[Elecraft] K3 Roofing Filters - Zerobeat

[Vic K2VCO]

waltk8cv4612amos at att.net wrote:
> Use the shape factor to predict the 60 db down response or band width. 
> example = the 200 Hz filter has 4 to 1 so ................... multiply 
> with a calculator the 6 db response by 4 and that would equal 224 X 4 = 
> 896 Hz and the 8 pole 250 Hz has a 2 X 1 so it is 270 X 2 = 740  !
> 
> so the 8 pole is really 156 Hz narrower than the 200 in getting rid of 
> LOUD ADJACENT QRM !

This is very misleading, because the ultimate selectivity is provided by 
the DSP.

So even though there would a big difference if the selectivity were 
provided only by the filters, they are only needed to protect the DSP. A 
signal *within the crystal passband* has to be S9 +30 dB before it will 
activate the hardware AGC; so if a signal is already on the skirt of the 
crystal filter, it will have to be even louder before any desensing will 
take place.
-- 
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco