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Hi Joe,
the attached plot shows the
frequency stability of your opds trace received last
night http://df6nm.bplaced.net/LF/opds32_151110_0736.png .
Each data point is the interpolated peak of a
single 438 uHz FFT (i.e. 38 minute window). The average frequency seems to be
0.7 mHz below nominal, and there are small temporal variations up
to 0.5 mHz in either direction. Though some of that is
probably attributable to noise and propagation, I believe that there
is also a bit of inherent instability in the frequency
source. Actually a few parts per billion is quite good for a free
running OCXO (assuming the "D" stands for
"double oven" and not "discipled"), and that the calibration was done over a 10
MHz HF link.
Is this good enough for EbNaut? During the
experiments with IZ7SLZ we found that for full sensitivity the phase variation
over the EbNaut sequence duration should not be much more than 90°. Thus
for a half-hour message the frequency should be ideally constant and known to
about 0.14 mHz. If there is more variation we often can still
get decodes, but we will have to find the best frequency offset by trial
and error, which can be cumbersome if the signal is weak.
I also fed opds data files from your Sat/Sun
transmissions to the EbNaut decoder. The idea was to see if your Opera
transmissions would actually produce the all-asterisk decode which is
expected for a non-PSK-keyed straight carrier, and to observe the symbol phase
evolution over the 38 minute duration. Starting midnight UT, one decode was attempted every 30 minutes,
using a setting for 464 raw symbols (4 characters) and 5 seconds per
symbol. A frequency offset of -0.6 mHz was selected to obtain
flat phase during the first decodes, and that offset was kept for
all later attempts. Despite the Opera gaps
which degrade or drop half of the available symbols, 12 out of 13
decode attempts produced the
correct "****" message, e.g. http://df6nm.bplaced.net/LF/r0317.png
. However at times the frequency drift was quite visible on the phase
plot, e.g. in http://df6nm.bplaced.net/LF/s0317.png (flat
phase around 3:17) versus http://df6nm.bplaced.net/LF/s0347.png (upward
phase slope around 3:47).
Scaling down to VLF, deviations from the same
oscillator will be 16x smaller (~30 uHz at 8.3 kHz). This will be probably good
enough even for narrow FFT bins, corresponding to several hours coherent
carrier integration or slow EbNaut decodes.
All the best,
Markus (DF6NM)
Sent: Saturday, November 07, 2015 3:45 PM
Subject: Re: LF: VO1NA opds-32
LF Group,
Again, thanks to SV8RV DF2JP PA7EY and DF6NM for
the reports.
In my last post I reported the drift was constrained to
within 1 Hz.
This should be 1 mHz. Thanks Markus for your assurances
that this
should be adequate for EbNaut which I hope to use to send send our
cat's name on LF. The little curls appear often on the QRSS traces
on
your high res spectra. Doppler variations were removed from the 10
MHz
WWV signals with harmonic regression when the DOCXO was calibrated
so
this effect is seen on LF but not as strongly.
After reading about
EbNaut on Paul Nicholson's pages it seems I have
much catching up
ahead. Fasinating stuff!
Can the EbNaut phase inversions be done
without an absolute time
reference? I'm cheap and lazy so hoping GPS
will not be needed.
The Opera will be doing an encore tonight and if the
wind stays low
a tower climbing expedition is planned for the
afternoon.
73
Joe VO1NA
On Sat, 7 Nov 2015, Markus Vester
wrote:
> Hi Joe, LF,
>
> during the last couple of nights,
opds detected all of your transmissions between about 22 and 7:30
UT.
>
> Attached is a zoomed 40 mHz section from my opds-32
spectrogram, showing your central coherent carrier which contains half of the
transmitted average power. It seems to be about 0.5 mHz below 137477 Hz
(possibly due to a calibration error in my receiver). The 2.7 mHz spaced
sidelines during the early part (bottom) were caused by 6-minute-periodic
interruptions from my own MF WSPR transmissions. However the little curls near
the carrier can presumably be attributed to Doppler-shifted multipath
components. The carrier was still weakly visible for another hour after the last
detection.
>
> Comparing the looks of the carrier to earlier IZ7SLZ
transmissions, the stability of the signal and the path seems quite sufficient
for half-hour EbNaut PSK transmissions. Unfortunately I missed the opportunity
to save the raw data and analyze phase evolution using the EbNaut decoder itself
- will try that next time.
>
> All the best,
> Markus
(DF6NM)
>
>
>
>
> From: [email protected]
> Sent: Saturday, November
07, 2015 12:03 AM
> To: [email protected]
>
Subject: Re: LF: last night opera opds detection in
km07ks
>
>
> LF Group,
>
>
> I am grateful
to Joe DF2JP for his captures QRSS and Opera
> Dionysious SV8RV for his OP
decodes to Domenico IZ7SLZ
> for his OPDS decodes, and thanks to DF6NM for
opds!
>
> I do not know if my DOCXO is stable enough for
EbNaut.
> It was set a couple of years ago and has not drifted more
than
> 1 Hz since then. It seems the phase decoherence has
stopped
> haunting me.
>
> I am hoping to replace the halyard
for the other 100m wire soon.
>
> OP32 again tonight as send this
email.
>
> 73
> Joe
>
>
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