565 manual
September 2003
Part #74279
Printed in USA
45
Of course, if you have a pileup 1 kHz away,
perhaps a tighter roofing filter is warranted
to keep those loud callers from
compromising close-in performance.
That’s fine – but make sure that the
crystal
roofing filters are centered.
DSP noise reduction in the Orion interacts
somewhat with the digital AGC system.
Here is why: when turning on the noise
reduction, without changing digital AGC, the
receiver will get very quiet, very quickly.
This is not the desired effect
of noise
reduction! The desired effect is to identify
what is signal and what is noise and improve
the ratio between the two, rather than
making the whole receiver quieter, signals
and noise. When DSP NR is selected,
digital AGC artificially reduces the threshold
setting of the digital AGC – and you will
notice that the overall noise level can
increase –
but
signal-to-noise ratio
improves
and that is the ultimate goal of the
NR system.
In practical terms, how does it work? There
are 9 different settings, and each of the 9 is
used to determine only
how aggressively
(quickly) the NR adapts and identifies
what is signal and what is noise.
Here is
the rub (and it’s logical): with weak signals, it
is harder for any DSP NR system to
determine what is noise and what is signal.
When turning DSP NR on with a setting of
“1” with a signal that is very weak, it’s going
to take a
very long time
for the algorithm to
figure out what is signal and what is noise.
For a somewhat louder signal (20 dB or
more above the ambient noise level, still
fairly weak) – a setting of “1” will adapt very
fast! What is needed for weak signals is
more aggression
. By starting the NR at “3”
or “4”, the NR will more quickly adapt to
what is signal and what is noise for a weak
signal.
Once the NR has adapted, for the
same signal, no change in NR will be
made when adjusting the value!
If you
turn the DSP NR on, and just cycle through
the values looking for “best” – nothing will
happen. Because the DSP NR builds a
bandpass filter to automatically reduce
noise, it produces the same effect as
manually selecting a very narrow DSP
passband filter.
Perhaps nothing is more important than
AGC setting, and
for single signal weak
signal DXing purposes the
programmable AGC value is probably the
only setting that the operator should
consider using.
Put the AGC hang at 0.00
– turn it off. Why? Because with AGC
hang, AGC will grab onto the most recent
noise peak and will adjust gain to it for the
hang period. After that, decay will begin –
this is
not good
if you’re trying to listen to a
signal at the noise floor and band noise is
varying on top of the signal! The
threshold
value in the AGC system acts like an IF gain
control for the receiver chain – turn the value
low (like to .37 uV) and the receiver gain
comes up, as do the signal levels. The
crucial part is adjusting
decay
and
threshold
.
Decay
is used to determine how
fast the IF gain increases in the absence of
a signal above the
threshold
value. For a
conventional setting like “fast”,
either on
Orion or any other transceiver
, the AGC
can actually clip both a weak signal and the
noise! Want an example? Tune to a point
on the band where there is no signal, only
band noise. Set AGC hang at 0.00,
threshold at .37 uV, and then start decay
rate at 5 dB/s. Turn it up to 2000 dB/s
(fastest setting). Hear the background noise
change? That’s AGC clipping the noise at
the fast decay setting.
If there’s a weak signal in there at that level
– you guessed it, the signal gets clipped too.
How to use the programmable AGC for
optimum performance for weak signal
DXing: There are two possibilities. For
each, set hang value at 0.00. The first
example we’ll call “quick decay, variable
gain” Set the
decay
to 60 dB/s (which is still
fairly fast) and the
threshold
at .37 uV while
listening to a weak signal. As you increase
the
threshold
to higher values, it is possible
that the weak signal will come out of the
noise as the system gain decreases and the
AGC no longer clips the weak signal and the
noise! The other method is “low threshold,
varied decay” to set
threshold
at a low or
the lowest value for maximum IF gain, and
then use the
decay
control to adjust, starting
from the slowest setting of 5 dB/s and
working upward. As decay goes faster, it
introduces clipping. When listening to a
weak signal with
threshold
low, advance
the
decay
until the point clipping starts
(audible change in the noise component
and/or loss of copy of chopping of the weak
