Hi to each interested in Orion performance parameters.
Wrote to Doug Smith a few days ago regarding the AGC
and how it works. He has sent two responses to a group
of us; seemed appropriate to share same with all interested:
On Nov. 18th:
" Orion's digital AGC is a peak detecting
system. The threshold adjustment is one
in which gain is altered according to the
level set. You can alternately look at it as
an IF gain adjustment. [KH7M note: but
it works inversed to such an IF gain control;
"raising" the threshold, lowers the gain]
In that alternate view, the threshold
remains constant and the gain varies.
Note that the SNR doesn't change: It is
only the output level that changes. Also
note that the AGC speed has some effect.
If you set AGC to fast and threshold low,
noise would be accentuated because the
AGC would follow the envelope of the
noise.
We recommend that you set the AGC
threshold high and the attack time to medium
or slow for weak signal work. We have
worked extensively with ON4UN and others
on this. The Ten-Tec Web site contains
elaboration on the issue. Check it out.
Your feedback is welcome. Please
e-mail me with your comments.
73,
Doug Smith, KF6DX"
and, on the 21st., in response to my wondering about how
to set the AGC Attack time as he suggests above:
"The attack time for the Orion
digital AGC is a few milliseconds.
In the finish, we decided not to let
users adjust it because it could
cause troubles if made too long."
I wonder, however, if the Attack times are not different
for the Fast, Medium, and Slow selections; seems to me
they ought to be, but Doug "seems" to imply that the
Attack times for all four AGC selection options are the
same. Note that in my RX-340 Ten Tec rcvr, the Attack
time can be changed at the Prog setting to vary from
0.01 mS/dB to a max value of 1.00 mS/dB. Not sure
how this correlates with Doug's remark above that
"The attack time for the Orion digital AGC is a few
milliseconds.", and they decided to NOT make it
operator adjustable......? Rather than a "few milliseconds"
and worrying about users setting "too long", why not
the range as provided for with the RX-340? Anyway,
as it is digital, it is in the firmware, and "could" be
made available via software update, IF advisable,
I guess, hi. That would just introduce another parameter
for us to wiggle and worry about, however!
With the Orion AGC attack time fixed at "a few milliseconds"
per Doug's note, it is certainly a slow attack, at least when
compared to that offered with the RX-340 Prog AGC setting.
And this would satisfy Doug's suggestion that the Attack
be slower and that the Threshold should be "high" for
weak signal reception optimization.
Doug continued the 21st note with a reference to Scotts
explanation about Orion and weak signal reception posted
at the TT web site, this one:
http://www.tentec.com/565optimize.htm
However, as Doug suggests above in the note of the 18th,
and as many of us have learned with use, setting the
Threshold to a much higher level than suggested in the
TT "optimize.htm" piece often is better. And, it seems to
me that often, contrary to Doug's note, as I raise the
Threshold level the S/N ratio does "seem" to improve;
at least for me the signal stands away from the noise
better than with the threshold lower and the noise louder.
Still learning daily about this Orion rig!! And I HOPE I
am interpreting Doug's notes correctly, hi.
73, Jim KH7M
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