[Elecraft] K3: use of 1.8 filter - clarification

[Bill W4ZV]

Hi Doug,


DOUGLAS ZWIEBEL wrote
> 
> The roofing filters are used for reducing IMD.  
> 

I agree IMD reduction is not a big issue on SSB but blocking dynamic range
(BDR) is the primary reason I sometimes need a narrower filter.  If you have
an S9+25 SSB signal CQing next to you (sometimes the case in crowded
contests), a narrower filter will prevent it from de-sensing your receiver. 
If part of the interfering signal falls inside your 2.7k filter, signals on
your frequency will be gain modulated by his signal (commonly called AGC
pumping).  In this case a narrower filter will prevent the problem.  Of
course when this happens, we still have to contend with his transmitted
"splatter" (typical SSB TXs run -35 dB IMD which would result in ~S7
splatter from an S9+25 signal), but at least you can still copy signals at
S7 or above (and lower when he's not transmitting).

73,  Bill

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