John wrote:

Besides the measured ADEV plot I posted at
http://www.febo.com/pipermail/time-nuts/attachments/20111007/48d1ab68/attachment-0001.gif

Attached is another way I've measured Phase noise of the Tbolt, to optimizing its antenna system.
This LH plot shows a total phase noise (GPS, + TBolt + Osc) of 0.087 ns RMS reading to reading variation at one second update, over a time period of 26 minutes using a one second disiplined loop.  This is the same as 0.87 e-10 RMS freq noise if using a 1 second time base.

On this test, I set the Tbolt's Time Constant to 1 second and its damping to 10.  (The Dac gain must be set right on to work right)
This causes the Tbolt's discipline loop to correct any phase error due to noise on the very next 1 sec update by stepping the Oscillator's  frequency.
This Is an easy way to measure the reading to reading phase difference using just LadyHeather.
The data can also be interpreted.as the average RMS frequency variation over 1 second, which is approximately equal to the ADEV value at a tau of one second (1e-10).

example: If the first phase reading where zero and the next one is +1ns then the control loop will change the Osc freq by way of its EFC, by 1e-9 so that the very next phase difference is  zero again. This makes it into a 1 sec delayed TPLL (Tight Phase Lock Loop).

I ran this same test on John's Online Tbolt. Its phase noise measured 0.13 ns RMS.
Most of the difference was caused by satellites switching during the test. Each switch causes a ns or so noise spike when the number of satellites changed.
I also tried several other test including using just one bird with no switching. That was more than twice as noisy depending on which satellite bird I selected.

I'd like to see what the Phase noise is of other Tbolts using this same method, especially when using a good choke ring antenna that has a good sky view.

ws

ws at Yahoo wrote:

The noise data is my measured values which I do several different ways. Some
of which are:

The GPS engine value was calculated from measuring the UNFILTERED RMS noise
of the freq plot data using LadyHeather, backed up by the independent way of
looking at the  UNFILTERED 1 sec ADEV values obtained when plotting the ADEV
from that data using an external low noise osc.
The other proof that the data is unfiltered was done by black box testing of
small near instantaneous freq changes of 1e-10 and measuring and how long it
took the Tbolt plot to settle to the new freq value using different filter
setting.
The answer is that it knows the correct freq (within it's nose limits) in
the next 1 sec sample period when the filter is turned off.

As for the ns phase noise that is the RMS Phase noise value from LH using a
good LPRO osc with it's Time constant set to many hrs.  (Phase correction TC
was 100K sec). The RMS noise value is very insensitive to the filter setting
up to 1000 seconds because most of the phase noise is slower than 1000
seconds.

As far as the 4 to 10 ns day to day USNO data , that has nothing to do with
sub ns short term noise which I generally limit to more like a few minutes
of sampel time, and if there is a satellite change during the test run, then
I start the test over because I'm looking at GPS engine noise and not the
GPS noise causes by changing satellites etc.

As far as the 4 to 10 ns over a two day period, that agrees pretty well with
what I see some times on a bad day.
On a good day I can get more like 2 to 3 ns, with a 500 sec filter, on a bad
day up to 5 or 6 ns.
For some periods lasting up to 5 to 6 hrs, I've seen numbers as low as 1.5
ns RMS.

ws

From: "John Ackermann N8UR"

In that test I was just capturing the ADEV table from the TSC-5120 so don't
have raw phase data.

I'm curious where you got the noise data for the TBolt gps engine -- that's
far better than I've seen quoted before.  The Trimble data sheet that I
found specs the system PPS accuracy at 20 nanoseconds one sigma; they don't
separately spec the GPS engine.  (The data sheet for the current Thunderbolt
E data sheet says 15 nanoseconds.)

The USNO says that their filtered, linear fit time transfer measurements
over a two day period, over the entire constellation, have an RMS residual
of 4 to 10 nanoseconds without SA (http://tycho.usno.navy.mil/gpstt.html).
That may not be apples-to-apples methodology, but it implies that
sub-nanosecond results may be difficult to obtain.

John

Your work is very interesting, now I wonder what is the Tbolt
single-shot resolution? Does the Tbolt use the analog interpolator
method? I don't have the Tbolt, I have an HP58503A at work as the only
reference.

On 10/13/11, WarrenS warrensjmail-one@yahoo.com wrote:

I know very little about the HP58503A. Any chance it is using the old 6
channel Oncore GPS engine?
If it is like the Oncore I tested long ago, that noise was about a decade or
so higher than the Tbolt's phase noise.

Not sure what you can call single-shot resolution. The data is reported with
Pico second  resolution.
The cycle to cycle max phase varation, If there is not a satellite change at
the same time, is around  0.4ns  max error.
With the very high resolution that is output, averaging  provides a lot of
benefit.
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

----- Original Message -----
From: "Azelio Boriani" <azelio.boriani@screen.it

The HP58503A has the Oncore 8-channel GPS receiver. The single-shot
resolution capability is the ability to resolve the time interval
without any averaging. For example, the Fluke/Pendulum PM6681/CNT81
has a 50pS resolution, the HP5370 has 20pS, the Racal Instruments 2351
VXI TIC has 8pS single shot maximum resolution, the Wavecrest SIA3000
signal analyzer has 200femtoS hardware resolution at 3GHz.

On 10/13/11, ws at Yahoo warrensjmail-one@yahoo.com wrote:

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

I'll try again.
My last post was completely garbled somewhere along the line.

Using the 1 sec ADEV noise floor from the plot at
http://www.febo.com/pipermail/time-nuts/attachments/20111007/48d1ab68/attachment-0001.gif

This shows the RMS sum of the short term GPS signal's noise Plus the Tbolt
engine, plus the Osc, for the achievable resolution at 1 second.

The "one shot", 1second resolution of the Tbolt's phase data is 100 ps or
less
The "one shot", 1 second resolution of the Tbolt's PTT detector is 10 ps or
less

What is not too clear is how much of that is due to the Tbolt engine and how
much is the "GPS Reference".

the GPS.
In these cases the Tbolt's external Osc noise is low enough as not to
contribute significantly to the RMS sum.

This is not necessarily an equal comparison to your 1 shot numbers,
because the Tbolt is using high speed averaging to get its low "single shot"
1second resolution by averaging over many cycles of a higher frequency.

Using this basic technique and averaging over 10,000 cycles of the 100ns
10MHz signal,
the "one shot" resolution achieved by the TPLL2.0 is under 10 femtosecond in
1ms.

ws

From: "Azelio Boriani"

The HP58503A has the Oncore 8-channel GPS receiver. The single-shot
resolution capability is the ability to resolve the time interval
without any averaging. For example, the Fluke/Pendulum PM6681/CNT81
has a 50pS resolution, the HP5370 has 20pS, the Racal Instruments 2351
VXI TIC has 8pS single shot maximum resolution, the Wavecrest SIA3000
signal analyzer has 200femtoS hardware resolution at 3GHz.

On 10/13/11, ws at Yahoo  wrote:

Hi Tom,

I propose to also look at packet 0x5A Raw measurements, especially the
doppler measurement. The Tbolt actually does not output carrier phase
measurements, it gives the related doppler measurement.

Is it established that the 0x8F-AC and A7, 10 MHz and PPS offset
measurements are not the output from some internal filter estimation?

It would be interesting to relate the 0x5A individual doppler measurements
to the individual 0x8F-A7 SV clock biases.

I propose an experiment with two Tbolts running from the same antenna and
sharing the same oscillator, ie at least one of the Tbolts modified to
accept external 10MHz. This would take out most/all GPS system errors,
leaving receiver measurement noise.

When speaking of 1-shot resolution I think a dual-Tbolt configuration,
with one driven by the DUT and the other by the house standard, is closer
to a fair comparison with a TIC instrument. Since this configuration will
eliminate the GPS system and propagation errors and the TIC is always
limited by the house standard.

Does anyone have two working Tbolts modified for external oscillator
available for testing?  I have one Tbolt and a loaner unit from a friend.
I am not very keen on modifying the two I have in my lab right now.

kind regards,

 Björn

Hi Warren,

Do you have two working Tbolts with their orginal oscillator removed?

I think a dual Tbolt configuration would eliminate GPS instabilities and
rely more on the house standard. As proposed in previous email.

--

Björn

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: