[time-nuts] Measuring ADEV using TBolt-Tic tester
Tom Van Baak
tvb at LeapSecond.com
Thu Oct 13 12:53:31 EDT 2011
> The noise of the Tbolt's freq (PPT) output data measured about ten times
> lower than it's phase output data at 1 sec.
> How it does it is anyone guess, but looks to be some sort of high speed
> averaging going on, taken over a one second time interval.
The short-term frequency values could be aided by carrier phase
while the longer-term timing from code phase. I am trying to dig
out the work I did on packets 0x8F-AC and 0x8F-A7 some years
In those runs I recorded all the raw data from a TBolt (not from
LH) at the same time as comparing the 10 MHz output against
a stable reference. That helped separate internal tracking effects
from actual 10 MHz performance.
Yes, the switching in/out of different satellites is quite an effect,
as you have observed. This is exposed in 0x8F-A7.
Note the long-term (say hours to one day) TBolt performance
is a couple ns RMS; which is close to a plain M12+T receiver.
I'm looking for some parallel runs I did with various Oncore and
TBolt receivers. I think you'd enjoy those data sets, but I have
to locate the old PC first.
There's one nice long TBolt data set that I posted:
and described here:
Put that into TimeLab and play around with it.
The stdev of this several day run is 2.6 ns.
The peak to peak variations are around 10 to 15 ns.
TDEV is about 1 ns.
One other trick you can pull with this data set is zoom in on
the ADEV. If you look closely you can see that the ADEV for
a GPSDO is ever so slightly better near tau 86164 (sidereal)
than it is as tau 86400 (solar).
This, because of the GPS SV orbital period.
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