GPS-based reference clock

Hello All,

How can I make reference oscillator (clock) out of generic consumer GPS Receiver? I hope to get stability 10-8 seconds over 8 hours.

There is off-the-shelf equipment doing just that, but prices are way over my budget.

Appreciate your advices.

Sincerely,

Andrey Gleener

Reply to
Andrey
Loading thread data ...

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.

Tim.

Reply to
Tim Shoppa

(snip)

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.

Reply to
budgie

Tim pointed to the Z8301A -- still available on eBay, for $300 to $400, and still very accurate. They have a GPS receiver and a double-oven VCXO.

--
Namaste--
Reply to
artie

... and worse phase noise.

However, the OP is only interested in "stability 10-8 seconds over 8 hours" so this may not matter.

Regards, Allan

Reply to
Allan Herriman

I have no intention of entering a debate on that, but my jury is still out.

Indeed.

Reply to
budgie

Thank you all very much for the advices.

I checked for Z8301A on EBay did not find it so far. I will keep looking.

far shorter settling time in a hardware-only device, resulting in a much simpler configuration.

This is also interesting statement unfortunately not supported by specific references. For my purposes 10 MHz is preferable output.

Sincerely,

Andrey

Reply to
Andrey

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:

formatting link

Reply to
budgie

A simple 50ppm tolerance crystal oscillator (less than a dollar) will meet your requirements.

At 50ppm error it will drift about 1.5 seconds in 8 hours

Slurp

Reply to
Slurp

I suspect he meant "ten to the minus eighth power", not "ten minus two".

Reply to
Ancient_Hacker

Ahh OK, I thought he said 10 or 8 seconds, not 10^-8!!!!!

Major problem even with GPS as you will only get an accuracy of ~10^-7 seconds absolute anyway without very expensive kit.

Only option is a phase tracked Rubidium - or if you have megabucks a caesium!

Reply to
Slurp

There are other similar OCXO and rubidium clock devices out there on the surplus market. If you look at

formatting link
the Z3801A-related stuff at
formatting link
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.

Tim.

Reply to
Tim Shoppa

looking.

a

simpler

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.

Cheers.

Ken

Reply to
Ken Taylor

Oops, I believe I misinterpreted your post.

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.

Regards, Allan

Reply to
Allan Herriman

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.

Reply to
budgie

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.  
 Click to see the full signature
Reply to
Joseph2k

In the New Zealand Power industry, we use a Tektron Clock to synch our relays, this is more accurate than you described, but that cant be bad.

I dont know if its comercialy available, nor the schematics, but it could be a starting point?

Tim Strange.

Reply to
nzstranger

The correct term is plesiochronous.

Reply to
Don Bowey

That's what i get for not verifying every term a telco uses.

--
JosephKK
Gegen dummheit kampfen Die Gotter Selbst, vergebens.  
 Click to see the full signature
Reply to
Joseph2k

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.

Thanks for reading.

Reply to
tony

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.