[Elecraft] adding 250Hz filer, any improvement?

[Jim Brown]

On 2/9/2011 8:23 AM, Scott Ellington wrote:
> The additional noise reduction of a narrow crystal filter is negligible.

Right.  While a narrower filter reduces the noise bandwidth, the ringing 
of a filter with steeper slopes may increase the effects of impulse noise.

>   (QRM is another matter.)

Yes. When there are two filters in the signal chain with electronics 
between them to prevent interaction, the rejection capabilities of those 
filters will ADD.  This addition is called CASCADING.

Putting some hypothetical numbers to it, let's say that the IF filter is 
down by 6 dB 125 Hz from center and 20 dB down at 300 Hz. If we add a 
roofing filter that is 6dB down at 125 Hz and 15 dB down at 300 Hz, the 
cascaded response of the receiver will be 12 dB down at 125 Hz and 35 dB 
down at 300 Hz.  The difference in rejection of slightly off-frequency 
QRM  is quite significant, and is clearly heard in crowded band conditions.

Cascading is most dramatic when the two filters have the same 
bandwidth.  As one filter is made more narrow, the addition of the wider 
filter still adds rejection, but it adds less rejection, and that added 
rejection is greatest farther off frequency.

73, Jim K9YC