[time-nuts] Measuring ADEV using TBolt-Tic tester

bg at lysator.liu.se bg at lysator.liu.se
Sun Oct 16 18:26:34 EDT 2011


Hi Warren,

> Something to be careful about when doing what you suggest is that the two
> Tbolts will not switch birds at the same time, so need to not use that
> part of the data if you want to remove the effect of the GPS. Treat that
> part like outliners.

Right that is a problem when looking at combined measurements. If you use
per satellite measurements instead, ie 'Clock bias per satellite' in
0x8F-A7 or doppler in 0x5A, then you are fine as long as you compare
'Common view' satellites. What is nice is also that you can use all
satellites that are tracked on both receivers.

The 2Tbolt-system is really a degenerated Common View time transfer
system, with both receivers beening colocated and using the same GPS
antenna signal.

Moving the receivers to different time-nut labs, we have a traditional
time-transfer system. Where we perhaps only lack L2 measurements, compared
to the serious time tranfer systems used by national labs.

     http://tf.nist.gov/time/commonviewgps.htm

--

    Björn

> To check for Tbolt noise floor, Good idea to use just one external one and
> an internal one as long as it has a good low noise Osc in it.
> Can using the 10 MHz output of the internal one to drive the input of the
> external one.
>
> Also to remove as much variation as possible, put BOTH of them in the
> disable mode (but not holdover mode)
> and manually adjust their setting so they both read about the same values
> for their Phase and freq.
>
> Now you're suggesting making a better Tbolt Tic by using two and comparing
> their differences.
> Have to think that over a little, sounds like a good plan, But so far have
> not seen anyone else that will even try it with one.
>
> Thanks,
> I'm now thinking about how to use two external Tbolts in a setup more like
> a
> DUAL Phase thing.
> The GPS signal can be the Offset osc and therefore will have little
> effect.
> The GPS will just act as a near zero offset frequency osc, and watching
> the
> phase differences between the two Tbolts, should work fine.
>
> Thanks, so many fun things still to play with.
>
> ws
>
> *****************
> Hi Warren,
>
>> What is not too clear is how much of that is due to the Tbolt engine and
>> how much is the "GPS Reference".
>
> Do you have two working Tbolts with their orginal oscillator removed?
>
>> From what I've seen in my test, a large amount of that noise floor is
>> due
>> to the GPS.
>
> I think a dual Tbolt configuration would eliminate GPS instabilities and
> rely more on the house standard. As proposed in previous email.
>
> --
>
>    Björn
>
> *****************
> snip
>
> I propose an experiment with two Tbolts running from the same antenna and
> sharing the same oscillator, ie at least one of the Tbolts modified to
> accept external 10MHz. This would take out most/all GPS system errors,
> leaving receiver measurement noise.
>
> When speaking of 1-shot resolution I think a dual-Tbolt configuration,
> with one driven by the DUT and the other by the house standard, is closer
> to a fair comparison with a TIC instrument. Since this configuration will
> eliminate the GPS system and propagation errors and the TIC is always
> limited by the house standard.
>
> Does anyone have two working Tbolts modified for external oscillator
> available for testing?  I have one Tbolt and a loaner unit from a friend.
> I am not very keen on modifying the two I have in my lab right now.
>
> kind regards,
>
>      Björn
>
>
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