[time-nuts] 1 pps Accuracy in two locations

[Magnus Danielson]

Many double-frequency GPS receivers have about 2 mm noise in them, that
for the 1,57542 GHz (19 cm) and 1,2276 GHz (24 cm), so about 1/100 of
the wavelength. It is not very strange that you can measure a fraction
of a cycle, this is about 3,6 degrees on a cycle.

So, to get that resolution you do not need to support full cycles in BW.
If so, our 10 MHz reference oscillator counters would only support
resolution of 100 ns, and that used to be the resolution. Improved
counters used higher clocks, such as 100 MHz to get 10 ns resolution.
Since then we learned to interpolate and can accurately resolve phase of
a cycle in the range of 1000 to 10000 today without too much trouble.
The good old trusty 5370A has 256 in interpolation gain, and that from
200 MHz / 5 ns, but you benefit from it as you measure 5 MHz and 10 MHz
signals... so, for 1 ps resolution you do not need THz bandwidth. I have
counters with about 2 ps RMS trigger jitter and resolution of 800 fs and
200 fs, and their BW is just above 1 GHz.

Cheers,
Magnus

On 2019-12-04 17:56, djl wrote:
> Maybe Grace Hopper's aid will help? 11.8 inches = one nanosec, so a ps
> is .0118 inches. You really want two clocks not occupying the same
> space to be correlated to that accuracy?
> Don
>
> On 2019-12-04 08:55, Tom Van Baak wrote:
>> Martyn,
>>
>>> I'm always being asked to provide equipment that can produce two 1
>>> pps outputs aligned to each other to within a few ps.
>>
>> They should look at their best 1PPS on a 'scope. You can get ns with
>> care; I doubt ps is possible. I mean, that's THz BW isn't it?
>>
>> Can you share with us what their application is?
>>
>>
>>> So they are asking for two of my GNSS frequency standards with 1 pps
>>> outputs.
>>>
>>> The 1 pps outputs being derived from the rubidium oscillator (which
>>> is aligned to GPS/GNSS)
>>>
>>> The best I think I can achieve is in the low ns range.
>>
>> Right. It will be ns, not ps. Forget about using GNSS for ps level
>> timing.
>>
>>
>>> Does anyone know how this can be achieved?
>>
>> Google for papers by high-end national timing laboratories. Words
>> like: active temperature stabilized (phase stabilized) bidirectional
>> optical fiber links.
>>
>> Very possible, very expensive, quite common now. I'd guess most of the
>> timing centers in Europe are linked this way.
>>
>> /tvb
>>
>>
>>
>>
>> On 12/4/2019 1:40 AM, martyn at ptsyst.com wrote:
>>> Hello,
>>>
>>>   I'm always being asked to provide equipment that can produce two 1
>>> pps
>>> outputs aligned to each other to within a few ps.
>>>
>>>   These two 1 pps pulses are not in the same location and could be
>>> 100 metres
>>> to a few km away.
>>>
>>>   So they are asking for two of my GNSS frequency standards with 1 pps
>>> outputs.
>>>
>>>   The 1 pps outputs being derived from the rubidium oscillator
>>> (which is
>>> aligned to GPS/GNSS)
>>>
>>>   The best I think I can achieve is in the low ns range.
>>>
>>>   Does anyone know how this can be achieved?
>>>
>>>   Regards
>>>
>>>   Martyn
>>>
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>>
>>
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>