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)
|
