[Orion] Orion's AGC THRESHOLD is in error by 21 (9) dB

[Sinisa Hristov]

Orion's AGC Threshold control is useful for
avoiding AGC action on noise peaks.
Such an action tends to intermodulate a weak signal with noise,
significantly reducing readability.

The same benefit was available to receiver users for decades.
One simply turns RF GAIN counterclockwise
until the S meter stops responding to noise peaks.
Contrary to popular belief, no sensitivity is lost
by reducing RF GAIN control this way, because the actual
gain would have been reduced by AGC anyway.

I've already written that Orion's AGC Threshold control
is less adequate for the purpose than an ordinary RF GAIN
(not Orion's) because of:
  * lack of correlation between AGC THRESHOLD values and S meter readings;
  * lack of dedicated knob.

Both disadvantages can be overcome,
but not without substantial price in time and effort,
which is important in contests.

Particularly cumbersome is to know "how much of it" to apply
(vs. RF GAIN method where one simply looks for a non-moving S meter).

Menu readings are given in microvolts. The manual is not explicit
where are these microvolts measured. One is tempted to assume
that they are measured at antenna connector and can therefore
be correlated with S meter indication on the basis of S9 = 50 uV
and +1 S unit = doubling the voltage.

However, such an assumption is wrong.
The measured AGC threshold occurs at signal levels
~21 dB below set values with preamp on,
and ~9 dB below set values with preamp off.

Correlation with S meter indication was found to be as follows:

  AGC THRESHOLD   S meter
    191.48          8.8
     95.74          7.9
     47.87          6.8
     23.94          5.6
     11.97          4.3
      5.98          2.9

The table above is valid independent of preamp and attenuator settings.

The moral from this is twofold:

  * adjust preamp, attenuator and RF GAIN controls (in this order)
    to avoid noise peaks going above approx. S 8
    (don't be afraid, no sensitivity will be lost);

  * adjust AGC THRESHOLD slightly above S meter noise peak readings,
    according to the above table.

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.


73,

Sinisa  YT1NT, VA3TTN