[time-nuts] Venus838LPx-T PPS stability measurements

Lars Walenius lars.walenius at hotmail.com
Tue Oct 18 04:45:40 UTC 2016



Hi



I see a similar behavior of my Venus 838LPx-T as Nick. I enclose two phase chart for Venus838T and M12 against a LPRO rubidium. Both charts were done in parallel. As the ripple of the Venus is so much higher I scaled the graphs myself and didn´t find any good way so excuse for the scales but they are about 10ns/div.



The LPRO is part of a GPSDO with the M12 and an Arduino controller. The GPSDO was set in holdover just before the test. The LPRO was still temperature compensated during the run with 1E-12/C. The LPRO is not pressure compensated but the air pressure was within 2mbar during the run. The pressure sensitivity is about 7E-14/mbar for the LPRO. The last two months the drift of the LPRO has been around 5E-14/day. As can be seen from the graphs the frequency offset became about 3E-13 when I set holdover. During the 1 ½ Days run the drift was 40ns due to this. I think this isn´t to bad in holdover. Also note that the M12 was measured by the Arduino with 1ns resolution. I was glad to see that it could measure 1E-9 in the TDEV chart enclosed.



I also see excursions of around 50ns pp in the short term similar to what Nick sees. I have one possible explanation why my Venus 838 (that I got from Nick) show these excursions and not other timing receivers I have like the M12 and LEA6T. I Think the TCXO in the Venus 838 might be digitally temperature compensated and that it makes jumps in the 1 to 2E-8 range?? That is 10-20ns per second. After just a few seconds the NCO (?) inside the Venus corrects the error. Today I tried just to breathe (blow) on the Venus and I could see more than 100ns pp excursions. Always around the mean.



Note that the data isn´t saw tooth corrected, but I doubt it will help at all for the Venus.



Lars



From: Nick Sayer via time-nuts<mailto:time-nuts at febo.com>
Sent: den 7 oktober 2016 16:55




Buh…

No graphs?

Um… “I’m a little teapot, short and stout…” :)

I’ll link it here instead:

http://www.kfu.com/~nsayer/Venus838LPx-T_graphs.zip

> On Oct 7, 2016, at 7:46 AM, Nick Sayer via time-nuts <time-nuts at febo.com> wrote:
>
> This is a bit overdue, but I finally got around to making at least an attempt to measure the stability of the Venus838LPx-T timing module’s PPS stability.
>
> The results are a bit of a mixed bag.
>
> The module under test is one built into one of my OH300 GPSDOs. It’s inside of a closed chassis, mounted about a half inch away from an OCXO, sitting on my workbench inside an unconditioned garage. My guess is that over the course of the test the ambient temperature varied by perhaps 15°F. The PPS output of the module - as well as being fed into the controller and PLL - is fed into a buffer before being presented on the diagnostic port. From there, it went straight into input 1 of my 53220A. Input 2 came from the PPS output of a Thunderbolt. The 53220A’s 10 MHz reference comes also from that same Thunderbolt. No effort has been made to apply the sawtooth corrections indicated by the PSTI,00 sentences. Both receivers are fed from the same antenna and splitter. Reception is nearly ideal, with the Thunderbolt having 7 or 8 satellites at all times, most of them with S:N > 45.
>
> The first surprise is that although both PPS signals are ostensibly synced with GPS, there’s a 135 ns residual between the two. The residual (after ~15 hours or so) has a ~2E-13 slope. This graph is the phase difference with that residual removed.
>
>
>
> There are three variances visible. Firstly, there’s about a ±6ns “fuzz” around the center on almost all samples. That can be explained by the quantization error indicated in the NMEA output. Plotting those errors shows a similar fuzz with a range of ±6 ns. Secondly, there’s a much slower wander that’s mostly confined to a ±10 ns corridor. I attribute this to GPS itself. How much of the wandering is due to which receiver is something I haven’t attempted to figure out. Running this test with an undisciplined rubidium oscillator might help, but the short term stability of the 5680A isn’t very good, so I didn’t want to make it the first test standard.
>
> The third variance is more serious. Periodically the variance is much larger - sometimes ±25ns or even more. These variances are not accounted for in the sawtooth correction values. The only good thing that can be said about them is that they’re fairly well balanced and can be easily averaged out.
>
> If you take a closer look at one, they sometimes appear adjacent to hanging bridges:
>
>
>
> This isn’t always the case, but it’s often enough to potentially be more than coincidence.
>
> All that said, I believe the resulting ADEV is in line with expectations for a GPS receiver:
>
>
>
>
> I’ve made an inquiry with SkyTraq to ask about the excessive excursions. I’ll report back what their answer is. Given that the excursions are easily averaged away, and given how inexpensive these modules are, I’m not bent out of shape about it. And if it’s something SkyTraq can figure out how to update in the firmware to report in their QE messages (so that it can be corrected for externally), I won’t mind at all.
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.

_______________________________________________
time-nuts mailing list -- time-nuts at febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: M12-LPRO phase chart.PNG
Type: image/png
Size: 86744 bytes
Desc: M12-LPRO phase chart.PNG
URL: <http://febo.com/pipermail/time-nuts_lists.febo.com/attachments/20161018/bfd3a548/attachment.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: TDEV M12+Venus838T-LPRO.PNG
Type: image/png
Size: 86099 bytes
Desc: TDEV M12+Venus838T-LPRO.PNG
URL: <http://febo.com/pipermail/time-nuts_lists.febo.com/attachments/20161018/bfd3a548/attachment-0001.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Venus838T-LPRO phase chart.PNG
Type: image/png
Size: 133355 bytes
Desc: Venus838T-LPRO phase chart.PNG
URL: <http://febo.com/pipermail/time-nuts_lists.febo.com/attachments/20161018/bfd3a548/attachment-0002.png>


More information about the Time-nuts_lists.febo.com mailing list