Thanks Tom very interesting.
I like your guess on how they get such a low noise for the PPT data.
I would like to know why they would go to the trouble, because I don't see
anywhere that information is used except for the PPT output.
Alos the Tbolt's PPT freq data, long term is usually offset by a couple of
parts in e-12 on LH plots.
Any reason you know of why it would bottom out long term, beside software
rounding?
I see this in LH plots as well as the ADEV plot.
Is there anyway to get the M12+t's 1 sec resolution down below 1ns?
I'd expect the Tbolt and M12+t to be the same over the long-term (say hours
to one day), or even a few minutes, the noise should be completely dominated
by the GPS and not by the engine of a good receiver by then.
The Satellite switching phase jumps gets much worse with a poor antenna set
up, such as if the location is not set right or there is mutipath etc, etc.
I got mine down from what use to be 10 ns to around 1/2 ns.
I would like to see how low that noise is with a near "perfect" antenna
setup using a choke ring etc, to know how much more improving I still need
to try for.
ws
******************
Tom wrote:
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
ago.
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:
http://leapsecond.com/pages/tbolt/log36116.dat.gz
and described here:
http://www.febo.com/pipermail/time-nuts/2011-January/053621.html
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).
http://leapsecond.com/pages/tbolt/log36116-adev-sidereal.gif
This, because of the GPS SV orbital period.
/tvb
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