[evlatests] Master LO frequency references and the 1.7ms discrepancy

[Walter Brisken]

Are you sure that it is not running on a Mickey Mouse watch from a vending 
machine?  That drift rate sounds horrid!  Maybe we could upgrade to a Seth 
Thomas metronome.

-Walter

On Thu, 18 Apr 2013, Keith Morris wrote:

> Yes, my mistake.  I remeasured this morning and I find a rubidium drift
> rate of about 100us per day.
>
>
>
> Vivek Dhawan wrote:
>> A short VLA-PieTown fringe test was done last night.  I can confirm that the
>> 1.7ms is gone, the VLA was at ~2.5usec offset from UT.
>>
>> There is however a fierce rate of drift, ~5Hz at C band or  10^-9, so about
>> 10 times more than Keith's 9.8us/day
>>
>> There are also various warts on the phase - and a lower fringe amplitude,
>> though I have not yet done a careful comparison - from the short-term
>> (seconds) instability of the rubidium.
>>
>>
>> On Thu, April 18, 2013 07:30, Keith Morris wrote:
>> | As it stands now, the L356 firmware version that is installed at the site
>> | accurately measures the drift, but not the offset, between the GPS and the
>> | synthesized pulse.  We have been testing a new firmware version in the lab
>> | in which we attempt to address the offset problem, but it is not yet ready
>> | to deploy.
>> |
>> |
>> |
>> |
>> |> It seems to me that the monitor point design must be changed to
>> |> measure the lag between the L356 and the GPS, so this doesn't happen
>> |> again.
>> |>
>> |>> We located the source of the 1.7 millisecond timing discrepancy.  The
>> |>> GPS
>> |>> 1PPS signal and the 1PPS signal generated in the L356 were
>> |>> mis-synchronized by exactly this amount, with the L356 lagging in
>> |>> time.
>> |>> This is due either to the L356 having been synchronized to the wrong
>> |>> mark,
>> |>> or not having been synchronized at all.  We resynchronized the two
>> |>> pulses
>> |>> this morning at 11am.
>> |>>
>> |>> Then, at 11:40am, we switched the master frequency reference from
>> |>> Hydrogen
>> |>> Maser to Rubidium, as planned.  The PPS that is synthesized from the
>> |>> Rubidium 5MHz is drifting relative to the GPS at a rate of about 9.8
>> |>> microseconds/day.
>> |>>
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>> |>
>> |>
>> |
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>
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