[Elecraft] APF and audio limiting

[Don Wilhelm]

  Mike,

The AGC works at the IF level, not the audio level.
What that means is if you crank the AF gain way up to listen to a very 
weak signal, AGC action is probably not even started because the signal 
level is at or near the level of the band noise - In other words, the 
receiver is operation at "full gain, all stops removed".

Under those conditions, if a very strong signal suddenly comes into your 
passband, yes, it will be loud simply because the AF Gain is advanced.  
Yes, the AGC will respond to that new signal - just how much depends on 
how you have the AGC menu parameters set.  If it does cause an AGC 
response, it will reduce the gain of the receiver, and you will no 
longer be able to copy the signal at or near the band noise floor.

So perhaps what I am saying is that everyone might run their AGC 
parameters so that all stations, no matter what their relative strengths 
will have the same audio level - that would cure the condition you have 
stated - but I do not really believe that is a good solution - for one, 
it will make band noise the same audio level as the signals - result is 
"noisy K3", and we have been there before.

Myself, I prefer to be able to determine the relative strength of 
various signals "by ear" rather than "by S-meter" - that means the 
strong signals will have greater audio levels than weaker stations - 
that is just the way it is.

Since very weak signals (which is what APF is all about) are well below 
the level that AGC action starts, when trying to dig out such a weak 
station, it may be wise to turn off AGC altogether (provided you have 
set the AF limiter beforehand).  With AGC off, the Audio limiter is 
active to protect you from ear-shattering events such as a strong signal 
suddenly coming on frequency.

An additional benefit to copying very weak signals with AGC off is that 
a strong signal coming on within the passband will not de-sense the 
receiver (as it would with AGC on), so you may be able to use the 
"filter between the ears" to continue copying the weak signal.

73,
Don W3FPR

On 11/6/2010 7:46 PM, Mike wrote:
> Thanks Tom.
>
> Well, I kinda thought it shouldn't blast my ears that hard. Audio *WASN'T*  up too
> loud for the signal I was after. Maybe I don't understand what AGC does. That's why I
> asked the questions I did.
>
> On 11/6/2010 6:21 PM, Amateur Radio Operator N5GE wrote:
>> On Sat, 06 Nov 2010 18:04:18 -0400, Mike<nf4l at nf4l.com>    wrote:
>>
>> Sounds normal to me.
>>
>> Audio up too loud and APF on, then a strong signal transmits on the frequency
>> you are listening to....
>>
>> What do you think it SHOULD have done?  You didn't say.
>>
>> Tom
>> Radio Amateur N5GE
>>
>>> On 11/4/2010 9:48 PM, Mike wrote:
>>>> I'm playing with the new APF, and trying to get a REALLY weak QSO to pop. I have the
>>>> volume cranked up pretty good, and a REALLY strong station throws in their call. I
>>>> thought my speakers had blown. Glad I wasn't wearing cans.
>>>>
>>>> My CONFIG:AF is 18, AGC = F.
>>>>
>>>> Was I doing something wrong? Anybody else experience this? Do we need/can we have
>>>> better audio limiting?
>>>>
>>>> 73, Mike NF4L
>
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