[time-nuts] A question about anomolous Alan Deviation Measurements.

[Azelio Boriani]

First take a look here:

<http://www.ko4bb.com/Timing/FAQ-2.php>

then you have to better illustrate how you take your measurements. Are you
using the time interval measurement? If you are using that method: how are
arranged your start and stop signals? The counter itsef has a minimum of
1.6ns signal spacing and width capability and a 100ps single shot
resolution. If the two oscillators are spaced 0.5ppm apart then their "time
spacing" is advancing (or slowing) at a rate of 0.05ps every cycle. The
counter can take measurements when the signals are spaced at least 1.6ns so
you have, roughly, 32000 "blind" cycles and 1936000 valid cycles, that is a
16% of the measures are not valid (cycles of 100ns). Maybe the counter
retains the last valid measure during the blind spot, so the overall result
will look better (no variation -> better adev).

On Mon, Sep 22, 2014 at 4:55 AM, morrie at iometric.com <morrie at iometric.com>
wrote:

> I am using a Fluke PM6690 counter measure the Alan Deviation
> of various pairs of 10 MHz OCXO's in use in my lab.
>
> I discovered that I can measure a (tau = 1 sec) low value of say 6.0E-12
> on a good set, but only If the crystals are tweaked so the delta is some
> 0.5 ppm.  If they crystals are tweaked to be within a mH, then the Alan
> Deviation zooms to about 5 times higher.
>
> I suspect a flaw in my PM5590 unit or in its design, and I guess will
> hunt around for something better.
>
> Does anyone have experience with this phenomena?  Does one always
> have  to separate the clocks being compared to avoid this "beating"?
>
> Thanks,
> Morrie Altmejd
>
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