[Elecraft] Filter Selection Approach

Wed Apr 15 11:17:44 EDT 2009

> I would disagree regarding the value or benefit of the 1 KHz 
> filter. Originally I had, for CW, 1800, 500 & 200 filters.  
> Since I normally set the width to 700 Hz for normal non 
> contest CW operating, it seemed that I was having problems 
> with loud signals within about 900 to 1800 HZ hitting the 
> AGC.  So when a 1000 HZ filter became available to trade for 
> my 1800 HZ filter I jumped on it and have not looked back!  

As I responded to someone who raised this same point privately, 
the wider filters are not centered on the desired signal.  Thus, 
if you favor a pitch of 500 Hz the 1000 Hz filter provides a 
"window" that runs from approximately 600 Hz BELOW the desired 
signal to about 400 Hz ABOVE the desired signal.  The 1500 or 
1800 Hz filter simply extends the window DOWN.  

With the asymmetric "window," if one is receiving QRM on the 
"wide" side changing to CW REV is just as effective in eliminating 
that QRM as switching to a narrower filter.  If switching to the 
opposite sideband results in QRM from other signals, one would 
need to use a still narrower filter - e.g., 500 Hz - to eliminate 
the QRM from both sides in any case.  

Because of the asymmetric window, there is little performance 
difference among the 1000, 1500, 1800 and 2100 Hz filters in 
CW.  Nobody has bothered to report the fact that 2 KHz IMD 
with the 2.7 or 2.8 KHz roofing filters is DIFFERENT if you 
measure above and below the interfering tones ... this is due 
entirely to the asymmetric window.  The same differences will 
be observed with 1 KHz or 500 Hz IMD DR in CW with any filter 
wider than 500 Hz. 

There is no doubt that an 800 Hz filter would be a good option 
for "wide" CW or that the 1000/1500/1800 Hz filters might have 
some benefit if the IF DSP were modified for "single signal" 
operation and the filters "centered" on the desired CW signal.  

> Last year in CQ WW CW I have had a chance to sit down on 20 
> meters with a K3 with 400 & 250 HZ filters.  I was very happy 
> to switch radios back to my K3 with 1000, 500 and 200 HZ filters.  

The 500/400/250/200 Hz filters have been discussed at length.  
With real bandwidths of 480 Hz (measured in four filters in my 
K3s) for the 500 Hz 5-pole filter, 450 Hz for the 400 Hz filter 
(Inrad web site), 350 Hz for the "250 Hz" filter (Elecraft web 
site) and 205 Hz for the 200 Hz filter, my choice will be to 
move from the 500/200 Hz combination to the 400/200 Hz pairing. 
The added skirts will improve performance where the wider filter 
is useful (particularly RTTY) without being significantly more 
narrow than the current 500 Hz 5-pole filters and the 200 Hz 
filter is the best truly narrow option.

73, 

   ... Joe, W4TV 

> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of .k8dd.
> Sent: Wednesday, April 15, 2009 12:10 AM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] Filter Selection Approach
> 
> 
> 
> 
> 
> 
> That is a lot of filters ... 
> 
> 2) Unless you are in very heavy QRM/pile-up situations the 
>    roofing filter does not provide a lot of "value added" 
>    since the final bandwidth is determined by the DSP.  The 
>    1 KHz filters add little benefit vs. the 1.8 KHz or 1.5 
>    KHz filters.  
> 
> 
> 
> I would disagree regarding the value or benefit of the 1 KHz 
> filter. Originally I had, for CW, 1800, 500 & 200 filters.  
> Since I normally set the width to 700 Hz for normal non 
> contest CW operating, it seemed that I was having problems 
> with loud signals within about 900 to 1800 HZ hitting the 
> AGC.  So when a 1000 HZ filter became available to trade for 
> my 1800 HZ filter I jumped on it and have not looked back!  
> Last year in CQ WW CW I have had a chance to sit down on 20 
> meters with a K3 with 400 & 250 HZ filters.  I was very happy 
> to switch radios back to my K3 with 1000, 500 and 200 HZ filters.  
> To my ears the 500 & 200 filters were a definite advantage 
> sorting out the weaker European signals in between the loud 
> east coast U.S. stations. This was not at all a scientific 
> test - just how it sounded to me! 
> Both K3s were using the same firmware.  
> And I really think if I had purchased a K3 with no roofing 
> filters, like I've seen suggested, I would have most likely 
> sold the K3 and gone back to a K2! Just my thoughts.
> 73    Hank    K8DD
> -- 
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> http://n2.nabble.com/Filter-Selection-Approach-tp2630347p2636834.html
> Sent from the Elecraft mailing list archive at Nabble.com.
> 
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