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| Subject: | Re: LF: VO1NA frequency |
| From: | Markus Vester <[email protected]> |
| Date: | Thu, 12 Nov 2015 06:41:26 -0500 |
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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) 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?
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)
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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