Hi
I need a GPS module which will give me: ...an accurate 10 MHz clock output ...an accurate 1 second pulse output ...a serial interface to read UTC.
(Position isn't important. I know where I am.)
I know there are probably dozens of modules which will do this, but I have no experience in this area, so any advice would be welcome. UK supplier preferred but not strongly. Will need only a few.
Cheers
-- Clive
What do you mean by "accurate"?
Bye Jack
Am 11.07.2018 um 11:02 schrieb Clive Arthur:
Search the archives of the time nuts list. That has moved to a new server last week, so g**gl results may be outdated.
The list is located at <
Myself, I'm running a HP58540A and a Lucent Z3811A/Z3812A pair. Both are not very mainstream.
I have modded the Z3811A to drive a frequency doubler right from the
5MHz crystal oven. <Cheers, Gerhard
Interesting, but a lot of big heavy metal..
I was wondering, I have several cheap ebay GPS modules with 1 Hz output that support both Chinese, Russian, and US GPS.
30 $ or so on ebay, but no 10MHz output. Been probing around in one of those (needed fixing) there _is_ a crystal, but not sure it is locked. Common chips... datasheet is available, cannot remember the exact number.I am thinking it should not be impossible to build a cheap divider and xtal oscillator and then phase lock it to the 1 pps (needs oven temperature stabilization). Depends indeed what you want to use it for. No need for that here, have a Rubidium 10MHz standard, also ebay.., good enough, and available 24/7 if needed, unlike GPS that may come and go some times, jamming.... There is also a 10MHz radio signal..
It depends a lot on the accuracy you actually require (it cannot be indefinitely "accurate") and aspects like
- has that accuracy to be documented and verified or is "chinese spec" OK
- what behaviour do you require when GPS sync is lost for a while (which, together with the above, determines if you need some cesium or rubidium referenced unit or if OCXO or even TCXO is good enough)
- are you looking for a small module to be used inside equipment, or a 19" module for a rack
- how much money you want to spend
For hobby/inexpensive I like the boxes made by Leo Bodnar, but he does not offer a model that fits your needs. Depending on what you actually require, you may be able to use one of them.
Inexpensive modules are sold by the Chinese. Professional stuff costing thousands of dollars is widely available from professional equipment manufacturers working in this field.
Normally, in the context of GPS, the word "accurate" would normally be regarded as being somewhat redundant. Presumably an accuracy measured in nanoseconds is being sought. :-)
-- Johnny B Good
GPSDO
Cheers
Clive Arthur wrote in news:pi4h30$92u$1@dont- email.me:
Devices like that are typically in the form of a 1U rack mount package. One will not find many portable or handheld units providing such features.
And if you do it will likely be prohibitively expensive.
uBlox has modules for that
-- Reinhardt
The fundamental GPS output is the 1 PPS pulse. It jitters and wanders around a lot, as satellites come and go and the atmosphere changes. It's accurate long-term, so you can lock a very good oscillator to it, slowly, and get a good 10 MHz. The slower the lock loop and the better the oscillator, the better 10M. A rubidium is good.
Cheap GPS receivers can have 1PPS jitter in the ballpark of a microsecond.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
These look very good, thanks.
Cheers
-- Clive
The 1PPS and serial (4800 baud) outputs are easy. The 10 MHz clock output is not. For that, you will probably need to build or buy a GPSDO (GPS diciplined oscillator). There are designs for a GPSDO available that are really crude and simple if you don't need the ultimate in stability, and others that take a week to stabilize and offer more accuracy and stability than you could possibly need. I built an OCXO (oven controlled crystal oscillator) with a 1, 5, and 10 MHz outputs to run my pile of test equipment.
You can buy Chinese OCXO based GPSDO bricks on eBay for about $120 and up:
As others have mentioned, the place to get accurate info is the Time-Nuts mailing list: Archive:
-- 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
These look like they might do it well enough for me...
...what do you think?
Cheers
-- Clive
uBlox makes good boards and it will probably work. However, you didn't mention if you are designing a product, or building a single unit for your own use. uBlox seems to sell modules, not complete systems. I guess their development system would be considered complete, but that probably costs more than you want to spend. I think you can buy these modules on a PCB which should be easier.
I looked at the specs and found that the NEO-M8T and LEA-M8T have 2 outputs which can be individually set to 0.25Hz and 10MHz. The diagram shows a 1PPS output, but it looks like it's shared with the two other outputs. I don't have time to dig deeper right now. I think it best read the 35 page data sheet, check for additional documents: and then ask for advice on Time-Nuts mailing list for confirmation or alternatives.
Also, I don't know your application for the 10MHz, but it has been my experience that when the satellite(s) disappear, the 10MHz output will tend to drift around. If can be a problem if you're using it to control test equipment clock frequency. You could add a slow PLL to stabilize it, or use a much more stable external OCXO to provide the
10MHz. If it looses lock for a few minutes, the output will not drift very far or very much. Anyway, check to see what the 10MHz output does when the satellite(s) disappear.-- 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
More:
Perhaps you should be looking for this module on a PCB.
-- Jeff Liebermann snipped-for-privacy@cruzio.com
150 Felker St #D
What is so "fundamental" about the 1PPS output. After all the GPS signal repetition rate is 1 kHz, so a 1 kHz output would be more useful for faster locking.
As long a the receiving station remains in a fixed position, this should not really be an issue.
You need weeks to lock to 1PPS signals.
Or even worse. The 1PPS is often software/firmware derived.
I built one of the VE2ZAZ 10 MHz GPSDO boards, a few years ago... very flexible, can be configured to work with various GPS boards. I originally used a Motorola UT. When that board died, a couple of years ago, I replaced it with a uBlox board (an eBay cheapie apparently intended for use in a drone) and it's back to working just fine.
I (and another guy) heavily reworked the VE2ZAZ firmware - I turned my version into a PI-type controller. It worked out quite well.
That's basically what the VE2ZAZ and the earlier Brooks Shera designs do, using a PIC or similar micro plus some discrete components.
snipped-for-privacy@downunder.com wrote in news:rkmckd1jegcpfoo75pank6p4nt8kkguieh@
4ax.com:
It is fundemental inasmuch as it is the standard adopted by the scientific community and the military for time synchronisation of equipment.
There is no "repetition rate". The C/A codes are sent at rate of
1.023 Mbit/sec which is modulated by navigation messages at 50 bits/sec.Ummm... The satellites are moving. However, even if both the satellites and receiver are stationary, atmospheric delays are quite erratic, unpredictable, and noisy. From my experience living in a forest, where foliage blockage by trees is a problem, when the data source moves from one satellite to a different satellite, there's a small but noticeable phase glitch. If my GPSDO loses lock, the OCXO clock oscillator drifts off frequency for the duration of the outage and then takes about the same amount of time as the outage to return to a stable lock condition. However, when I played with a Cesium secondary clock GPSDO, where I could theoretically get to 1 part in
10^14 accuracy (about 4 nsec/day), it did take days for things to settle down. I never got close to this level of accuracy because 60Hz power line and 120Hz power supply noise was wrecking my measurements. If it had works, it probably would have taken several days to stabilize.Locking to 1pps was the way GPS time and freq clock receivers worked before built in higher frequency clock outputs were available. The time to gain lock was roughly proportional to how far out of phase the PLL clock was with the 1pps output. I don't recall the exact time, but I think it was less than 2 mins if the oscillator had warmed up, and maybe 20 mins if it was from a cold start. These use the 1pps output: "1pps locking module" "GPS-Locked Frequency Standard"
Not a problem because it has 1 full second to do the computation necessary to run whatever DSP filter algorithm (usually Kalman) is needed to reduce the noise and speed up locking by predicting the Doppler shift from each satellite.
-- 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
Sigh. I found my notes. Looks like my memory is failing. Some corrections.
With a Cesium clock, possibly. With an OCXO, much less.
Faulty memory. About 14 hrs from a cold start for the OCXO oscillator using a 1pps reference. This probably varies radically depending on ambient temperature and equipment temperature, but I only did one run. About 30 minutes if the OCXO had warmed up and had been locked for a while. Sorry about the memory failure.
Drivel:
Visual GPS Software:
NMEATime2 - PC GPS Time Synchronization $20.48USD, 30 day free trial.
-- 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
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