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

[Stephan Sandenbergh]

I'm now wading through the GPS data I logged to see if anything happened at
the time of the jumps. Will post once I find something.


On 29 November 2013 15:35, Stephan Sandenbergh <ssandenbergh at gmail.com>wrote:

> 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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