[time-nuts] Sub Pico Second Phase logger

[Joseph M Gwinn]

Bruce,


time-nuts-bounces at febo.com wrote on 12/10/2008 07:58:49 PM:

> Joseph M Gwinn wrote:
> > Bruce,
> >
> >
> > time-nuts-bounces at febo.com wrote on 12/10/2008 04:46:50 PM:
> >
> > 
> >> Joe
> >>
> >> Another limitation on using too low a beat frequency is imposed by 
the
> >> increasing equivalent input noise spectral density of the sound card 
as
> >> the frequency decreases.
> >> 
> >
> > Yes, although people made good use of 1 Hz in DMTD instruments despite 
the 
> > junk near DC.
> >
> >
> > Joe
> > 
> Joe
> 
> The low frequency noise of the components used in their unoptimised
> slope amplifiers is significantly smaller than that of a sound card.

Who is "their"?  But I would hazard that people designing for operation at 
1 Hz chose components with low flicker noise, to the extent then possible.


> A Collin's style optimised slope amplifier limiter may be useful for use
> with a sound card (particularly the lower resolution ones) if one is
> timetagging the beat frequency zero crossings.
> Less slope gain will be required than when driving a counter input.
> In the flicker noise region choice of beat frequency is relatively non
> critical.

With the vast amount of data available from a soundcard, I'd be tempted to 
do a least squares three-parameter fit of a sine wave to the (decimated) 
measured data and use only the resulting fitted parameters in subsequent 
computations.  Like estimating where the zero crossings are.  Then the 
noise averaging is over the entire fitting window, not just near zero 
crossings.  (I have been following the talk of using a sin x over x 
fitting function, but I don't know if this really works any better than a 
simple fit to a sine, given this much data.)

Joe