[time-nuts] Measuring ADEV using TBolt-Tic tester
ed_palmer at sasktel.net
Fri Oct 14 23:36:48 UTC 2011
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.
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
> 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 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
> 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
> 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.
> 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.
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