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-...@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 > ---- > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.