[Elecraft] [K3] Filter Cascade Effect

[Joe Subich, W4TV]

> > Another point -- some have noted that at least some of the
> > roofing filters are actually wider than their nominal value. 
> 
> By design, INRAD makes the actual -6 dB bandwidth greater 
> than the filter name.  See the actual analyzer plots on the 
> Elecraft web site.

This is a throwback to the practice by Kenwood and Yaesu in the 
first transceivers with cascaded (8 MHz/455 KHz) filters.  The 
filters were specified based on the effective bandwidth of the 
cascaded pair.  For example, the "Standard" Yaesu "2.4 KHz" 
filters were actually 2.6 KHz wide (note the original model 
numbers were XF-ffff-262-xx) - the cascaded pair resulted in 
an effective bandwidth of 2.4KHz.  

The same effect can be observed with Inrad's filters.  Looking 
at the 250 Hz pair for the TS-940, the 8.83 MHz filter is 367 Hz
wide at -6dB but it is down about 2dB +/- 125 Hz from the center. 
when this is combined with a 455 KHz filter that is 265 Hz wide 
at -6db and 250 Hz wide at -4 dB, the effective bandwidth is 
slightly less than 250 Hz.  

73, 

   ... Joe, W4TV