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Re: LF: Coherent BPSK on LF using EbNaut

To: [email protected]
Subject: Re: LF: Coherent BPSK on LF using EbNaut
From: DK7FC <[email protected]>
Date: Thu, 01 Oct 2015 16:29:52 +0200
In-reply-to: <[email protected]>
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Hi Domenico,

Thanks for your report.
I see you on my OPDS32 grabber, as you say.

2015-09-28 21:00:47 IZ7SLZ  1159km 137502.120Hz   1mHz -46.7dBOp  93% 17.9dB
2015-09-28 20:00:47 IZ7SLZ  1159km 137502.120Hz   2mHz -41.1dBOp  90% 20.4dB
2015-09-28 19:00:45 IZ7SLZ  1159km 137502.120Hz   5mHz -44.1dBOp  93% 18.8dB
2015-09-28 18:24:51 IZ7SLZ  1159km 137502.121Hz   2mHz -45.0dBOp  75% 15.2dB

Is 137502.120 Hz the frequency of your choice? If your TX and my RX is GPS locked (or tracked) then where does the 1 mHz offset come from? Maybe a phase variation due to changing propagation? Which offset is acceptable to avoid SNR losses in EbNaut (at a symbol rate that allows a QSO (in one evening! ;-) )) ?

73, Stefan

PS: Unfortunately not QRV on LF at the moment. I need to find a new loading coil body because my glass coil fully broke during maintenance attempts. Now i need something to come out with a stronger signal because each modification on the system has to be an improvement, by definition. So maybe i can come closer to 1W ERP then...

Am 01.10.2015 01:53, schrieb IZ7SLZ:
Hello LF and all,

here's my "2 cents" about coherent-bpsk transmit point of view. I will briefly describe the setup used by me during the tests and QSO with DF6NM.

I have employed an old analog SSB Exciter that has a resolution of 100 Hz. So, in order to set more exactly the frequency, exciter has been programmed on dial 136.0 kHz and then modulated with a 1502.120Hz tone to obtain the TX QRG used during QSO with Markus.

The tone is generated by Spectrum Lab having sample rate calibrated with 1_pps signal coming from a GPS (Rockwell Jupiter). Exciter is referenced to an 10 MHz OCXO (MTI-Milliren 230 series) having a thermal stability of 2.5E-8.
https://www.dropbox.com/s/wg0h78x31vdqr2i/OCXO.pdf?dl=0
There is a plan to replace the OCXO with a 10 MHz GPS disciplinated oscillator. This latter has been already ordered and it is on the way.

Before entering in the SSB exciter, audio tone is passed through a simple ring-modulator built with four diodes and two audio transformers.
https://www.dropbox.com/s/gsf74bz8e1cqnmt/ring%20modulator.jpg?dl=0
Ring modulator is driven by EbNaut-TX via RTS-DTR signals of the PC com port. There is an option in EbNaut-TX to use RTS and DTR in differential mode.
137 kHz output of the exciter, drives the 100 W PA.

The use of SSB analog exciter, of course, is not a good solution due to the possible soundcard glitches. Markus suggests to generate the carrier directly at RF i.e. using an AD9850 DDS, and apply the DBM modulation there. This solution will be also tested by me as soon as the GPS Oscillator becomes available.

Clock of EbNaut-TX Pc is corrected every minutes by a simple VB program that reads the gps nmea, extracts the time from GGA "sentence" and updates the PC clock at 1_pps rising edge. In fact 1_pps is also connected at CTS of the RS232 com port dedicated for the GPS.

Prior of any EbNaut sessions, i made a check of the frequency by sending some OP-32 sequences and looking my signal on the very precise DF6NM and DK7FC OPDS grabbers (Thanks also to Stefan!). Markus has kindly open a right window in his OPDS Spectrogram centered on 157502.120 Hz. Then i just modify, in SL, some mHz in the frequency of the tone generator, to compensate the error.

In these days, observing DF6NM and DK7FC OPDS grabbers, i have noted some stations with a very steady-frequency signal, i.e. G6AVK and PA0OCD. I noted their signal during the night also on my receiver setted in OPDS session mode. Unfortunatly their level is not enought for an OPDS detection by me but i'm sure that is ok for a small EbNaut message !

So i want to invite this OM to try to bpsk-modulate their carrier using EbNaut-TX.

I'm at disposal for any sked.
Most probably i will carry-out some EbNaut TX sessions during next week-end. In case i will inform via this mail reflector some enought time before.

It's a great fun to play with these stuffs!

73 and GL all,

Domenico (iz7slz)



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