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

[Bob kb8tq]

Hi

The typical answer to this issue is to first try to clean up the supply to the
DAC. If that does not fix the issue, you likely need a better DAC. Unless the
TCXO has a crazy large tune range, rational parts should be able to do the job.
If the TCXO has a crazy large range then maybe a different TCXO is the answer.

You can measure this or that to confirm / deny that the problem exists here or 
there in the circuit. An audio analyzer that gets down to a bit under 1 Hz should
be able to tell you what’s what. Various sound card based approaches likely 
would be “good enough” ( yes, ground loops will be fun to take care of ….)

Bob

> On May 20, 2022, at 9:26 AM, Erik Kaashoek via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 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<DAC_Filtering.png>_______________________________________________
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