On Fri, 16 Mar 2012 03:08:47 -0700
Hal Murray <hmur...@megapathdsl.net> wrote:

> 
> p...@phk.freebsd.dk said:
> >> Hmm... do you mean you want to store all samples of an hour and then
> >> avarage over it?
> 
> > That would be the ideal way to do it, since it would make one heck of a comb
> > filter and eliminate pretty much anything else. 
> 
> That only works if your reference clock is stable enough over the collecting 
> period.

Not necessarily. The noise of the VLF signal is much higher than the
noise of your crystal, hence the limit will not be the stability of
your crystal. Beside, integrating over such long periods tend to eliminate
noise sources. 

> I'm far from a DSP wizard.  Assume I have a signal at 100 kHz with a 
> bandwidth of 1 kHz.  (or pick any numbers you like)  How many samples per 
> second do I need?  How stable does my sampling clock need to be if I collect 
> data for for N seconds?

You need to get at least two times the bandwidth (Nyquist criterion).
Ie in the example above a sampling rate above 2kHz would be enough.
But:
* you need to filter out all noise outside the band you are interested
in, otherwise it gets folded in and degrades your SNR.
* you need a low jitter ADC, as the jitter requirements are set by the
signal frequency, not by the bandwidth.
* the analog part from the input to the sample and hold circuit of the
ADC need to be able to handle the high frequencies of your signal.


> Is "stable" the right term?  What's the right way to think about the question 
> I'm trying to ask?

That i cannot say, as i dont know what you are asking :-)

In this case, i think you are asking for phase noise as well as
"short" term stability (tau < averaging time)

Calculating effects on signal processing in the precense of noise is
nothing trivial. Dont worry if you dont understand everything on the
spot or cannot form an intuitive understanding.


Hmm.. i someday have to look for a good introdcution into this stuff
that doesn't rely on a lot of math. All the books i have rely at least
on Laplace.. often on z-transformation as well. And that math isn't 
high school level anymore.


                        Attila Kinali

-- 
The trouble with you, Shev, is you don't say anything until you've saved
up a whole truckload of damned heavy brick arguments and then you dump
them all out and never look at the bleeding body mangled beneath the heap
                -- Tirin, The Dispossessed, U. Le Guin

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