[time-nuts] ADEV vs MDEV

Magnus Danielson magnus at rubidium.dyndns.org
Sun Feb 7 00:13:15 UTC 2010


Bruce Griffiths wrote:
> If one has a high end sound card then it could be used to implement the 
> bandpass filter and replace the zero crossing detector.
> It may be necessary to insert a pilot tone to calibrate the sound card 
> sampling clock frequency.
> A noise floor of about 1E-13/Tau should be achievable.
> This simplifies the DMTD system by replacing the zero crossing detector 
> with a low gain linear preamp.
> 
> If one analyses the resultant data off line then one can also try out 
> different techniques such as a Costas receiver rather than a simple 
> bandpass filter plus zero crossing detector.
> However 1000 seconds of data for 2 channels of 24 bit samples at 192KSPS 
> will result in a file with a size of at least 1.15GB.

First step would be to implement a real-time first-line receiver 
processing. Should not be too hard. Costas-loop isn't too complex to 
implement, but for full repeatability using the same signals for 
different set of parameters... but then again, it is fairly 
straight-forward to implement.

Using either a separate channel or FDM a reference 1 kHz (from say 
TADD-2 divider) should work for reference tone. A good board can be 
word-clock locked without too much hazzle.

Not too happy about the quality of the real-time spectrum apps I have seen.

Cheers,
Magnus



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