[Elecraft] K3 2.7 k Filter vs. 2.8 k

[ab7r at cablespeed.com]

The following cut is from the K3 FAQ - Elecraft on Roofing Filters.

"In order to achieve a K3 blocking dynamic range (desense) in the 140 dB+ range, 
you -must- use a narrow crystal filter (400 Hz for closer interfering signal 
spacing) in front of the DSP. We use hardware AGC after the narrow crystal filter 
and ahead of the DSP to protect the DSP when signals inside the crystal filter 
exceed a 100 dB dynamic range. If you only use the 2.7 kHz stock filter for CW or 
data operation you will be significantly desensed once signals within that filter's 
bandwidth exceed about S9+25. This is before phase noise from the transmitting 
station becomes a factor. Not uncommon on 40M at night, during a contest or at a 
multi-op station -- Or every day in major cities.  Changing to a 400-500 Hz filter 
reduces blocking from signals 1-5 kHz away. I've personally confirmed this on the 
air with my K3 and the other commercial rigs we have here. when I've operated with 
the K3, or another DSP rig, on CW without using a narrow 400-500 Hz filter ahead of 
the DSP filtering, I frequently experienced desense (BDR) from nearby signals. 
Putting in the narrower crystal filter immediately cleaned it up. Using narrow 
crystal filters ahead of the DSP also reduces AGC pumping from static crashes on 
80/160M etc.

This was from Eric's portion of the article.


-------------------------
73,
Greg - AB7R
Whidbey Island WA
NA-065

On Tue Sep 18 17:42 , K4IA at aol.com sent:

>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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