[Elecraft] An AGC story

[K4IA at aol.com]

As I understand the graphs and  operation:

AGC Threshold is the point at which AGC action  starts.  The higher the AGC 
THR number, the higher point at which AGC  starts.  The highest AGC THR (8) 
starts AGC action at S3.   That  is a pretty low level but any setting less 
than THR 8 starts the AGC action with  even less signal. 

AGC SLP is how aggressive the AGC action is as  signals rise.  A highest 
AGC SLP number means the AGC flattens  everyone.  AT SLP = 15 all signals 
above S3 are brought to roughly the same  level.  Heard one at a time, two 
signals - one at S3 and one at S9 will  sound the same level.

But, since AGC works on every signal in the  passband, if there are two 
signals - one S9 and the other S3 appearing at the  same time, the S3 signal 
will disappear because the AGC is knocking down the S9  signal to an S3 level 
and thereby also reducing the S3 signal to the  noise.  Using a less 
aggressive (lower) slope number will help but the  dynamic range between and S3 and 
S9 signal is compressed.  The core problem  is the AGC starts at such a low 
level.  Background noise alone on 80 - 40  is almost always more than S3. 

The suggestion is an AGC THR  setting to allow higher than an S3 level 
before AGC action starts.  That  makes sense but does it start to degrade the 
high-end dynamic range of the  radio?  Are we in danger of overloading the 
intermediate amplifiers/mixers  generating IMD and other unwanted issues?  
Seems like there is a balancing  act here.

Another way to accomplish this is to use the Attenuator  and/or reduce the 
RF gain to reset our starting point so that the S5 signal at  the antenna 
becomes an S3 signal at the AGC level and our AGC action doesn't  really begin 
until S5.  I would like the ability to switch in even more  attenuation for 
high noise conditions.  Can that be done through the  software?

Still I would like to try a higher AGC THR setting and see how  it plays.  
The beauty of this radio is its ability to adjust to different  conditions - 
one size does not fit all.  Unfortunately, its also  complicated.  It has 
taken me three years to figure this little bit  out.  

Comments?  Do I understand it  right?

Buck
k4ia
K3  #101








In  a message dated 12/9/2010 10:11:19 A.M. Eastern Standard Time,  
barry.n1eu at gmail.com writes:
Thanks Don.  I just realized that I can do  some quick experiments to get my
answer using an XG2's s3/s9 capability and  noting audio level increase with
AGC Off and then with AGC On and reducing RF  Gain until same audio increase
is accomplished.

An aside to KE7X and  W4TV:  I too wish we could try experimental firmware
that significantly  raised the max AGC THR value.

And to K6LL: Yes, I too fear loss of weaker  signals

73, Barry N1EU

On Thu, Dec 9, 2010 at 3:00 PM, Don Wilhelm  <w3fpr at embarqmail.com> wrote:

>  Barry,
>
>  Without doing any measurements it may be difficult to say precisely, and 
 I
> would think there would be a bit of difference from one K3 to  another.
>
> However, there is a quick and easy way to accomplish  that - note that the
> S-meter increases as the RF gain is reduced.   So if you want to increase 
the
> AGC Threshold by (say) 3 S-units,  disconnect the antenna, and reduce the 
RF
> Gain until you move the  no-signal S-meter up by 3 S-units.  That should 
be
> close to your  goal.
>
> 73,
> Don W3FPR
>
> On 12/9/2010 9:07  AM, Barry N1EU wrote:
>
>>
>> Cady, Fred  wrote:
>>
>>> I made some measurements and have posted the  results for your perusal
>>> and information at  www.ke7x.com.
>>>
>> Very interested findings -  Fred  has a graph of AGC action with AGC 
THR=8
>> labeled figure 3 on  http://www.ke7x.com/home/the-k3-agc-story  :
>>
>> So I  ask this group: is it possible to raise the AGC Threshold from s3 
to
>>  s6
>> or s9 by simply reducing the RF Gain control?  Can anybody  estimate what
>> settings of the RF Gain control would accomplish  that?
>>
>> Barry  N1EU
>>
>>
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