suspect that he misinterpreted your stability requirements, which I think you could spend a little time clarifying. There are GPS-locked-oscillator applications which do require the fast lock time that he was addressing but I don't think your application falls in that range.
specific references. For my purposes 10 MHz is preferable output.
You can pick up a Ball-Efratom rubidium standard on Ebay easily enough; I think even their older GPS units are available from time to time. Failing that an HP via the same source - they all pop up from time to time.
You are suggesting that locking an external VCXO to the 10kHz output instead of the 1pps output will give better results. That is a good suggestion, both in terms of implementation ease and phase noise performance.
It certainly provides implementation ease, as shown in the G3RUH project I referenced elsewhere in this thread. It also provides for incredibly good cold start capability, with final stability within three minutes of power application (even using 1998 ROM data).
When your *reference* frequency is from a division chain , then the higher up that chain you take the feed to the phase comparator the better the loop parameters can be and the better the phase noise also. In the case of the GPSR I employ (Conexant Jupiter) the 1pps rising edge is cited as being coincident with the edge of the 10k stream. But whether the 10k stream is simply a local loop locked to the 1pps or a higher point in a division chain which produces the
1pps is not explained by the data sheet, and Navman (who acquired the Rockwell-Conexant product line) decline to provide any support for the product.
Regarding phase noise, I started a thread under the heading "Jitter measurement" about three weeks ago in this group because I wanted to quantify that area of my unit.
In the case of my GPS-derived 10MHz unit using the 10khz stream, comparisons against the 10MHz OCXO in my frequency counter have been very promising, although I am unable to quantify the performance.
Actually, if you really get into the GPS signal structure all of the various clocks are all plesiosynchronous. All of the signals from 1 pps to 10.23 MHz chipping rates and even the L1 and L2 carrier frequencies all are synchronous (at each transmitter). This was done explicitly to allow post processing phase accumulation error corrections. And resultant accuracy improvements. The real trick is in compensating for Doppler and relativistic effects.
--
JosephKK
Gegen dummheit kampfen Die Gotter Selbst, vergebens.
If anyone's still on this thread, I'd appreciate any tips or links to help me get whatever I need to generate a hardware clock that stays within, say, 1ms, over 1 or more days. I can build/program any required additional electronics, as this clock system will be part of a larger production-line calibration facility anyway, including some PCs. I imagine GPS would be a good way to go, but what hardware/software should I look for, and where? I also imagine it might be a PC peripheral, and therefore possibly also supply a time reference to our network for other things. Cheap would be good, but time is money.
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