Hello Jim (and all GPS/SL users),
I have just uploaded a new version of Spectrum Lab (currently V2.92 b01)
which supports 'very short' GPS sync pulses.
Tested with 10 us GPS pulses. This isn't descibed in the manual yet :
Simply set the "Pulse Width" on the SR Calibrator panel to "10 us" (the
parser understands this simple 'tech' notation).
SL will automatically use a modified (*) centroid timing algorithm
instead of the pulse edge detection.
It now gives a standard deviation of less than 60 ns at 48 kSamples/sec,
and between 22 and 30 ns at 192 kSamples.
(*) for the curious: the modified centroid algorithm can be seen in the
C source, which is at
www.qsl.net/dl4yhf/t/GpsPulseDetector.cpp
(it's ugly and bulky because I modified it back and forth a few
dozen times,
the interesting part is in GPS_Det_ProcessInterpolatedShortPulse() .
The key for improvement at low sampling rates was the modified
centroid calculation area,
as explained in the above function.
There is also a lengthy note of 'test results' which may be
interesting. )
It's important not to overload the soundcard's input, even if the sync
pulse is a rectangle.
I set the gain pot of the E-MU 0202's line input to the center position,
and feed a pulse with about 200 mV peak into the jack.
All the best,
Wolf DL4YHF .
Am 30.10.2016 um 06:16 schrieb [email protected]:
Wolf,
Thank you, that is great news (10us pulse compatibility, and 25ns standard
deviation). I have an E-MU 0202 so I will use the (192kHz centroid 4x
interpolated windowed-sync-filtered) setup that you mentioned.
On my previous attempts I was using a Scarlett 2i2 at 96kHz and probably didn’t
have the 1pps channel configured correctly (in Windows and/or SpecLab); I'll provide
for some hardware termination in case the Trimble rise time, coax length, and Z_in of
the sound card don't agree.
I'll also figure out what I did wrong with the Scarlett 2i2; and try the Roland
SuperUA to see if it has some favorable attributes.
Thanks again,
Jim AA5BW
-----Original Message-----
From: [email protected] [mailto:[email protected]]
On Behalf Of Wolfgang Büscher
Sent: Saturday, October 29, 2016 7:14 PM
To: [email protected]
Subject: Re: LF: Re[2]: LF: Idiot's guide to receiving Ebnaut? - Tutorial part2
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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