[time-nuts] Segal law: freq difference for GPSDO

Henry Hallam henry at pericynthion.org
Tue Mar 29 18:53:00 EDT 2016


If you put the PPS outputs of the two GPSDOs into an oscilloscope and
trigger on the leading edge of one of them, what does the other look
like?

Henry

On Tue, Mar 29, 2016 at 12:56 PM, Vlad <time at patoka.org> wrote:
>
>
> Hello,
>
> I am looking for for the advise regarding some strange difference in
> frequency between of two GPSDO: Trimble Thunderbolt and Datum Starloc II
>
> Measuring by HP 5386A counter it shows something around 0.0000014 delta.
> If I connect TB as REF source then I am getting numbers like this:
> 10.000001396 Mhz
> If I connect StarLoc II as REF source then I am getting following:
> 9.999998612 Mhz
>
> May be somebody could recognize, what 10.0000014 Mhz could be used for ?
>
> TB Lady Heather screenshot
> http://www.patoka.org/OCXO/TB-LH-Mar.png
>
> Allan Deviation for several hours looks like this:
> http://www.patoka.org/OCXO/TB-SL.png
>
>
> I connected both GPSDO to Rigol DS1102E Oscilloscope to see the phase
> difference. And its shows me the ellipse which change its orientation left
> to right and vice versa.
>
> Its classical case of Segal's law: "A man with a watch knows what time it
> is. A man with two watches is never sure". This is exact situation. Now I am
> not sure which GPSDO (if any) is more accurate.
>
> I have no third GPSDO (or equivalent) to compare. TB and Starloc connected
> to the same GPS Antenna distribution amplifier. So basically both using the
> same antenna. Both of them was turned on at relatively the same time and
> both was warmed up for few days.
>
> I have a theory, that may be StarLoc II was customized somehow to produce
> the freq. slightly different from 10Mhz. But its just a theory.
> Is it worth to compare 1PPS outputs using something like AD8302 + DAC + MCU
> ?
>
> --
> WBW,
>
> V.P.
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