[TenTec] guide to adjusting Orion receiver; filters and performance

[Eric Scace K3NA]

Hi Sinisa --

   I have read through your guide.  It's very good.  One observation that you might want to check.

   According to KF6DX's description of the Orion AGC systems, once a signal has crossed the AGC threshold, it may be either (a)
amplified up towards the target AF output level; or (b) attenuated down towards the target AF output level.  In other words, the AGC
system is not limited to just reducing gain -- it may increase gain!

   According to my understanding, this is the key to the "weak signal reception" technique that W4ZV and others have described.  If:
   -- a weak signal is hovering just a little bit above the ambient QRN level, and
   -- one sets the AGC threshold to a value between the ambient QRN and the desired signal's level, and
   -- keeps the AF gain relatively low
then the AGC will add gain while the signal is present (and for a period of time determined by hang), but not add gain while the
signal is absent.  This draws the signal up out of the noise.

   Of course, the operator needs to ride the AGC threshold pretty attentively to maximize this effect.  (Hang should be minimized
and decay should be rapid.)  In a contest, this is not (usually) the proper setting.  But it is an important (and perhaps unique)
feature for weak signal DXing.

   I would be very interested if you can confirm this behavior.

==========

   Your guide also mentions the problem of 500/250 Hz roofing filters coming after an amplifier stage.  According to my
understanding, when the 500 or 250 Hz roofing filters are employed, the signal is routed first through the wide L-C filter (I think
it's 12 kHz?)... then through this amplifier that causes the problems... and then into the 500 or 250 Hz filter... and finally
through another amp to make up for the filter loss.

   I have just received my Orion recently, and haven't taken it apart to examine the PIN diode control signals to confirm this
report.  But, if it's true, then the following two potential modification may correct the problem:

   1) Change the routing of the PIN diode control signal, so that the 1 kHz filter position is selected instead of the L-C filter
position in the first stage.  This could be done in firmware.

   2) Replace the L-C filter stage with a 600 Hz bandwidth 4-pole filter centered on 9000.75 kHz.  One may surrender the ability to
transmit wideband modes such as AM.  For many of us, that is no sacrifice.

   Again I would be interested in your thoughts on this.

73,
   -- Eric K3NA