[time-nuts] Setting loop TC using thunderbolt internal data.

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Jan 21 05:04:55 UTC 2009


Tom

I turned of the OCXO discipling using Thunderbolt monitor menu:

Control:Disable Discipling


Then I used  the Window: Logging Dialog box to enable logging of

1) Time of week (sec)
2) PPS offset (ns)
3) 10MHz offset (ppb)
4) DAC voltage
5) Temperature

to a data file for about 40K sec.


The Thunderbolt keeps the DAC voltage constant and continues to measure
the oscillator offset and PPS offset even whilst the OCXO disciplining
is disabled.

The measurement noise on the LHS is that of the Thunderbolt receiver etc
itself.
The RHS is essentially OCXO drift whilst discipling is turned off.

The thunderbolt has its own internal TIC or equivalent thereto.

To increase the certainty that the receiver is behaving as I believe it
is when configured like this, it would be nice to simultaneously measure
the ADEV of the undisciplined OCXO output by other means.

Unfortunately I am not at present able to do this even though I do have
another GPSDO availble.

Bruce

Tom Van Baak wrote:
> Hi Bruce,
>
> Couple of questions. To me the plot looks like it is dominated
> with measurement noise (negative slope on the left) and OCXO
> frequency drift (positive slope on the right).
>
> You mention "GPS receiver noise" but I don't see that in the
> plot. Is this a free-running TBolt? If so, what 1pps source are
> you comparing it to?
>
> The title of the plot says, "Undisciplined Thunderbolt OCXO vs.
> Thunderbolt GPS receiver"; so is this maybe a plot between
> two different Thunderbolts; one locked, the other unlocked?
> What TIC was used to collect the data?
>
> /tvb
>
> ----- Original Message ----- 
> From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
> To: "Tom Van Baak" <tvb at leapsecond.com>; "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Sent: Tuesday, January 20, 2009 6:04 PM
> Subject: Setting loop TC using thunderbolt internal data.
>
>
>   
>> If one turns of the disciplining of the Thunderbolt's internal OCXO,
>> then one can log various parameters such as the 1 pps error.
>> A plot of the resulting OADEV (all tau) using such data is attached.
>>
>> For short tau the GPS receiver noise dominates.
>> For large tau the OCXO noise dominates.
>>
>> If the OADEV plot is symmetric about the minimum then the value of tau
>> at the Allan intercept coincides with the value of tau at the minimum in
>> the OADEV plot.
>> In this case the plot has greater slope after the minimum than before so
>> the value of Tau at the Allan intercept is about 500 sec rather than the
>> 400sec corresponding to the minimum in the OADEV plot.
>> The optimum  loop time constant for this particular Thunderbolt is thus
>> around 500sec.
>>
>> In the absence of any other means of measuring the performance of the
>> Thunderbolt as a function of the loop time constant, this technique
>> allows the loop time constant to be set near the optimum for each
>> particular Thunderbolt.
>>
>> Bruce
>>
>>     
>
>
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