Re: LF: Idiot's guide to receiving Ebnaut? - Tutorial part2

[Paul Nicholson <vlf0403@abelian.org>]

Wolf wrote:
> measured the standard deviation in the GPS sync pulse
> timing. Used an E-MU 0202 at 192 kSamples/second.

Ran a quick trial here.

With the Trimble 10uS directly into an E-MU 0202 at 192k/sec I
get 100nS std dev of the raw intervals.  The timing system here
uses smoothed intervals which reduces the jitter to below 30nS.

Same setup at 48k/sec is useless, the raw std dev is around
6uS.  Watching the pulse on vtscope, the waveform changes
significantly from one pulse to the next.  This is so at both
sample rates but seems worse at 48k.   Looks like waveshape
depends on just when the pulse arrives with respect to the
soundcard's internal A/D sampling.

With the centroid method, we must avoid sharp edges and flat
tops.  Some sort of analog filtering before the card input
is required.  A low pass RC is usually used.  I always recommend
running the card at its full sample rate and down sample if
required after the timing system has done its stuff.

--
Paul Nicholson
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