I'm sure a lot more can be done with coherent operation at LF.
A few BPSK messages have been sent, distance record is 5418 km
(VO1NA to IZ7SLZ, Nov 2015). Little use has been made of
very narrow bandwidths and it is not clear what the LF band
can support in that respect.
My VLF receivers are intended to also work at LF but not yet
tested on-air. When I get the chance, I'll tee off some signal
from the VLF rxs into a BT878 card to see how reception looks
on E and H.
Now I'm thinking about a 3-channel mixer.
After some thought I decided the best option for a coherent rx
for 137kHz is to use a NEO-M8T. This has two timepulse outputs:
one can be configured for 1PPS to discipline the sound card,
the other provides the local oscillator. The M8T has a timing
mode where it assumes the position is fixed and thus produces
more accurate timing signals.
The M8T are not so easy to buy, or as cheap. I just ordered
one of these boards -
http://www.csgshop.com/product.php?id_product=218
I think it comes from Latvia. It's a lot cheaper than the
Ublox development board.
To go with it, I ordered another Octo sound card for the RPi,
https://www.amazon.com/gp/product/B06Y3PZ6MF/
No longer listed on Amazon UK, these are getting harder to buy
and may soon be obsolete. There's some talk of an upgraded
version. Traditionally, upgrades to a good soundcard produce
a card which is not so good for radio work (but has a lot of
extra useless features), so I'm not hopeful.
That's the easy bit done. I think the main challenge will be
to work out how to do some tight filtering without introducing
nasty temperature dependence of phase and amplitude response.
--
Paul Nicholson
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