[time-nuts] Sub Pico Second Phase logger

[Bruce Griffiths]

Joseph M Gwinn wrote:
> Bruce,
>
>
> time-nuts-bounces at febo.com wrote on 12/15/2008 05:31:27 PM:
>
>   
>> Joseph M Gwinn wrote:
>>     
>>> Bruce,
>>>
>>>
>>> time-nuts-bounces at febo.com wrote on 12/15/2008 04:34:34 PM:
>>>
>>>       
> [snip]
>   
>>>> I need to build a noise source to check the absolute level.
>>>> Will use the amplified Johnson noise of a 150K resistor.
>>>>
>>>> By Allan deviation do you mean calculate it from the sequential 
>>>>         
> 96KSPS
>   
>>>> ADC output samples?
>>>>
>>>>         
>>> Yes, although some decimation may be needed to keep compute times 
>>>       
> under 
>   
>>> control, at least for the larger values of tau.
>>>
>>>
>>>
>>>       
>>>> I can do this, but since the dominant noise source is white the Allan
>>>> deviation will scale with the measurement bandwidth.
>>>>
>>>>         
>>> Would modified Allan deviation be better?
>>>
>>> I'm more interested in the general shape of the Allan curve than its 
>>> absolute value, one issue being the effect of thermal variations in 
>>>       
> your 
>   
>>> laboratory dungeon.  We had speculated as to the relative size of 
>>>       
> thermal 
>   
>>> effects in these sound cards, and this would give us some idea.
>>>
>>>
>>> Joe
>>>
>>>
>>>       
>> Joe
>>
>> Modified ADEV, ADEV etc are possible, although the maximum usable record
>> length probably depends more on the limits of Plotter and Windows 2K.
>>
>>
>> I'll look into doing this.
>> Real time filtering and decimation may be impractical, in the short term
>> at least, as most signal processing libraries only process 16 bit 
>>     
> samples.
>   
>> Most real time spectrum analysis programs are similarly afflicted in
>> that they only process 16 bit samples.
>>     
>
> I don't see why we would need realtime filtering.  Data reduction can be 
> offline, so we ought to be able to use 32-bit or 64-bit arithmetic.
>
> Given that we will inspect Allan Deviation data in a log-log plot, one can 
> save much processing time by spacing the tau values to be computed 
> uniformly in log tau.  I've played with this in Mathematica, and it does 
> work and yields a large speedup factor.  It should also help with Plotter 
> and Win2K limits.  One trick is to ensure that one computes each tau value 
> at most once.  This check is needed because with close spacing, the round 
> function will yield the same tau values multiple times for small values of 
> tau.
>
> Joe
>
>   
Joe

Real time processing certainly isn't required to characterise the
performance.
However some may be tempted to do this, its probably possible with a
sufficently fast machine.
I was just highlighting a problem with some available signal processing
libraries which may have been developed before sound cards with
resolutions of more than 16 bits became available.
Some (perhaps most) real time spectrum display software also has this
problem (eg baudline, Virtins etc).

It isnt necessary to use a pair of mixers and an offset source to
characterise the sound card, driving both sound card inputs from the
same audio source should suffice.
The audio source need not have low ultra low distortion (the IF output
signals in a dual mixer system wont have ultra low distortion) or very
high frequency stability ( the IF output signals in a dual mixer system
wont necessarily have particularly high frequency stability).

A standard RC audio oscillator with distortion lower than 1% or so
should suffice.
At least the resultant frequency fluctuations should thoroughly exercise
the phase extraction algorithms.

Another option would be to low pass filter the output of a divider.
Using a sound card to generate the test signal is also possible but it
can potentially introduce extraneous noise and other artifacts such as
phase truncation spurs.

Bruce