[Elecraft] K3 Filter Configurations and Explanations

[Joe Subich, W4TV]

> 1. the interfering signals causing IMD don't have to be more 
> than 20dB down to stop the IMD. 2. 

On the other hand the 2.8 KHz 8 pole filter is actually narrower 
than the 2.7 KHz 5 pole (2.888 KHz vs. 2910 KHz from previously 
published data) with significantly sharper skirts so the -30 dB 
widths are roughly 3.755 KHz for the 2.8 KHz filter vs. 5.675 KHz 
for the 2.7 KHz filter. 

> If I use the DSP filtering, why add 8-pole filters if 1. applies 
> 3. I read that the roofing filters can introduce IMD (because of 
> steep skirts or...? Is there a chance that 5-pole filters have 
> less chance of introducing IMD? 

The signal level thorough the roofing filters is quite high.  With 
high signal levels, the crystals need to be exceptionally clean 
and well behaved.  Eric tells me that the 8 pole filters are 
specially selected by INRAD for low IMD characteristics (the lower 
grade filters go into FT-1000/FT-1000MPs).  Elecraft to a special 
"burn in" of their crystals before building the 5 pole filters. 
Still, the filter IMD (due to non-linearity in the crystals) may 
be up to 2 dB better with the 8 pole filters than with the 5 pole 
units. 

73, 

   ... Joe, W4TV 
 
 

> If I use the DSP 
> filtering, why add 8-pole filters if 1. applies 3. I read 
> that the roofing filters can introduce IMD (because of steep 
> skirts or...? Is there a chance that 5-pole filters have less 
> chance of introducing IMD? Anybody can comment on this?)



> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Arie 
> Kleingeld PA3A
> Sent: Tuesday, April 01, 2008 5:32 PM
> To: 'Elecraft Reflector'
> Subject: RE: [Elecraft] K3 Filter Configurations and Explanations
> 
> 
> Thanks to all for the comments. (I am using a 2x 400Hz and 2x 
> 250 Hz Inrad combo in my MP for selectivity at this time, and 
> a  wiiiiiide roofing filter)
> 
> The subject is very interesting to me since I expect my K3 
> sometime in June.
> 
> 
> How about this one for the K3:
> 2.7 standard
> 2.1 8-pole  SSB contesting
> 500 5-pole  CW contest running
> 200 5-pole  CW S&P especially 160m and 40m
> 
> The rationale: you don't need 8-pole since:
> 1. the interfering signals causing IMD don't have to be more 
> than 20dB down to stop the IMD. 2. If I use the DSP 
> filtering, why add 8-pole filters if 1. applies 3. I read 
> that the roofing filters can introduce IMD (because of steep 
> skirts or...? Is there a chance that 5-pole filters have less 
> chance of introducing IMD? Anybody can comment on this?)
> 
> I sure would like to hear the difference between 400 8-pole 
> and 500 5-pole with fixed DSP bandwidth in a crowded strong 
> signal band.
> 
> How about that?
> 
> 73
> Arie PA3A
> 
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