[time-nuts] Strange 100ns jumps on Motorola M12+T

[Stephan Sandenbergh]

Hi All,

I have now redone the original experiment (same settings), and this time
round no jumps at all. Strange. This means I cannot reproduce what I
originally measured, making it all the more difficult to hunt down the
issue with certainty.

Thank you for the good advice thus far. For future measurements I'll stick
to microsecond offsets and stay away from multiples of the various clocking
frequencies.

I'll post again should I find something that might explain the original
result, or if I perhaps manage to reproduce it.

Regards,

Stephan.






On 22 November 2013 14:06, Bob Camp <lists at rtty.us> wrote:

> Hi
>
> Something is going wrong somewhere. The question is where.  Three ideas /
> targets here:
>
> The counter is a 100 ns (10 MHz input) beast, so it *might* be the issue.
>
> The offset source or the GPS might also be the issue (thus avoiding
> 1/10.24 MHz).
>
> The idea with the larger offset is that there is no significant accuracy
> degradation with a modest increase in the offset. The exact value isn’t the
> issue there, just making it larger. That way you will still properly
> capture stuff in the 100 (or more) ns range.
>
> Bob
>
> On Nov 22, 2013, at 4:38 AM, Stephan Sandenbergh <ssandenbergh at gmail.com>
> wrote:
>
> > Hi,
> >
> > Ok, the 53131A trigger settings: I've turned auto trigger off and set it
> to
> > trigger at 2V (I think this is right, but I'll have to go and double
> check
> > the exact threshold setting) threshold. Sensitivity is set to high.
> >
> > Location is GPS surveyed and all M12+'s are set to position hold mode.
> >
> > I did record the time stamped GPS data as well so will go have a look at
> > what happened to the constellation at the time of the jumps. Will post
> when
> > I have results.
> >
> > Sorry if I'm a little slow here, but why is it better to use larger
> > offsets? Also I get that 100ns is exactly one cycle of 10MHz, but why
> would
> > the 53131A have trouble with this? Surely it uses linear interpolators
> > along with digital counters to calculate the result. Also, I assume the
> > counting doesn't happen at 10MHz, but at a much higher multiple. Had it
> > been done at 10MHz I'd understand that skipping a beat would result in
> > 100ns offset. Granted I don't know much about the innards of the 53131A.
> >
> >
> >
> >
> > On 22 November 2013 02:25, Bob Camp <lists at rtty.us> wrote:
> >
> >> Hi
> >>
> >> To be clear - the idea of going to a non-100 ns multiple is a good one.
> >> You probably should avoid multiples of 1/10.24 MHz as well.
> >>
> >> Bob
> >>
> >> On Nov 21, 2013, at 8:00 AM, Azelio Boriani <azelio.boriani at screen.it>
> >> wrote:
> >>
> >>> Yes, do not use tiny offsets, go to 1us: I use microseconds offsets to
> >>> take PPSes measurements .
> >>>
> >>> On Thu, Nov 21, 2013 at 1:34 PM, Bob Camp <lists at rtty.us> wrote:
> >>>> Hi
> >>>>
> >>>> The counter and offset generator both should be quite accurate at a 1
> >> us offset. That’s large enough that you are outside the range of most
> GPS
> >> jumps. If you are going to move things around, you might as well move
> out
> >> to that vicinity.
> >>>>
> >>>> Bob
> >>>>
> >>>> On Nov 21, 2013, at 6:20 AM, Tom Van Baak <tvb at LeapSecond.com> wrote:
> >>>>
> >>>>>> Below is a plot so you could see exactly what I measured. What is
> >> peculiar
> >>>>>> is that the time jumps by exactly 100ns to 200ns. Almost as if the
> GPS
> >>>>>> receiver decides to offset the time by twice the amount I set it to.
> >> Which
> >>>>>> is why I initially thought it might be a firmware thing. I suppose
> >>>>>> multipath is a good explanation, it is just odd that the time error
> is
> >>>>>> exactly 100ns.
> >>>>>
> >>>>> Hi Stephan,
> >>>>>
> >>>>> A quick test you could perform is set the offset to 125 ns instead of
> >> 100 ns and see if the jumps still occur, still occur at 100 ns, or now
> >> occur at 125 ns.
> >>>>>
> >>>>> Since you have three M12's offset the third one by 150 ns and see if
> >> it experiences jumps too.
> >>>>>
> >>>>> Question -- are you using the external 10 MHz reference input or
> >> output for any of your 53131A counters?
> >>>>>
> >>>>> /tvb
> >>>>>
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