[time-nuts] Setting loop TC using thunderbolt internal data.
Tom Van Baak
tvb at LeapSecond.com
Wed Jan 21 00:23:53 EST 2009
Ah, OK. So you keep the antenna connected and you keep the
GPS receiver in position hold mode, still receiving fixes.
All you're doing is disabling the software disciplining algorithm.
That sounds like a good test. Let me try this too and see if I
agree with your conclusions.
I'll try to simultaneously measure the 1PPS or 10 MHz outputs
as you suggest.
Has anyone hacked a TBolt yet to find which internal pin has
a raw 1PPS from the GPS engine (as opposed to the 1PPS
divided down from the OCXO)?
----- 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 9:04 PM
Subject: Re: [time-nuts] Setting loop TC using thunderbolt internal data.
> 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
> 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.
> 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?
>> ----- 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.
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