[Elecraft] Dumb question .....

[Don Wilhelm]

Mike,

Since you understand 'filters' - consider that the ultimate selectivity 
of the K3 is determined by the DSP.
Now think about the input to the DSP - if one restricts the number of 
signals entering the DSP (actually the ADC front end for the DSP), the 
DSP will not have to deal with as much 'garbage'.  The more narrow the 
roofing filter, the less 'garbage' will be present at the DSP front end.

Consider also that the DSP can handle an S-9 +20 signal input from an 
unwanted signal - above that level it will be overloaded.  In situations 
where signals greater than that level are encountered within the 
passband of the roofing filter, the Hardware AGC will limit the signal 
level presented to the DSP.  Even though the DSP may be cranked down to 
hear only the desired signal, the signal gets through to the DSP front 
end and will reduce the gain of the receiver because the Hardware AGC is 
being activated by the strongest signal within the passband of the 
roofing filter - the result is AGC "pumping" as a result of a signal 
that one may not even hear.  Using a more narrow roofing filter can 
eliminate this unwanted signal and eliminate the AGC pumping.

Think of the roofing filter as a roof on a house - the wide filter 
allows the DSP (and hardware AGC) to respond to all the rain falling on 
the roof.  To reduce the amount of rain falling on the roof, a more 
narrow roofing filter is required.

If you are interested in contesting and/or heavy DXing, you are liable 
to encounter nearby unwanted strong signals within the passband of the 
roofing filter which will cause the hardware AGC 'pumping', and you 
should invest in narrow filters that will eliminate that effect.  For 
those whose operation is more casual and do not wish to dig for weak 
signals in the presence of stronger nearby signals, a wide roofing 
filter will do the job nicely.

73,
Don W3FPR

Michael D. Heit wrote:
> This may seem like a really dumb question but I have to ask....
>
> What are the "roofing filters" options for the K3 radio?  I understand
> filters but the term roofing filters really confuses me and why they just
> aren't built right into the radio from the start.
>
> Mike Heit AD7VV
>
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