[time-nuts] Trueposition Antenna Location

[Bob kb8tq]

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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