Hi Markus,
Convinced that the oscillator had drifted since calibration 2 years ago,
I tweaked the DOXCO up 0.05 Hz and was happy to see the result on your
OPDS page.
Thanks for the tips on getting started on EbNaut. I'll tinker with it
and hopefully make some progress. Our cat seems anxious lately!
73
Joe
On Thu, 12 Nov 2015, Markus Vester wrote:
Hi Joe,
an interesting change happened last night: as far as I can see, your frequency
has shifted to nominal 137477.000 at 22:45 UT, and appeared to be rather stable
thereafter.
Regarding EbNaut transmit: I haven't tried it on our RasPi yet. Both Domenico
and I have run the Windows version of ebnaut-tx, in conjunction with
USB-to-serial converters. RS-232 lines are normally current-limited to 20 mA so
they can be directly connected to the DC-coupled IF input of any ring mixer. On
137 kHz, I used a Minicircuits ZAY3. As I have a linear PA I inserted a simple
500uF/100ohm shaping filter to reduce keying sidebands, but this may not be
essential. This mixer won't work on VLF 8.3 kHz so I used an old telephone
relay there, but due to contact bouncing the transitions were less than
perfect. Domenico uses an SSB exciter on LF, and he made a dedicated diode
mixer with audio transformers to apply PSK on 1502 Hz.
Our current receiving method re-uses opds32 data files and is thus limited to
sequence durations of a bit over half an hour. For example, using EbNaut 8K19A
encoding with 1 second symbols would fit 31 characters into 29.3 minutes, and
the message could be repeated half-hourly. Yesterday your opds carrier was
weakly visible until midday in Europe, so daytime TA decodes might be possible
using slower rates (e.g. 5 seconds).
Our cat seems to be very curious about radio, so I thought he might feel a
desire to communicate with his fellows overseas. Rumour has it that at least in
one instance, Uwe's cat successfully managed to take control of the DJ8WX VLF
transmitter ;-)
All the best,
Markus (DF6NM)
-----Ursprüngliche Mitteilung-----
Von: Markus Vester <[email protected]>
An: rsgb_lf_group <[email protected]>
Verschickt: Mi, 11 Nov 2015 6:12 pm
Betreff: Re: LF: VO1NA frequency
Joe,
more of the same from last night, this time with frequency and level plots.
Again we see some frequency variation of similar size as yesterday. It is not
clear how much of that is due to propagation, I only think that
the rise near the end is due to Doppler from the descent of the ionosphere
at sunrise. Russian Loran lines on 137475 and 137482.5 seemed to have less
overnight variation (~ 0.1 mHz), but then of course the intra-European path
from Slonim is very different from yours.
Regarding EbNaut carrier analysis: for some reason (lack of coffee?), I only
saw Domenico's earlier mail after I had sent mine, so please excuse me for
basically having described the same procedure again. Anyway it's good that
we both arrived at the same conclusion: If Joe started sending EbNaut PSK
with his oscillator as it is now, we would very likely be able to decode it
here.
Joe so all you need now is a diode ring-mixer for PSK modulation and Paul's
ebnaut-tx program. Symbol timing will be derived from the PC clock, so
with NTP available there's no need to involve GPS at all. If Paul brought his
LF receiver back to life, he could probably copy you even if you reduced TX
power to 0.1 Deccas.
How many cats do you have? We want to know names, colours, sex and age for
all of them ;-)
All the best,
Markus (DF6NM)
PS. SpecLab question: Wolf is there a way to extract peak frequencies from
past spectrogram lines (semi-)automatically, without having to
manually move and read the mouse cursor over each point, e.g. something like
a peak_f(f1,f2, time) interpreter function?
-----Ursprüngliche Mitteilung-----
Von: Markus Vester <[email protected]>
An: rsgb_lf_group <[email protected]>
Verschickt: Mi, 11 Nov 2015 1:24 am
Betreff: LF: VO1NA frequency
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)
From: [email protected]
Sent: Saturday, November 07, 2015 3:45 PM
To: [email protected]
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
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