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

Azelio Boriani azelio.boriani at screen.it
Thu Oct 13 06:21:40 EDT 2011


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 at 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 at screen.it
>
>
>> 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 at yahoo.com> wrote:
>>>
>>> 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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