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From: =?UTF-8?Q?Wolfgang_B=c3=bcscher?= <dl4yhf@freenet.de>
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Date: Sun, 30 Oct 2016 01:13:32 +0200
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Subject: Re: LF: Re[2]: LF: Idiot's guide to receiving Ebnaut? - Tutorial
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Am 27.10.2016 um 21:31 schrieb Paul Nicholson:
>
> I have used the Trimble Thunderbolt with its fixed
> 10uS pulse.  It worked (vlfrx-tools, M-Audio 192) but
> timing jitter improved after a pulse stretcher increased
> the width to 1mS.  As Andy says, the 10uS pulse energy
> was a bit low giving poor S/N and a jittery centroid of
> the smoothed pulse.
>
> Now I use a Ublox for timing VLF reception and this works
> fine.  The Thunderbolt is used for calibration and as a
> reference for comparing my growing collection of GPSDOs.
>
> I'll be interested to see Wolf's comparison of edge
> and centroid timing.  I've never had any luck with
> edge timing.
>

I compared both now, and measured the standard deviation in the GPS sync 
pulse timing.
Used an E-MU 0202 at 192 kSamples/second.
Test result:
  - 100 ms pulses, using *edge* detection (in the fourfold interpolated, 
windowed-sinc filtered signal):
    standard deviation about 40 to 50 ns/second

- 10 us pulses, using *centroid* detection (also with fourfold 
interpolation):
    standard deviation a very respectable 25 ns/second.

The interpolated waveform of what used to be a 10-us-pulse is here:
http://www.qsl.net/dl4yhf/t/10us_pulse_EMU0202_192kS_interpolated.png
  (the green line in the center marks the length of a sample *from the 
soundcard*, the orange segments are interpolated).

BUT...
After reducing the sampling rate from 192 to 48 kHz :
   - standard deviation 200 ns for the edge-detection method

   - standard deviation 600 ns for the centroid method, reasons not 
understood yet
   (this may be a bug, or it may be caused by using integer array 
indices as 'x' coordinate for the centroid detection area.
   I will refine the algorithm for these 'very short' pulses by using 
polynominal interpolation for the centroid edges as well.
   That would be a fairer comparison.)

Even with the above restrictions, 10 us sync pulses seem to be ok, if 
the soundcard uses a good delta-sigmal ADC (which results an almost 
ideal low-pass filter and thus the text-book pulse response as in the 
screenshot linked above).

So, short answer for Jim:
You can use the Thunderbolt E's 10-microsecond sync pulses as they are, 
no need to shape or stretch them.
I will upload a new version of SL soon, but first I will try to improve 
the pulse timing for lower sampling rates.

Cheers,
   Wolf .
(back in "MEZ" / CET now - farewall, nice daylight-saving time.. sigh.. !)