Stefan wrote:

> http://www.iup.uni-heidelberg.de/schaefer_vlf/VLF/record.tar

Interesting.  The first thing I notice is some curious effects
on the background noise, on ch1 only, eg

 vtcat -S49 -E2 170708-095003 | vtrsgram -ox -g200 -- -:1

(ctrl-C in the shell to quit)

or with vtsgram which uses sox as a back-end for the
spectrogram

 vtcat -S49 -E2 170708-095003 | vtsgram -p200 | display

There are lots of these:  For a very wide image

 vtsgram 170708-095003 -p50 | display

(Use the pan icon for panning and q to quit).

Looking closely at the one at 24 seconds

 vtcat -S23.5 -E1 170708-095003 | vtrsgram -ox -g200 -- -:1

and

 vtcat -S23.5 -E1 170708-095003 | vtscope

gives http://abelian.org/vlf/tmp/170708a.gif

We can just see a slight change to the ch1 amplitude
which lasts about 100mS.   Can't really see anything
obvious on the scope when zooming in on the start,

 http://abelian.org/vlf/tmp/170708b.gif

The amplitude just seems to go up a tad at about 442 mS.

Lets look at the phase during that same 1 second window

 vtcat -S23.5 -E1 170708-095003 |
   vtnspec -f1000 -w0 -r20 -k |
   awk '{printf( "%s %.1f\n", $1, atan2($4,$3)*180/3.14159)}'

produces

 1499507427.027 121.1
 1499507427.077 120.9
 1499507427.127 120.8
 1499507427.177 120.6
 1499507427.227 120.5
 1499507427.277 120.3
 1499507427.327 120.2
 1499507427.377 120.0
 1499507427.427 119.9
 1499507427.477 119.7
 1499507427.527 119.6
 1499507427.577 119.4
 1499507427.627 119.3
 1499507427.677 119.1
 1499507427.727 119.0
 1499507427.777 118.8
 1499507427.827 118.7
 1499507427.877 118.5
 1499507427.927 118.3
 1499507427.977 118.2

Just shows a steady phase drift, we're not quite on frequency here
with this recording - did it go through vttime?  I see from

 vtstat -i 170708-095003

that we have

 sample rate: 24000, correction 0.99997722

The correction should always be 1.00000000 if the stream has been
through vttime.

Curious.  Signal goes up, background noise drops. I wonder if
there is a dodgy input capacitor or dry joint.  Not sure about
that explanation, it would be odd to see the same on more than
one channel.

Wonder if the effect occurs at the same time on ch1 and say ch4?
Or instead the effect is independent on the effected channels.

I will set my Octo up with the same test and look for the same
effect here.

--
Paul Nicholson
--