Dear Andy, LF Group,
At 08:52 10/07/2002 +0100, you wrote:
137k, on the other hand is characterised by a much more constant noise
background and does not behave like HF divided by ten - if fading is
present,
it covers a much longer period of tens of minutes or hours
This is true under quiet winter-time conditions, but these are in the
minority - the rest of the time, a large proportion of the total noise
power is in the form of QRN spikes. If you look at an RX IF output on a
scope under noisy band conditions, you see a fairly low background level
with much larger spikes - the noisier the conditions, the greater the rate
of spikes occuring. The spikes are 10s of dB larger than the background
level, and usually overload the receiver for the duration of the spike - if
you manually reduce the IF gain, the background noise gets smaller, but the
peak amplitude of the spikes on the scope stays much the same. When you use
PSK31under weak signal conditions, the effect is that each time there is a
spike, a character is corrupted, which matters little if there is only 1
spike every few seconds, but when the noise is clattering away like it is
at the moment, the signal will probably be unreadable even if it is well
above the background noise level. So while PSK31 is good under quiet LF
conditions (and "PSK08" better still), I don't think it is the optimum mode
for typically noisy LF conditions.
Some sort of error correction would seem to be a good idea, since the data
bits between the noise spikes will be uncorrupted, so a fairly large
proportion of the data will be received OK - I have not tried the QPSK
variant of PSK31 due to lack of suitable TX hardware, But I recall VE2IQ's
"Coherent", which does include error correction, worked well under noisy LF
conditions. This is actually very flexible software - the only real
drawback of this is that it requires a computer running DOS, and some
external hardware. Oh, and as with other PSK modes it requires TX envelope
shaping - but there is the "variable phase" modulation technique which
could help there.
Since the spikes are of short duration, another approach might be to use a
hardware or software noise blanker in conjunction with a low baud rate so
that each bit was much longer than the noise impulse. This would require an
RX IF bandwidth much larger than the bandwidth of the signal for the
blanker to work effectively, but still narrow enough to eliminate adjacent
channel QRM like DCF39 which would cause dynamic range problems - but with
8 or 10 bauds this should not be a problem.
Cheers, Jim Moritz
73 de M0BMU
|
