[time-nuts] Re: ADEV interpretation

[Magnus Danielson]

Hi,

The 1/tau slope in ADEV is... a bit messy to explain. Often it ends up
being an limitation in the instrument. The simple rule is that it is
single-shot resolution limit. The fine-grained detail is that it is a
combination of resolution quantization and that of noise. I did research
on it, and it does some funky stuff. I should redo that one into a more
readable article some day, to many loose ends to clean up and explain.

Anyway, most often this instrument noisefloor is of great annoyance.
Often, as ADEV breaks away from it you start to see the ADEV of the
reference and source. The actual instrument will also have some
systematics that show up, but usually they are not too strong.

As you do MDEV and TDEV, the slope of white noise and indeed
quantization noise goes steeper, so you can get real measurement values
for lower taus that way.

Cheers,
Magnus

On 2021-04-04 13:52, Marek Dorsic wrote:
> Hello,
>
> I would like to ask your knowledge to help me interpret this basic ADEV result.
> Assume following measurement setup:
> PPS output of Ublox RCB-F9T connected to channel 1 of a HP53131A counter.
> 10MHz output of the counter's internal oscillator (opt. 010, medium stability oven) divided with PICDIV to 1 PPS connected to channel 2 of the counter.
> Counter set to measure time interval from channel 1 to channel 2.
> After processing the readings, the floor of ADEV is 5x10^-10 at about 30-100 seconds interval.
> Which of the following is true?
> a) ADEV shows the real performance of the internal oscillator,
> b) ADEV floor is 5x10^-10 because of the restricted counter time interval resolution (500ps),
> c) ADEV floor is 5x10^-10 because of the ZED-F9T PPS jitter (after saw correction) of the gate start input channel.
>
>
>
> Thanks in advance
>      .marek
>
>
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