Magnus
I probable was not very clear in my posting. (what else is new?)
There where TWO completely different subjects, goals and techniques in the
same Posting.
#1 was how to Log data for valid ADEV plots. That takes setting the filter
OFF for the reasons you state.
The ADEV tau axes provides the filter function.
Different subject altogether, the one you copied has NOTHING to do with
ADEV plots or data.
#2 was measuring and Plotting the AVERAGE frequency (Not ADEV) and to do
that effectively, one uses LP filtered data.
For Low pass filtering, can use LadyHeather's filter function and / or the
trend line.
By looking at the freq plot at different filter settings, one can see what
type of noise is there.
And as you point out, the noise type does make a difference to the
improvement in resolution that is obtained.
I was just giving some general values that are valid for my typical type of
noise in my setup, which is mostly white over short time periods using lots
of care.
And yes can not be overstated, need to be carefull when taking and using
data.
ws
*****************
On 07/10/11 19:00, WarrenS wrote:
Use the (J J) command to rezero the Phase plot (plus the cable delay if
desired)
and then read or adjust the Osc freq on the ppt plot.
With LadyHeather's filter off (F D 0) useful resolution is about 1e-10
in one second,
With the filter set to 10 sec (F D 10) useful resolution is near 1e-11
in well under one minute
With the filter set to 100 sec, resolution is around 1e-11 in a few
minutes. adding a trend line helps (G O L)
Be a little careful here. Pre-filtering will scale white-noise ADEV
linearly, please see the Power noise table I put in the ADEV wiki article:
http://en.wikipedia.org/wiki/Allan_variance#Power-law_noise
f_H is the system bandwidth and changing the pre-filtering you do change
this property. The end result is that your h_2 amount of white phase
noise is the same but only gives different ADEV values, and is not the
result of improved noise floor.
However, you can use various settings to separate between WPM and FPM
noise levels.
Cheers,
Magnus
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