566 manual
Version 4 – October 2006
Part #74410
Printed in USA
46
low (like to .37 uV) and the receiver gain
comes up, as do the signal levels.
The threshold value can also be used to
determine (limit) the top end of available
receiver gain for the digital part of the
receive chain. Setting the threshold value
higher in turn imposes that higher limit on
the available range of the RF GAIN control.
Example: Listen to band noise with the
threshold set to the lowest value (0.37 uv)
and the RF GAIN set at 100. With these
settings, receiver gain is at maximum.
Adjust threshold value to a higher value, like
31.91 uv. Note how overall receiver noise
has dropped with the gain reduction. The
top end of the available range of the RF
GAIN control (which is still set at the highest
value) is now limited by the threshold value
rather than the entire actual available range
of the receive chain. 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 II 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 1 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 or 0.04. 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
signal) – ideally, you want to adjust this to
just before the clipping point for maximum
copy of the weak signal.
So, all that being said, how about a “set-it-
once-and-leave-it” value for programmable
AGC for weak signal DXing?
Set AGC hang at 0.00, decay at 40-60 dB/s,
and the threshold at .5 uV. This ought to
provide a good enough combination of
settings to allow just about anything that can
be audible to be detected by the radio (and
likely copied as well) and allow for plenty of
gain. If you need just a little more, you can
go in and change the AGC parameters for a
given situation as described above.
While you can have an optimal starting point
for programmable AGC – every signal and
every noise situation is unique! This is
precisely what makes this system so
advantageous over traditional settings of
“fast” and “off”.
Updated February 2005 in the original
ORION manual and reproduced here for
ORION II owners’ benefit:
Anecdotal
evidence from weak signal DXers using the
original ORION transceiver has suggested
that for the very weakest of signals, some
AGC hang may be desirable to hold the
AGC level constant in the presence of
varying noise. As every noise situation and
signal condition is unique, it is possible for
some operations that the weak signal (rather
than the most recent noise peak) can hold
the AGC constant and allow the very
weakest of signals to be copied. This will
vary with the conditions, and with the
operator themselves for desirability.
A practical example from a leading low band
DXer for listening to very weak signals in
band noise (no lightning or unusual
atmospheric QRN) on 160 meters is to
adjust the RF GAIN control upward until
normal band noise is present. Set decay at
a very low value (e.g. 5 dB/s) and adjust
hang between 0.3 and 1.0 for best signal-to-
noise ratio. Lightning QRN would be a
different scenario – perhaps using fast AGC
