[time-nuts] Question about SoundCard stability?

David McClain dbm at refined-audiometrics.com
Thu Oct 14 07:08:58 UTC 2010


> Or, now that I think about it, it's similar to what we do when  
> measuring ADEV.. you can do a crude "how many zero crossings in the  
> time window" or you can do a "fit a sinusoid to a series of ADC  
> samples".  One has an uncertainty of "one count/epoch", the other  
> can be substantially better.


How could it be substantially better for the same analysis period?  
Unless the frequency under test is an integral number of periods  
during the analysis period, you will have a variation in the sine  
fitting due to starting phase.

OTOH, as admonished in Horowitz & Hill, if the frequency to be  
counted is substantially below your counter timebase, then you should  
count zero crossings of the higher timebase frequency in the period  
of the lower frequency under test.

Dr. David McClain
Chief Technical Officer
Refined Audiometrics Laboratory
4391 N. Camino Ferreo
Tucson, AZ  85750

email: dbm at refined-audiometrics.com
phone: 1.520.390.3995
web: http://refined-audiometrics.com



On Oct 13, 2010, at 22:30, jimlux wrote:

> Jim Lux wrote:
>> That's not precisely true.  You can get a frequency estimate that  
>> is substantially more precise than 1/T if the snr is high.   
>> Consider super-resolution in an interferometer which is  
>> mathematically similar.  What you give up is ambiguity.  Probably  
>> one of the oldest techniques is that of Prony, but there are lots  
>> of others
>
> Or, now that I think about it, it's similar to what we do when  
> measuring ADEV.. you can do a crude "how many zero crossings in the  
> time window" or you can do a "fit a sinusoid to a series of ADC  
> samples".  One has an uncertainty of "one count/epoch", the other  
> can be substantially better.
>
>
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