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From: =?UTF-8?Q?Wolfgang_B=c3=bcscher?= <dl4yhf@freenet.de>
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Date: Sun, 27 Sep 2015 11:02:32 +0200
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Subject: Re: LF: Coherent BPSK on LF using EbNaut
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Status: O

Sounds like an application for one of those Cortex-M development boards 
with built-in fast multi-channel ADCs, like the one Alberto recently 
mentioned here. I have a stm32f429-"discovery" board here. It has an 8 
MHz crystal oscillator which is "PLLed up" internally for all other 
clocks (core, timers, ADC, DAC). If that 8 MHz clock would be replaced 
by a GPS-synced signal, much of the ugly work would already be eliminated.
The board connects to the PC via USB (host or on-the-go) so it may be 
useful as a data acquistion unit with built-in display. Another USB port 
is used for debugging and firmware development (they call it "ST-LINK", 
and it's actually a kind of JTAG replacement). A kind of disadvantage 
are ST's libraries and C header files, which are created by a monster 
named 'STM32Cube'. The code spit out by this utility is an extremely 
manufacturer-specific 'framework' which makes sure that an application 
originally developed for am STM32F4 cannot easily be ported to another 
Cortex-M (by another manufacturer), so maybe GPS-locking an entire 
Raspberry PI should be tried as well....

Have a nice sunday everyone,
    Wolf .

Am 27.09.2015 um 09:13 schrieb Paul Nicholson:
>
> The soundcard becomes a liability for coherent signalling
> at LF.  The fiddly software is all concerned with coping with
> the wandering soundcard conversion rate.
>
> Andy wrote:
> > direct I/Q downconversion from LF to baseband, low pass
> > filtering  then I/Q sampling into an A/D at something like
> > 1kHz (or even lower),
>
> Yes, a small amount of hardware replaces a lot of software
> once you move away from the soundcard.
>
> And because the signal is shifted to a carrier frequency of
> zero, the A/D sampling of the baseband I/Q doesn't need to be
> super accurately timed - it merely affects the symbol timing,
> which needs only 1/10th second accuracy.
>
> No doubt there is scope for some direct conversion LF
> electronics to take over most of the work (certainly the
> timing critical part).   An opportunity here to revive some
> traditional techniques that are simple and ideal for this.
> Time to revisit some projects from a couple of decades ago.
>
> Just needs a GPS-locked DDS for the LO and a simple A/D for the
> I/Q.  Probably not many bits wide - 8 plenty, maybe 2 enough.
> 100Hz sampling is more than fast enough considering bandwidths
> are 1Hz or less.
>
> Even the DDS probably doesn't need to be very good.  Needs the
> long term stability of a GPS lock but it probably doesn't
> matter if it has 5 degrees or so RMS phase noise.
>
> I suppose at LF, you reach a point where your antenna is as
> large as it will go and the house lights dim when you key.
>
> How do you go further from there?
>
> One avenue might be this coherent signalling where you try
> to maintain an almost constant phase for the duration of a
> transmission.  The rx does likewise and the decoder expects the
> signal to be so weak it can't rely on picking out a reference
> phase, so it guesses.  Then you're suddenly several dB more
> capable.   You'll find yourself exchanging messages that are
> not just 50dB short of being audible, but can't even be seen
> on a spectrogram.
>
> Hmm... Have I nearly talked myself into setting up something
> here?
> -- 
> Paul Nicholson
> -- 
>