[time-nuts] ADEV drop
magnus at rubidium.dyndns.org
Sun Aug 12 10:13:56 UTC 2012
On 08/12/2012 05:02 AM, WarrenS wrote:
> The basic problem is that one can not meet Allan's requirement
> of the integral of the instantaneous frequencies over tau0 time
> and Nyquist-Shannon sampling theorem requirement if taking just
> one raw phase sample per displayed ADEV tau0.
> The two requirements are then mutually exclusive.
> This is especially true when that sample is coming from a
> DTMD zero crossing detector.
> The way I get ADEV tau answers that do not droop at all near tau0,
> and that are independent of the displayed tau0, the oversample rate
> and the NEQ.BW filter (if BW > 2* tau0) without having to throw
> away the low tau answers or save data files with more than tau0
> number of samples, is by oversampling the raw data and then
> reducing it in an appropriate way before saving it as tau0.
> With high speed oversampling it is also very simple to avoid
> any aliasing problems.
> Using an external DC coupled sound card, oversampling at 48KHz
> for any tau0, both the TPLL2.0 and the XOR-LPD give non-drooping
> tau0 answers that are not a function of the oversample rate or the tau0
> reduction rate.
> That is, get the same ADEV tau 1sec answer if the tau0 is 1KHz, 1Hz or
> anywhere in-between.
The problem with counters is that they often sample to infrequently. An
audio rate sampling and proper conversion of the DM beat note will just
as your TPLL2.0 provide sufficient sample rate. If you where trying to
make ADEV plots all the way to your sample-rate, you would get droop
too, but since you don't you don't experience it. That is because you do
keep sufficient number of samples on the first displayed tau.
The same behaviour has been seen in the TimePod for instance.
So, it just illustrates how you need to handle your data, not that the
method itself is better.
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