[time-nuts] Trueposition Antenna Location

[Mark Goldberg]

I am starting with what should be a frequency measurement at the sample
rate. Timelab can import that and should be able to determine ADEV from
that. I agree that something is not right though. I unfortunately am trying
to measure with what I already have. I don't have a TIC.

Regards,

Mark


On Mon, Dec 3, 2018 at 5:10 PM Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> Let’s back up a bit:
>
> The input to an ADEV calculation is a record of phase (time between wide
> spaced
> edges). Often this is from a PPS output on a device. It could equally well
> be from
> 10 MHz edges spaced a count of 10,000,000 edges apart.
>
> The first part of the formula takes the difference between adjacent
> samples.
>
> The next step puts that difference into a standard deviation calculation.
>
> If you are starting with something else, then you likely are not
> calculating ADEV.
>
> Bob
>
> > On Dec 3, 2018, at 11:21 AM, Mark Goldberg <marklgoldberg at gmail.com>
> wrote:
> >
> > I have looked at it multiple times and so far cannot see a discrepancy.
> The
> > ration of the ADEVs for the two sample rates is very close to 2 for a
> wide
> > range of Tau in the 10-200 sec range. That can't be a coincidence. It's
> > between 2 and 3 for higher Tau but that may be due to temperature
> changes.
> >
> > Basically, the Perseus SDR, clocked by my Bodnar GPSDO is sampling the 10
> > MHz from my TruePosition and using the DDC to down convert to 48 kHz. I
> > then do either a 32768 or 262144 point FFT, resulting in window times of
> > 0.6816 or 5.453 sec and use a built in function in Spectrum Lab that as I
> > understand it interpolates the peak frequency from the highest two bins.
> > That calculated peak frequency is saved every 0.6816 or 5.453 seconds.
> That
> > data is fed to Timelab.
> >
> > I could do the same size FFT for both and decimate the input by 1 and 8
> > respectively, which should result in the same FFT window time and check
> > that.
> >
> > I'm scratching my head here.
> >
> > Regards,
> >
> > Mark
> >
> >
> > On Mon, Dec 3, 2018 at 6:28 AM Bob kb8tq <kb8tq at n1k.org> wrote:
> >
> >> Hi
> >>
> >> ADEV is pretty well documented and the results should *not* be dependent
> >> on the
> >> sample rate. The proper approach to dropping the sample rate is
> decimation
> >> of the
> >> data. I think you may want to look at the way you are doing ADEV.
> >>
> >> Bob
> >>
> >>> On Dec 3, 2018, at 12:30 AM, Mark Goldberg <marklgoldberg at gmail.com>
> >> wrote:
> >>>
> >>> I ran them again today and the results are almost identical. The slower
> >>> sample rate still shows a lower ADEV. I am not sure what the algorithm
> to
> >>> determine the peak frequency is but it is described as a linear
> >>> interpolation between the two highest peaks on the FFT.  I could see
> how
> >>> that could be subject to math errors. For the slower sample rate, the
> >>> frequency bins are smaller. I have not researched how all this could
> >> affect
> >>> the peak frequency and possibly introduce noise (or by truncation
> remove
> >>> small amounts of noise).
> >>>
> >>> I will run it a few times in the next few days and will also try to
> move
> >>> the antennas to a location with lower multipath.
> >>>
> >>> Thanks for the suggestions,
> >>>
> >>> Mark
> >>>
> >>>
> >>> On Sun, Dec 2, 2018 at 2:16 PM Bob kb8tq <kb8tq at n1k.org> wrote:
> >>>
> >>>> Hi
> >>>>
> >>>> Looking at the data, I’m guessing the slower sample rate was done
> last.
> >> If
> >>>> so,
> >>>> you may not be letting the GPSDO’s “warm up” long enough. Most designs
> >>>> take days (if not weeks) to get to their ultimate stability.
> >>>>
> >>>> Bob
> >>>>
> >>>>> On Dec 2, 2018, at 11:16 AM, Mark Goldberg <marklgoldberg at gmail.com>
> >>>> wrote:
> >>>>>
> >>>>> I broke down and drilled a couple holes in the wall and moved the
> >>>> antennas
> >>>>> for both GPSDOs outside with a better view of the sky, but still
> >> somewhat
> >>>>> shielded by the house and a metal RV cover. The number of sats is way
> >> up
> >>>>> and the TruePosition is in normal mode most of the time. The ADEV is
> >> not
> >>>>> improved and actually measured worse. I took data for twice as long
> at
> >>>> two
> >>>>> sample rates, 0.6816 sec and 5.453 sec. The ADEV from the slower
> sample
> >>>>> rates is lower, possibly due to the measurement method, but they all
> >> are
> >>>> in
> >>>>> the same order of magnitude. I've named the Heather screen captures
> for
> >>>> the
> >>>>> inside and outside locations and shown one combined Timelab capture.
> >>>>>
> >>>>> Regards,
> >>>>>
> >>>>> Mark
> >>>>>
> >>>>
> >>
> <TruePos_Outside_Ant_Heather.png><TruePos_Inside_Outside_Compare_Ant_Allan.png><TruePos_Inside_Ant_Heather.png>_______________________________________________
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