Electronics > Metrology

Frequency offset between GPS receivers!?

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nctnico:
Lately I have been doing some testing with a GPS receiver which disciplines an OCXO (uBlox LEA-M8F with a 16 bit DAC + OCXO). When I compare the 10MHz frequency with my BG7TBL GPSDO I notice that after a startup there is always a random but constant (over several hours) frequency offset (a few milli-Hertz). I'm getting a flat graph on the frequency counter.

For my test I use a cheap antenna under a non-metal roof; the antenna should have a clear view of the entire sky. It is right next to the antenna of the GPSDO (which is always on). Also I did not configure the GPS receiver to GPS only but I'm not sure whether it is worth trying.

My question is: am I doing something wrong or is this a normal effect?

tszaboo:
You should either check the matching of the antenna, or use an active antenna. Or both, if possible.
Also, that cheap antenna could be a patch antenna. That works much better if it is mounted on a metal plate.
My main issue with cheap antennas is the lack of documentation. For example, I played with the Simcom GPS eval board. It kinda sorta works out of the box, but reception was bad. Only later did I see that I had to mount it on a metal plate, and it improved reception significantly (dont remember numbers). They dont tell you that you need to do this, and dont tell you the type of the antenna.

CJay:
A few mHz could even be as simple/nasty as a little extra cable or even different velocity factor.

It may be possible to set a cable delay compensation parameter to calibrate it out

Remember the 'discovery' of faster than neutrinos a few years ago?

That was down to a loose fibre optic connector in the time measurement system...

Kleinstein:
A vekocity factor of the cable can give a shift in the absolute time, but alone would not give a frequency difference. It would take something like a constant drift in the delay to cause a difference in frequency.

The likely cause for different frequencies is using different satelites (e.g. one receiver may include 1 or 2 more because of better reception) and maybe a different weight (not sure if they use weights or just in or out) to different sats. So improving the antenna may help.

A mHz difference over a longer time would however mean quite a bit of difference - that would be seconds in the absolute time.  :-BROKE

CJay:

--- Quote from: Kleinstein on March 02, 2021, 10:11:53 am ---A vekocity factor of the cable can give a shift in the absolute time, but alone would not give a frequency difference. It would take something like a constant drift in the delay to cause a difference in frequency.

The likely cause for different frequencies is using different satelites (e.g. one receiver may include 1 or 2 more because of better reception) and maybe a different weight (not sure if they use weights or just in or out) to different sats. So improving the antenna may help.

A mHz difference over a longer time would however mean quite a bit of difference - that would be seconds in the absolute time.  :-BROKE

--- End quote ---

Ah, true, as it would be a constant, mea culpa, you are of course correct.

I wonder if there's scope for a distribution system so the antenna is shared?

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