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

bg at lysator.liu.se bg at lysator.liu.se
Fri Oct 7 18:01:15 EDT 2011


Hi Tom and Warren,

> With a Z3801A or TBolt (or any cheap single channel GPS) receiver
> you should expect maybe a 5 to 10 to 15 ns variation over a 12 or 24
> hour period. You should be able to see this with a 5065A or a good
> Cs reference.

Hope to get a working 5065A in a month or two... ;-)

>> The Z3801's GPS engine is far inferior and not even close to the 10 to
>> 100
>> ps, 1 sec noise that a Tbolt engine has.
>
> Careful, each does averaging. So to compare you have to include
> the time constant into the equation. The RS232 numbers the TBolt
> reports are heavily averaged so at 1 second they always look a lot
> better than they actually are. If you have hard data for your claim,
> I'd be interested in seeing it.

Where comes the averaging on the Tbolt messages?

Looking at these (old) plots of two Tbolts running on the same antenna a
few years ago. (I have not searched for the orginal datalogs.)

   http://www.lysator.liu.se/~bg/timenuts/sd_doppler.png
   http://www.lysator.liu.se/~bg/timenuts/csum_sd_doppler.png

Eying the single difference plot, having around 1cm/s rms noise, that
translates to 1/30 ns/s or 33ps 1 second noise. Does that look ok with
you?

Warren: "A really good Tbolt set up will hold around 2 ns of phase noise."

Where are you getting phase measurements out of the Tbolt? Are you
converting doppler measurements to phase? Or is your phase refereing to
something else?

kind regards,

    Björn






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