Re: [Orion] Question about noise floor

[Bill Tippett <btippett@alum.mit.edu>]

Hi Lee!

At 12:54 PM 11/11/03, you wrote:

> OK, yes the test works exactly as you said it would. I have been 
> experimenting with the AGC threshold values. I understand completely whats 
> happening here.
>
> AND that brings me to a question , or in otherwords educate me here a little.

        I'm definitely out of my league on explaining exactly how Orion's
AGC works but you can read more by Doug Smith KF6DX here:

http://www.doug-smith.net/digitalagc.htm

Actually I have several questions for TT myself about some AGC issues,
but I haven't gotten around to asking them yet.  Anyway, read the above
first and I'll do my best with your questions (W4PA correct me if I'm wrong).
Sometimes I learn something myself when trying to explain something I don't
completely understand!  ;-)

> 1) OK, to do weak sig DX work the book says to use .5 uV as a starting  point,
> OK, if I use a high value of threshold, does that not desensitise the rcvr 
> front end??
> Hence not god for weak dx work.

        No, it does not desense the front-end, it only affects the point at
which Digital AGC starts compressing the signal.  If the signal is
buried in noise, you should still hear it even if the AGC Threshold is set
above that level.  On 160 meters, where QRN is extremely high, I've found
I can copy weak signals best with VERY high settings of Threshold, even
the max of 191 uV sometimes!

> 2) OK, I have a Collins, Drake (FULL SHERWOOD) and an Icom 746 PRO.
> So, ( you may not have an opinion on the boat anchors functionality, if so 
> its OK)
>
> HOW DO THEY WORK?, obviously they have slow, med, fast AGC, with no other 
> avaiable parm adjustment.
>
> The 746 PRO has a relatively quiet noise level without NR (much quieter 
> than ORION with the .5uV setting) and hears dx well. So where would the 
> threshold be set at by the factory, or does it float based on conditions??

        If I'm not mistaken, I believe all the radios you mentioned only use
Analog AGC.  Orion has both Analog and Digital, but the Digital is the only
one you can adjust.  If you read KF6DX's article, you learned that Digitial
AGC works only within the 3rd IF DSP.  The Analog AGC is at the very front
end (1st IF) and its sole purpose is to limit over-driving the DSP.  DSP's
distort bandly if they are over-driven, and current DSP's do not have enough
dynamic range (~105 dB in Orion I believe) to handle the full 140 dB range
of the radio.  So if a signal exceeds approximately S9+30dB, the Analog
AGC activates to protect the DSP from being over-driven and creating
distortion products.  In other words, Orion's Analog AGC only comes into
play for signals within the roofing filter from S9+30 to S9+60, which is not
very often!

        As I said before, I seldom adjust Threshold below 3 uV, not the 0.5
recommended in W4PA's optimization article.  Note the following comment
in KF6DX's AGC article:

"A low threshold (0.35 uV) means all signals are boosted to a constant peak 
output level; a high threshold (191 uV) means signals must reach about 12 
dB over S-9 before compression occurs."

If I understand this correctly, setting a high Threshold is roughly 
equivalent to
turning AGC off for lower level signals, and activating it only for very 
strong signals.
On traditional radios, this is roughly equivalent to what we do in very 
high QRN,
i.e. turn AGC completely off, back off on RF Gain and let our ears do the AGC.
You will still hear weak signals, but the AGC does not come into play except
to limit very high static bursts.

        For weak signals on an atmospherically quiet band (like 10 meters),
you will still hear signals at the noise floor even if Threshold is set to 
3 uV.
If I recall, 2-3 uV is where I had mine set for the entire CQ WW SSB and I
think I heard most weak callers extremely well.  AGC boosts both weak
signals and noise equally, so setting the Threshold to the minimum really
does nothing to help Signal-to-Noise ratio which is what really counts.

        I'm not exactly sure where and how AGC levels are set in the radios
you mentioned, but I suspect they are somewhere above the 0.37 uV that
Orion allows.  Allowing us to adjust the Threshold can be both an advantage
and disadvantage depending on whether we learn how to do it properly.  My
advice would be to use the default setting of 3.03 uV that is in firmware rev
1.363 and up, and then vary it up or down for best S/N according to conditions.

> 3) To do weak sig. DX, for max sensitivity, do I have to use the low 
> threshold value? AND put up with the noise?  Remember I am talking about 
> using no NR. The NR works well to get the noise,,, but it makes the sig. 
> more bassy and comes in and out, like it is breathing???

        I am not the one to ask about NR, because I seldom use it, and I
do a LOT of weak signal work on both 10 and 160.  Why?  Because all
NR does (for CW at least) is to create a narrow DSP filter.  Since I am
likely already using both narrow roofing and DSP filters on CW, NR has
little if any effect on improving S/N for me.  It might help on SSB but
I never found the need for it in the CQ WW SSB, which is the grand
total of my experience using Orion on SSB!  I will add that I often use the
Hardware Noise Blanker which works quite well on both 10 and 160.

        Hopefully this answered some of your questions, and I certainly
hope W4PA or others will correct anything I got wrong.  There is really
no substitute for trying these things yourself and learning what works
best for you.  I've had my Orion for ~2.5 months and am STILL learning.

                                        73,  Bill  W4ZV 

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