[Elecraft] K3: narrow filter

[Jim Brown]

On Wed, 14 Jul 2010 11:45:01 -0300, Jorge Diez - CX6VM wrote:

>Hello,

>I have in my K3, in both MAIN and SUB this filters:

>KFL3A-400  400 Hz, narrow 8-pole  filter

>KFL3A-1.8K  1.8 kHz, 8-pole filter

>When I reduce width, I notice that it went down to 200 in CW. This is
>true? 

Yes, and it can be set as narrow as 50 Hz!  

>So that´s means I don´t need the KFL3A-250  250 Hz, narrow 8-pole  filter to
>narrow less than 400Hz?

Right. BUT -- let's clearly understand the function of these fixed filters. 
First, let's talk about how the K3 works. Older radios were built with fixed 
bandwidth IFs, and with crystal filters that could be switched in for wide and 
narrow bandwidths. The K3 does not work that way -- instead, it uses a DSP 
system to provide IF filtering, and you set the width of that IF when you tune 
the WIDTH knob. 

Those FIXED filters that you plug into a K3 are ROOFING filters -- that is, 
they sit in front of the DSP system to protect it from overload by very strong 
signals that are very close in frequency. That is their ONLY function in the 
context of general operation. 

In the context of heavy QRM (contesting or a pileup), the selectivity of the 
roofing filter does a lot more -- it ADDS to the selectivity of the IF. This 
only matters when the QRM is VERY strong, and is most useful when the IF is set 
to the same bandwidth as the roofing filter. 

For example, let's say that you've got a 40dB over S9 signal 200Hz away from 
your frequency, and you're trying to copy an S3 signal. The DSP IF set to 400 
Hz might reject that 40dB over S9 signal by 10 dB, and a 400 Hz roofing filter 
might add another 6 dB of rejection. If you narrow the DSP IF to 200 Hz, you 
might get 25 dB of rejection from the DSP IF. That same 400 Hz filter still 
gives you that 6dB more rejection, but a 250 Hz roofing filter might give you 
15-20 dB of additional rejection. This combining of filtering is called 
CASCADING. 

Until last fall, I also used only 400 Hz and 1.8 kHz filters. I added a 250 Hz 
filter to one of my K3s in time for the winter contests, and soon found that it 
really helped on weak signals in heavy QRM. I've since added 250 Hz filters to 
my other K3 (the one with the sub-RX). 

When you use the MENU to set up the K3 properly, you tell the K3 what filters 
are installed in which sockets, and the K3 automatically switches them in when 
you change the WIDTH. For example, my 400 Hz filter switches in when I reduce 
the bandwidth from 450 Hz to 400 Hz, and my 250 Hz filter switches in at 250 
Hz. You can also cause the filters to switch at different WIDTH settings by 
lying to the K3 about their bandwidth. For example, you could cause the 400 Hz 
filter to switch at 500 Hz by telling the K3 that it is a 500 Hz filter. Some 
very good operators do that and like it. 

73, Jim Brown K9YC