[time-nuts] Sub Pico Second Phase logger

[Joseph M Gwinn]

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