[Elecraft] 2.7 vs 2.8

[Jim Brown]

On Mon, 12 Apr 2010 11:24:54 -0700, Fred Jensen wrote:

>I thought the xtal roofing filter[s] are at the 1st IF [~8 MHz], and 
>their purpose is to restrict the passband coming out of the 1st mixer so 
>that strong adjacent signals don't make it to the 2nd mixer and DSP. 
>Thus, it seems to me that tweaking the roofing filters will only have 
>noticeable effects if you are plagued with very strong neighbors close 
>in on your frequency ... often.

You're sort of right, Fred, but there's a bit more to it. While the 
PRIMARY function of a roofing filter is to protect the DSP from overload 
by strong signals off frequency, the roofing filter cascades with the 
response of the DSP filter. That is, the total rejection by the filters 
is the rejection of the roofer PLUS the rejection of the DSP. When the 
two filters are of comparable bandwidth, the result is considerably 
better rejection of strong interfering signals. 

As a simple example, consider a signal 300 Hz off frequency with a 400 Hz 
roofing filter and 400 Hz DSP setting. If each filter is 7dB down at that 
frequency, their combined (cascaded) response would be 14 dB down. Take 
that out to 800 Hz and each might be 20dB down, combining to 40 dB. This 
can be a very big deal when the interfering stations are 40dB over S9 and 
you're trying to read an S5 signal, or when lots of big guns are squeezed 
into a narrow DX window. :)  

I started with 400 Hz filters as my narrowest roofers, and while I was 
reasonably happy, I recently added 250 Hz 8-pole filters. For the reasons 
noted above, I'm very glad that I did.  

BTW -- one good reason for using the 8-pole 2.8 kHz filter is that it's 
alleged to be a bit flatter in the passband (less ripple) than the 5-pole 
filter. Rigs with 2.7 kHz filters tend to have some incidental AM when 
transmitting RTTY, and a flatter filter would help that. Other than that, 
I can't think of a good reason for spending the money. I nearly always 
run with a 1.8 kHz roofer for SSB contesting. 

73,

Jim K9YC