[time-nuts] Trueposition Antenna Location

[Mark Goldberg]

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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