John wrote:
I'm curious where you got the noise data for the TBolt GPS engine
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
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