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

ws at Yahoo warrensjmail-one at yahoo.com
Fri Oct 14 20:17:17 EDT 2011


That is a great LH plot, AND Tbolt setup.
Noise is about the same as I'm seeing.

Can also use this setup to see if the Tbolt location is set correctly by 
doing a longer plot that includes many satellites changes and see what the 
peak noise spikes are.

One little minor thing you missed that I find very useful is to set the DAC 
Plot gain (LH command "GDS xxx"  to 1/Dac_Gain x K (K an integer constant)
In your case use -269 for Dac plot gain.  Then use "+" or "-" to put the Dac 
plot exactly on top of the other two plots and that way it makes it very 
clear and very sensitive to see any drift in the Osc.
Any Osc drift will shift the offset of the Dac plot.  Using the Tbolt filter 
"FD100" (or slower up to 1000)  and adding more plot gains, you can see Osc 
freq drift changes as small as 1e-12.


----- Original Message ----- 
From: "Ed Palmer"

> HI Warren,
> The attached picture shows how my Tbolt reacts in TPLL mode.  I lucked
> out and got about 35 minutes without a satellite switch.  The antenna
> consists of a VIC-100 + about 100 feet of RG-59 + 20db amp + HP
> splitter.  No choke ring or ground plane.  The antenna has good
> visibility E, W, S, and up, but it's on my balcony on the South side of
> a building so there's little reception from the North.  I realize that
> the big hole to the North is inherent in the GPS system.
> Ed
> On 10/12/2011 11:46 PM, WarrenS 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 
>> 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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