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Hi Paul,
looking at your spectrograms three weeks ago,
your direct-sampling LF receiver actually seemed to be doing rather well!
It's surely sensitive enough, and apparently not limited by ADC noise or dynamic
range. I think that the initial lack of IZ7SLZ daytime decodes is not
showing a receiver deficit but only due to sheer distance between the two
of you.
GPS injection into the antenna signal doesn't have
to be bad either. I have often used a cheap mono sound dongle for
VLF, and SpecLab with 1pps lock and absolute-phase NCO produced clean
traces down to 46 uHz FFT, even riding through multiple soundcard
dropouts. The 1 Hz harmonics from the 1pps were completely cleared by the
noiseblanker. One big advantage with direct RF
sampling is that you don't have to worry about the local oscillator as
another source of potential frequency and phase uncertainty.
Best 73,
Markus
Sent: Friday, November 20, 2015 12:55 PM
Subject: Re: LF: EbNaut LF test from iz7slz
Domenico wrote:
> transmission in progress (21:30
UT) is formed
> by 39 characters at 0.5s.
Sorry I was unable
to decode this one. Tried list 100k
with time offsets -0.1 to +0.9 in
0.02 second steps.
My receiving system is not very good. You guys
are doing much
better because you have proper receivers. Here I am
trying
to use the same method that I use on VLF: direct reception
through
a soundcard, and it is just not working out very well.
The only soundcard
I have which will clock fast enough has a
mono input which means I have to
mix the PPS and LF together
into the one input. That, and the card
itself being quite
noisy, makes it difficult to apply a precise time
stamp to
the signal samples. It is working, just not well enough
and
it looks like I'm losing a dB or two at least.
It means I can't
make accurate measurements of carrier phase
and although we're getting some
success with EbNaut messages,
we cannot tell yet whether the band will
support long distance
long duration coherent signalling when the received
signal is
too weak to extract a reference phase.
Hopefully someone
better equipped will be able to measure the
phase variations of a long T/A
carrier.
--
Paul Nicholson
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