To: rsgb_lf_group@blacksheep.org
From: Paul Nicholson <vlf0403@abelian.org>
Message-ID: <4d96a9d4-5f22-7650-872b-6344e9aea8e3@abelian.org>
Date: Sat, 25 Nov 2017 21:20:33 +0000
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Subject: VLF: Todmorden receiver
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It looks like I'm on the finishing straight now with this
new E-field receiver.  Started in March 2013 and now it's
almost working!    It took about a year to get the old rx to
work properly and it's been out in the field since September
2003, looking distinctly shabby now after 14 years of Pennine
weather and attacks by cows and sheep - but it's still working.

It's taken 3.5 years to improve on it.  How can it take so
long to build an audio amplifier?

The answer would make a long and perhaps interesting article if
I had time to write it.  It would mostly be a list of things
that didn't work and why.   Much time was spent trying to do
away with the isolating transformers but eventually I had to
admit defeat.  Transformers bring with them a lot of problems
but they are hard to beat.  Things that work fine on paper, and
even test fine on the bench, just don't hack it when deployed
and carrying real signals.  I wanted to cover ULF, down to 1Hz,
but that turned out to be very problematic indeed. Digitising
near the antenna would solve a lot of these problems but the
technology to do that reliably with sufficient quality isn't
quite there yet.

Right now the new rx is layed out on the bench, complete with
150m of cat5, running an end-to-end system test. System noise
measures 14nV/m equivalent E-field and Spice is predicting
18nV/m.  The output noise of the rx is only 9dB above the
M-Audio 192 sound card noise floor at 8kHz.

I'm making good use of vtcat ++ to send the full 192k/sec VLF
raw signal from one of the loops into the office to use as
a test input to the rx on the bench, so now I'm listening to
VLF that's been received twice!

Response is good at 137kHz too, I've no means to measure the
system noise up there but Spice is showing about 10nV/m.  I just
need to work out a way to T-off the 137 at the bottom of the
cat5 without complicating the VLF response and compromising the
isolation.   There's still a problem with the DC-DC converter
in the rx, it doesn't always start up when power is applied,
something to do with the series resistance of the cat5.
The calibration port works real well and a re-build of the
antenna has cured some microphonics.  The antenna models
accurately using LCnetgen, capacitance measures within 2pF of
the model which means I can trust the model's effective height.

Next on the agenda is a re-build of the H-field rx.  That should
be a lot easier, it's just some small changes to improve
the frequency response, a new line driver, better isolation,
and reduction of cross-talk between the two channels.

When it's all working, the online steerable spectrogram will
have a 360 degree compass and a few dB more sensitivity -
it will be interesting to look at VO1NA's signal then!

--
Paul Nicholson
--