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

WarrenS warrensjmail-one at yahoo.com
Fri Oct 14 13:30:10 EDT 2011


Thanks for the Tbolt vs. H-maser data log,
That's great data showing how good GPS can be out to 200,000 sec with SA 
Your data showing a little under 5e-14 at 1 day, makes a very good reference 
point to remember.
Any guess when your plot would flattens out or turn around?

It is also a very useful plot that can be used to see where to set the TC 
for a Rb disiplined GPSDO.
What I see with LH and a disciplined tweaked-out LPRO on a good run, for 
periods of time lasting a few hours is a RMS phase noise a little under 2ns.
With the Tbolt noise floor being well under 1ns, good to know it is not a 
limiting factor.

>Tom Posted on Oct. 4, 2011
>I have a couple of suggestions for you; in another posting.
Tom, did I miss anything?

Thanks for all the info,

Tom Van Baak tvb at LeapSecond.com
Posted Thu Oct 13 16:53:31 UTC 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.
> ws

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.


Tom Posted on Oct. 4, 2011
> What I'd like to find out is how accurate a GPS Disciplined_Rb_Osc can be 
> made compared to the typical Cs out there.


Most modern GPSDO will outperform rack-mount Cs in stability
as the tau grows beyond a day or a week. Of course, it depends
quite a bit on the particular GPSDO or the brand of Cs but in
general GPSDO's are really good and always "win" in the end
because they are locked to UTC and a free-running Cs isn't.

Some of my 5071A flicker out at 5e-15. It takes a 4 ns RMS
GPS timing receiver about 10 days to get to the same point.

Remember that a GPSDO is a really just a radio receiver, a
time-transfer device, a slave repeater; not strictly a "clock".
So their superior long-term performance isn't surprising. It
also explains why there's a huge market for GPSDO products
and a very small market for large stand-alone Cs standards.

The surplus Z3801A and Thunderbolts that hit the eBay market
this decade have been the greatest treat any amateur could
possibly have hoped for!

I have a couple of suggestions for you; in another posting.



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