[time-nuts] HP5065A environmental sensitivities

[Magnus Danielson]

Hi Wulf,

On 05/18/2016 08:57 AM, DG2OM at gmx.de wrote:
> This is my first contribution to the Time-Nut list, but I am a follower for several years.
> I have made 30 days phase difference measurements of an HP5065a versus a modified Thunderbolt GPSDO (got a MV180 oscillator) with a Fluke PM6681,
> and recorded the results with Timelap. Starting signal of the PM6681 was the 5MHz of the HP5065A, stop signal the 10MHz from the Thunderbolt. A single
> measurement was triggered once a second from the PPS of the TB.
> I also have a weather station at 6 km distance away, which records barometric pressure at time intervals of 10 minutes. Out from these measurements, I divided
> my phase measurements into 10 minute intervals, calculated the 10 min average frequencies and made a linear regression with the 10 minute pressure measurements.
> I got a slope of 3.86E-14/hPa frequency difference versus pressure.

Interesting!

> Then I corrected the original 10 min average frequency measurements with the result from the linear regression and imported the original and the corrected
> datasets into Timelap.
> The influence of the barometric pressure to the ADEV for long time intervals is shown in the picture.
> Just for fun, to render the pressure sensitivity visible for the human eye, I removed the noise of the GPSDO by generating a plot of overlapping 1 day
> frequency averages in 10 minute time intervals. On the second picture you can compare it with the 10minute pressure data from the weather station. I think,
> it is quite impressive. The calculated slope out of these overlapping averages was with 3.74e-14/hPa a little bit different compared with the non overlapping dataset.

Now, after the fact fitting of curve has a drawback in that you can make 
the fit as really good but there you measure the reference, hence you 
have a closed loop. The challenge now is to use a feed-forward scheme to 
take the measures from the pressure and use to adjust the C-field of the 
5065 and then measure against the Thunderbolt. I would guess it won't be 
exactly the same, not as good, but it will be then the actual performance.

Cheers,
Magnus