[Elecraft] K3: narrow filter

[Jorge Diez - CX6VM]

Thanks all for the info.

Jim your email is very clear for me, I not understand too much how works
roofing filers and DSP, buy you clarify me something.

I have the same filters you have, and seems will be good to add the 250Hz
filter, since I am in contests and I notice QRM of stations near my
frequency.

I not have too much 59+ station here, not too much hams here..., but seems
will be good to have also the 250Hz filter.

So now I have another question, I think I have room for only one more
filter, so how will affect DIVERSITY if I have 250Hz, 400Hz, 1.8kHz in MAIN
and 400Hz, 1.8kHz in SUB?

I cannot do DIVERSITY with a 250Hz narrow filters (because I will not have
it in both receivers), buy the K3 will work OK using the 400Hz I have in
MAIN and SUB?

Thanks,
Jorge
CX6VM/CW5W
K3 #4077

-----Mensaje original-----
De: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] En nombre de Jim Brown
Enviado el: Miércoles, 14 de Julio de 2010 12:52 p.m.
Para: Elecraft Reflector
Asunto: Re: [Elecraft] K3: narrow filter

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 that4s means I don4t 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