NM7X wrote:
> With today's technology, it's not necessary to use this
> indirect method.
> Let's bury this dinosaur once and for all.
NQ5T replied:
>Not so fast.
>The point of the S-Meter movement upward, as I recall the endless
discussions of all of this regarding the Orion, is to give you a notion of
the MINIMUM signal that will impact AGC, via the THRESHOLD setting. If you
leave the S-meter reading unaffected, what does it tell you ... Not much.
Exactly right. I still get questions from new users
to the effect of "Why is my Orion so noisy?" The simple answer
is usually that they have AGC Threshold set too low. Setting the
S-meter indication just above noise peaks is a very simple way to
do this in V2. Sinisa YT1NT first suggested this in March 2004:
http://www.geocities.com/va3ttn/UsingOrionRX.pdf
*****************************************************
"If the test is done by external instrumentation or by ear, AGC
THRESHOLD must be increased slightly above the
point where the external noise volume starts to decrease. This
ensures that AGC will stay inactive, and what is
heard (and measured) will reflects actual power ratios. The exact
point is not critical and may be much higher than
the minimum one. Of course, the selected setting must not be changed
during the test.
On "usual" receivers, RF GAIN control can be used to superimpose DC
control voltage (and move S meter)
making AGC loop inoperative for signals that do not move S meter
anymore. This is a very effective
weak signal technique. There is an Inrad flyer recommending its use
on FT1000MP. One simply turns
RF GAIN counter-clockwise until the S meter stops responding to noise peaks.
Contrary to popular belief, no sensitivity is lost by reducing RF
GAIN control in this way,
because the actual gain would have been reduced by AGC anyway.
Orion's RF GAIN does not control the AGC threshold directly. Orion
users have to activate menus
and then select and change the clumsy AGC THRESHOLD setting (which
does not move the S meter),
instead of simply turning a knob counter-clockwise as on "less
modern" receivers. Therefore, speaking
ergonomically, Orion's AGC THRESHOLD control is less adequate for the
purpose due to:
* lack of correlation between AGC THRESHOLD values and S meter noise readings;
* lack of dedicated knob.
TenTec would have done a much better job by displaying AGC THRESHOLD
values in S meter units,
with some care taken to ensure good tracking between them, which I
guess is easy, as both S meter
and AGC are DSP functions.
**********************************************
The above was also written before AGC Threshold (digital IF gain)
and RF Gain (analog IF gain) were integrated into the single RF Gain
knob (implemented in 1.372). After this change, plus the V2 linkage to
S-meter position, it's a very simple job to correctly set Orion for any band
situation. Now unnecessary to go into Menu to change AGC Threshold
or to go through the extensive procedure Sinisa used in his original paper.
Simply vary RF Gain knob until the S-meter rests slightly above noise peaks.
I feel this is another example of Ten-Tec listening and trying to make Orion
friendlier to users.
73, Bill W4ZV
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