Paul,
Thank you, this is very helpful. I had not considered the interactions of
the level tracker and the blanker with the strong continuous signals, and
accordingly I've noticed wide variation in the performance of the sferic
blanker. I had planned to head to a low-noise location this week in search
of better SNRs, now can't wait to see how a well configured sferic blanker
performs in the current location.
73, Jim AA5BW
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Paul Nicholson
Sent: Monday, March 3, 2014 10:55 AM
To: [email protected]
Subject: LF: VLF weak signals and sferic blanking
It is very important at VLF to use effective sferic blanking when looking
for weak amateur signals. There are 10 or 20 or more sferics per second and
the wanted signal will be lost in the noise when the energy of all those
sferics is distributed across your Fourier bins.
Let me illustrate with a plot. Here is a one hour spectrum of Bob's signal
with and without sferic blanking,
http://abelian.org/vlf/tmp/29499_140302n.gif
As you can see, without the blanking even this strong signal is completely
buried by the noise.
Effective blanking will discard between 25% and 35% of the incoming signal
but will boost the S/N ratio by 20dB or more.
The blanking threshold must be set very low to achieve this, roughly 1.2 to
1.5 times the mean level is the optimum.
The mean noise floor is conveniently tracked by an exponential moving mean
of the absolute signal value (I use time constant of 1 second or so but
longer is fine).
Some important caveats:
First, the input to the blanker must be free of MSK signals, hum harmonics,
and any other continuous signals when viewed in the time domain. Two bad
things happen otherwise: the blanker mean level tracking will be foiled by
the continuous signals, and the chopping action of the blanker will spread
the continuous signals across the band to contribute to the noise floor.
So, precede the blanker with a filter not wider than say 3 or 4kHz and
include notches for any significant mains harmonics or MSK signals that
remain within the passband. If you can see them against the noise in the
time domain then they need to be notched out.
Second - the blanker must see a clear sferic in order to work, so the
preceding filter should not be too narrow. 2kHz to 4kHz is enough. 1.5 kHz
is starting to get too narrow. The passband doesn't have to be centered on
the rx frequency, it can be offset to avoid including some inconvenient
continuous signal.
While a bit of lightweight sferic blanking is a nicety at higher
frequencies, at VLF some serious blanking is essential
for detecting weak signals. Nearly every amateur signal I
receive is well beneath the un-blanked noise floor.
--
Paul Nicholson
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