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.. !)
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