Andy and all,
I've been researching how a peak clipper might be implemented in an analog
LF receiver chain before the narrowband filters, similar to your suggestion
about taking noise pulses out using DSP. Such a limiting technique is
mentioned in the book "Communications Receivers," 2nd edition, by Rohde,
Whitaker and Bucher, but there is little about a practical design. My
feeling is that a limiter could work much like transmit RF clippers, where
post-clipping selectivity reduces the distortion products that might rise
from diode clippers and the like. I'm a little unsure about the remaining
amplitude-clipped impulses, though, as the rise time of these still may
overexcite later selective stages. Has anyone information about how such a
clipper or limiter could be built that would improve the signal to noise of,
say, QRSS signals?
Roger
AD5T
-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Andy talbot
Sent: Wednesday, July 10, 2002 7:59 AM
To: 'rsgb_lf_groupblacksheeporg'
Subject: RE: LF: Amtor FEC on LF
Importance: High
OK then, so we need a waveform that is immune to high level broadband
spikes.
Narrow filtering will remove the spike energy, but in turn will spread it
out
over the period of the filter response so won't help greatly with
arbitrarily
low bandwidth signalling. So some data repetition or convolution is called
for
to get the basic link operational before we start adding error correction by
repeats. ARQ can only make a mediocre link good; not a poor or non-existant
one into a mediocre link.
What hapenned to WOLF ? That had very heavy convolution and repetition and
would solve the problem very effectively by coding.
It ought to be easy to take out the noise pulses in DSP. If these really
are
sharp spikes, then an algorithm similar to that used for cleaning up old
vinyl
recordings (frequently set as a university third year project a couple of
decades ago) would clip the spikes before any narrow band filtering and
demodulation spread them out. Examine at the signal in the time domain
(the
raw samples from the A/D), and look for a sharp rise in energy, ie. a peak
in
sucessive samples. When a peak above a certain threshold is detected,
either
clip it, or replace by an interpolated version of adjacent ones. It will
now
help to be receiving and digitising in as wide a bandwidth as possible, so
the
spike affects relatively less sample periods.
Andy G4JNT
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