[Elecraft] K3 2.7 k Filter vs. 2.8 k

[Don Wilhelm]

Craig,

The AGC pumping will not happen if the undesired signal is within the 
spectrum of filter ultimate rejection.  Any filter worthy to be called a 
filter (even a poor one) will have an ultimate rejection of 60 dB or 
greater - the K3 filters plots show ultimate attenuation in the 78 dB or 
greater range.

As Vic pointed out in a related post, the hardware AGC only kicks in if 
the DSP will be overloaded, so as long as the undesired signal  is 
outside the filter passband (by definition, that is the area of ultimate 
rejection for the filter) the unwanted signal will be severely 
attenuated and should not activate the hardware AGC.  Yes, the K3 is 
different than the Orion.

If OTOH, you are concerned about a signal somewhere on the filter slope 
(say 20 dB down), then you may encounter an AGC pumping problem, but 
that has to do with the steepness of the filter slope and is much 
different than the ultimate rejection that you indicated you were 
concerned about.

73,
Don W3FPR

K4IA at aol.com wrote:
> In a message dated 9/18/2007 4:36:17 P.M.  Eastern Daylight Time, 
> w3fpr at embarqmail.com writes:
> If the roofing filter is  able to knock down the 
> unwanted signal to a level the DSP can handle, then  the DSP will not 
> overload and the DSP itself will provide great ultimate  rejection.  
>
> Well, sorta kinda.  If the unwanted signal gets in  and pumps the AGC, then 
> the DSP might knock down the signal but won't be able to  do anything about the 
> AGC pumping.  Then, you won't hear the signal if it  is weak.  That is the 
> whole issue of the DSP being in or out of the AGC  loop and it generated a 
> mountain of controversy with the Orion when TT moved the  DSP to within the loop.
>
> I believe I read the K3 did not follow that  mistake but a little 
> clarification from the gurus who know a whole lot more  about this issue would be helpful.
>
> Craig  "Buck"
> k4ia
> Fredericksburg, Virginia USA  
>
>
>
>
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