[time-nuts] Measuring ADEV using TBolt-Tic tester
warrensjmail-one at yahoo.com
Thu Oct 13 15:20:37 EDT 2011
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
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.
>> 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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