[time-nuts] Does filtering a TCXO Vtune to reduce low tau ADEV with max 50% make sense?

[Erik Kaashoek]

During testing a TCXO using a correct Vtune for 10MHz output, the ADEV 
at tau=1 second  was measured at 1.08E-10 (see attached plot[1], trace 
DAC=normal)
When the DAC controlling the Vtune was forced to zero volt output the 
ADEV at tau=1 s reduced to 5.43E-11 (see plot, trace DAC=0)
This clearly hinted at some noise present on the DAC output.
Adding substantial filtering between the DAC and the Vtune input of the 
TCXO and setting to 10MHz resulted in an ADEV at tau=1 s of 7.28E-11 
(see plot, trace DAC filtered)
All measurements where done with the controller for the Vtune disabled.
The plot contains error bars.

The extra filtering required for the reduction creates a lag of about 10 
seconds in controlling the TCXO and the controller without the filtering 
was nicely able to correct fairly quickly random walks so I'm a bit 
worried about the problems this 10 seconds extra lag may create in 
tuning the controller loop.

Is a reduction of the ADEV of a 10MHz reference output with 25% (or at 
most 50%) for low tau in practice of any relevance if this may cause an 
increase in ADEV for larger tau as the controller may have more 
difficulty to correctly adjust Vtune to correct random walks?

[1] Plot: http://athome.kaashoek.com/time-nuts/DAC_Filtering.png
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