566 manual
Version 4 – October 2006
Part #74410
Printed in USA
45
compromising receiver performance.
Listening to a weak signal on a quiet band
does not meet the definition of need for a
tight roofing filter. On a band where few
signals are present other than a desired
target weak signal, only a wide roofing filter
would be necessary. Using the 300 Hz
roofing filter when listening to a single weak
signal on the low bands gains nothing
additional. In fact, it could hurt your ability to
hear a noise floor level weak signal. Why?
Among other reasons, all narrow bandwidth
filters suffer from insertion loss – and in the
Orion, like other transceivers have for many
years, we use an amplifier to compensate
for crystal filter loss. A 300-Hz filter is going
to have slightly more loss than a 600-Hz
filter. Both of these narrow filters have
more loss than the 1 kHz roofing filter. The
loss, after preamp compensation, is about 2
dB.
No loud signals nearby = no need for
a tight roofing filter
. Regardless of
DSP
bandpass filter setting
, we’d recommend
using a wider roofing filter as acceptable for
weak signal DXing. The DSP bandpass
filtering does not vary in gain down to the
minimum setting of 100 Hz. Therefore, if
there is no offending signal within 1 kHz,
setting the roofing filter at 1 kHz and
narrowing the DSP bandwidth to any desired
value is by far the best setup for ORION II
for single signal DXing.
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 II
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.
We suggest using AGC
hang at either 0.00 or a very small value like
0.04. 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! It has been argued by
operators of the original ORION that a small
amount of hang may be desirable to keep
the AGC action ‘smoother’ than it would be
without hang. This is plausible – but again
depends on the individual and the situation.
The
threshold
value in the AGC system
acts like an IF gain control for the DSP
section of the receiver chain – turn the value
Содержание ORION II
Страница 58: ...566 manual Version 3 March 2006 Part 74410 Printed in USA 58 Figure 5 2 Subassembly Cabling...
Страница 59: ...566 manual Version 3 March 2006 Part 74410 Printed in USA 59 Figure 5 3 Plan View of ORION Chassis...
Страница 60: ...566 manual Version 3 March 2006 Part 74410 Printed in USA 60 Figure 5 4 Main Receiver Signal Path...
Страница 61: ...566 manual Version 3 March 2006 Part 74410 Printed in USA 61 Figure 5 5 Subreceiver Signal Path...
Страница 62: ...566 manual Version 3 March 2006 Part 74410 Printed in USA 62 Figure 5 6 Transmit Signal Path...
