[Elecraft] K3 Roofing Filters - Zerobeat

[Joe Subich, W4TV]

> 270 x 2 = 540, so  the difference is 356Hz.  ;-)

However, the 250 Hz filter is actually 370 Hz at the - 6dB 
points according to previous e-mail here.  

With the K3, the point of diminishing returns for IMD (and 
close in rejection) is perhaps - 30dB instead of -60dB or 
more in a conventional crystal filter only radio. 

If we use a straight line (linear slope) from the -6dB to 
-60dB points the 200 and 250 Hz filters look like this: 

              200           250 
  -------------------------------------------------
   - 6dB      224 Hz        370 Hz
   -60dB      896 Hz        777 Hz 
    slope     6.22 Hz/dB    3.77 Hz/dB  (one side) 
  
   -10dB      274 Hz        400 Hz    
   -20dB      398 Hz        475 Hz
   -30dB      522 Hz        550 Hz
   -40dB      647 Hz        626 Hz
   -50dB      771 Hz        702 Hz

By this simple calculation (or by plotting on graph paper) one 
can see that the 200 Hz filter should do a BETTER job as the 
most "narrow" filter.  It remains "tighter" to about -35 dB 
and the five pole design should result in less pulse stretching 
(ringing in the presence of static) than the 250 (really 350 Hz) 
filter. 

73, 

   ... Joe, W4TV 
 
 

> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of David Lankshear
> Sent: Monday, December 10, 2007 6:24 PM
> To: elecraft at mailman.qth.net
> Subject: [Elecraft] K3 Roofing Filters - Zerobeat
> 
> 
> 270 x 2 = 540, so  the difference is 356Hz.  ;-)
> 
> 73, Season's Greetings all.  DaveL  G3TJP