Electronics > Metrology
GNSS RB (chinese)
kpjamro:
Has anyone tested one of these GPS-disciplined rubidium frequency standards?
The spec says: "Final accuracy will reach 2E-11 (0.0002Hz at 10MHz), within 24 hours"
I have one and, when compared with my Z3801A, it measures ~3.8E-11 low in frequency. It is very stable, just off frequency.
This frequency offset never changes. It is always low by about that amount, as best I can measure.
As a test, I disconnected the GPS antenna and measured the frequency after an hour, or so, and found it had dropped to 5 or 6 E-11 low, so I guess the GPS adjustment of the rubidium is working. But it is not adjusting it to the right frequency, or not able to pull it as far as it needs to go (?)
The issue I have now is that this does not meet its own spec by a factor of about 2. On the other hand, its not that bad. Do I want to send it back to China and if they send me another one, will it be exactly the same?
Ken
SilverSolder:
How do you know which one of the two is actually right?
kpjamro:
Good question. I have reasonable confidence that the Z3801A keeps its frequency within a 1E-12 range over the time periods involved.
But you are right - how do I know the center frequency of the Z3801 is exactly what it says it is? The Z3801 is not supposed to gain or lose more than 1 microsecond per day while locked. That means its average frequency must be within +/- 1.157E-11 of the correct value. That is worst case and better than the 3.8E-11 difference I am seeing.
What's interesting is that multiple tests throughout the day always show the same offset.... it seems both devices are quite stable. Just that one of them is slower than the other.
Ken
edpalmer42:
FYI, the graph you posted shows the stability, not accuracy, of 13 quartz oscillators that are in Z3801A GPSDOs that are *NOT* locked to GPS. [1] So that graph tells you nothing about the frequency error of the GPSDOs themselves.
The frequency error of the Z3801A is spec'ed at only 1e-9 on the 10 MHz output averaged over one day. Like you, I suspect that it performs much better than that. But to actually test it you have to compare it to something of known accuracy. In this case, you'd have to use a Cesium or Hydrogen Maser or a calibrated Rb standard as a reference.
I've attached a couple of graphs showing measurements I made comparing an FTS-4065A Cesium to my Z3801A. The measurements start at 10 sec. because below that, you're seeing the limitations of my measuring equipment.
So, averaged over 100 sec., there were frequency variations in the range of +-1e-11 between my Z3801A and FTS-4065A. These variations could have been due to either device. More measurements with different devices would be required to narrow it down, but I suspect that both units are contributing noise to the result. Cesium is not known for being an extremely low noise technology and any GPSDO will show noise due to the random noise inherent in any system that uses radio waves that interact with the ionosphere.
So if your Z3801A is showing good status information, i.e. low HUP, good signal strength, etc. then the frequency error you're seeing could be in the GPS-disciplined Rb unit. But it would require more equipment and more measurements to be certain.
It's also possible that there is a frequency adjustment built-in to your unit. Does the manual discuss that?
[1] http://www.leapsecond.com/pages/z3801a-osc/
kpjamro:
Ed
Thanks for the graph.
Strangely enough :-+ the number you came up with is the same as my guess for the absolute accuracy of the 3801 (around 1E-11).
My rational was that the 3801 will have some worst case difference in absolute time and then you add to that +/- the Allan deviation, which will be a function of observation time.
If the Allan deviation is in the E-12 range (I think it is) and the absolute time difference is 1E-11, then the short term performance of the OCXO it could be ignored.
My 3801 is presently showing an average TI error of +/- 500ps/24 hours, day after day. That tells me the average frequency and absolute time must be pretty good. I mean the average amount of correction applied during the course of a day (86,400 seconds) is 0.5 ns. That works out to a frequency fraction of 5.8e-15.
So probably the Allan deviation *cannot* be ignored.
Edit - I realize now that the lower numbers I was seeing were because the phase had actually slipped an extra full cycle so, for example, the slip was not 48 ns but 148 ns.
Some folks on another reflector have suggested this may be a repeat of a bug in BG7TBL equipment ca 2015-2016. That bug was fixed with a firmware update.
Ken
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