AGC

[w7aqk]

Hi All,

Band conditions of late have made things pretty difficult.  Lots of QSB, and
I'm hearing a lot more noise than usual.  Of late, I've had problems getting
100% copy on some stations because the QSB, even when fast and not too deep,
has clipped characters I try to copy (on CW) making it easy to miss
something here and there.  I've been checking my AGC to make sure it was
fast rather than slow, but that didn't always solve the problem
satisfactorily.  Now, I do often just turn the AGC off, but that's usually
to avoid the pumping that the AGC can cause.  I've always been an advocate
of turning the AGC off at times, but I noticed something else recently.
Probably I've noticed it before and just didn't think about it.

The recent combination of QSB and higher noise were BOTH much improved by
just turning the AGC off.  The noise went down considerably!!!  That
improved everything, and copy became 100%, or close to it!  I don't know why
I didn't think to do this sooner!  Anyway, I am guessing that the AGC is
enhancing the noise, and it has nothing to do with the K3 itself as to any
oddity.  Somebody can probably explain this anomaly better to me.  I always
have to be very careful when turning the AGC off, especially if I am wearing
"cans", but it sure does make a big difference on weaker signals.

Anyway, it was sort of a "slap your forehead" moment here!  Hi.  Time for me
to read, or re-read, how to properly take advantage of AGC!

Dave W7AQK


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[w7aqk]

Hi again,

I got several very nice responses to my question already, and it seems I
need to do some adjusting.  My AGC threshold was set at 5, which apparently
is too low.  I've also increased by AGC slope setting as well.  I'll have to
check all this out with some weaker signals to see just how much difference
that might have made.  Apparently I need to pay a lot more attention to
these settings, and may have to adjust them more often depending on
conditions, etc.  It was also suggested I set these adjustments up in a
memory position, which makes a great deal of sense.  The K3's AGC seems to
work very well, and I just need to take advantage of the fact that it can be
adjusted a lot to accommodate different conditions.

Thanks for the advice folks, and I welcome any other suggestions as well.
With my modest antennas I have to dig signals out of the noise quite often,
so I obviously need to take better advantage of what control I do have!!!
Hi.

Dave W7AQK

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[Joe Subich, W4TV-4]

On 2015-03-19 12:59 PM, dyarnes wrote:
 > Anyway, it was sort of a "slap your forehead" moment here!  Hi.  Time
 > for me to read, or re-read, how to properly take advantage of AGC!

First thing to to do is get out your XG-2 or XG-3 and run the RF gain
calibration on the K3.   If the gain cal is off, the AGC may activate
well before the signal has a chance to quiet the noise.  Sherwood had
that experience when he tested a K3 with the new synthesizers that had
not had the RF Gain Calibration performed before it left the Elecraft
factory.

73,

    ... Joe, W4TV




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[Don Wilhelm-4]

Dave,

Yes, AGC settings can 'enhance' the receiver noise if set incorrectly.
When testing AGC setting, listen not for the signals themselves, but to
the pauses in the signals.
For more guidance on choosing the right AGC settings for your condition,
check out the "Noisy K3" article on my website www.w3fpr.com.

73,
Don W3FPR

On 3/19/2015 12:59 PM, dyarnes wrote:

> The recent combination of QSB and higher noise were BOTH much improved
> by just turning the AGC off.  The noise went down considerably!!!  
> That improved everything, and copy became 100%, or close to it!  I
> don't know why I didn't think to do this sooner!  Anyway, I am
> guessing that the AGC is enhancing the noise, and it has nothing to do
> with the K3 itself as to any oddity.  Somebody can probably explain
> this anomaly better to me. I always have to be very careful when
> turning the AGC off, especially if I am wearing "cans", but it sure
> does make a big difference on weaker signals.
>

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Dave,

That's good advice. I usually keep my AGC THR set to about 18. However,
the higher the threshold, the closer the dynamic range approaches that of
no AGC, but without the danger of overload or hitting the limiter that
will protect your hearing. If that isn't to your liking try dropping it
down to the mid to lower teens.

Also -- don't overlook something as easy as simply reducing your RF Gain
control. As you first begin to reduce it, the results are virtually
identical to what you'll get by raising the ACG THR level -- and it's
right there on the front panel. You may find that's all you need.

73, Dale
WA8SRA

> Hi again,
>
> I got several very nice responses to my question already, and it seems I
> need to do some adjusting.  My AGC threshold was set at 5, which
> apparently
> is too low.  I've also increased by AGC slope setting as well.  I'll have
> to
> check all this out with some weaker signals to see just how much
> difference
> that might have made.  Apparently I need to pay a lot more attention to
> these settings, and may have to adjust them more often depending on
> conditions, etc.  It was also suggested I set these adjustments up in a
> memory position, which makes a great deal of sense.  The K3's AGC seems to
> work very well, and I just need to take advantage of the fact that it can
> be
> adjusted a lot to accommodate different conditions.
>
> Thanks for the advice folks, and I welcome any other suggestions as well.
> With my modest antennas I have to dig signals out of the noise quite
> often,
> so I obviously need to take better advantage of what control I do have!!!
> Hi.
>
> Dave W7AQK

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[Cady, Fred]

Hi Dave,
Here is an easy test to see what THR level you should use.
First, set the SLP to 15.  That will give the most AGC action -- all signal strengths will sound about the same.  You might want to do this if you are listening to a round table of differing strength signals.  I keep my SLP = 0, to allow me to distinguish between strong and weak signal in a contest situation.
Next, choose a noisy band or the band that you want to set the AGC THR.  It very well could be different on different noisy bands.
Now, while listening to the noise, decrease the THR until you hear a decrease in noise.  You could also use the AFV and dBV measurements but that is a bit trickier.
If it were me, I'd set the THR a little higher than where it starts to be activated by the noise.  Actually, I like to run THR pretty high because I like to operate the receiver in a linear region to distinguish different strength signals.  Sometimes, though, I run the THR down a bit when there are a bunch of weak signals, like EUs calling, in addition to a bunch of strong US stations.  Then having the AGC clip the stronger signals lets me run the AF or RF gain up to be able to hear the weaker EUs without blasting my ears with the stronger stations.

The bottom line is that there is tremendous flexibility in setting up the AGC and one size doesn't fit all.
Cheers and 73,
Fred
KE7X

Author of:
"The Elecraft K3: Design, Configuration and Operation 2nd ed"
"The Elecraft KX3 - Going for the summit"
"The Elecraft KPA500 and KAT500 - the K-Line Dream Station"
Printed, coil bound copies of these are all available at www.elecraft.com
KPA500 and KAT500 Quick Set Up Guide
http://www.ke7x.com/home/k-line-introduction-and-set-up-guide
KAT500 and KXPA500 Tuner Operation
http://www.ke7x.com/home/kat500-and-kxpa100-tuners
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