[Elecraft] 250 Hz and 400 Hz Filter Measurements

[Joe Subich, W4TV]

 > Of course. But as contesters and DXers, we always look for more, and
 > the cascading of filter responses can certainly be beneficial.

Yes, cascading can be beneficial but the DSP filter set for 200 Hz
at the corners is narrower that the "cascaded" response of the
"250 Hz, 8-pole" filter and the DSP set to align with the corners
of the "250 Hz" filter.

 > Yes, but the laws of physics does result in cascaded response.   Just
 > because a tool is designed for one purpose doesn't necessarily mean
 > that it can't be used for others.

Yes, the laws of physics result in a cascaded response.  However, when
the DSP filter is significantly more narrow than the roofing filter,
the roofing filter provides very little in the way of "cascade gain."
Consider that the DSP filter set for 250 Hz is about 350 Hz wide at
-50 dB and the "250 Hz, 8-pole filter" response is only about -6dB
at 350 Hz bandwidth, the roofing filter is not providing much if any
selectivity ("cascade effect").

 > Repeating what I stated earlier -- when 100 big signals like W8JI,
 > W4ZV, N6RO, and K1TTT are crammed into 15-25 kHz to work EU or JA,
 > you can never have enough front end!

That's true but even the "250 Hz, 8-pole filter" is not doing much
except keeping them away from the DSP (and the hardware AGC) to the
extent that they are more than +/- 300 Hz off frequency.

The difference is one of receive (base band) selectivity vs. front
end (or 1st IF) selectivity.  The front end (IF selectivity) is
only important in its effects on blocking (HAGC "pumping") and
"wide" spaced IMD (where "wide" is defined as greater than the
DSP bandwidth).  In a multiple strong signal situation it is
convenient to have "front end" selectivity as closely matched to
the DSP bandwidth as possible to protect the IF, 2nd Mixer and
DSP.

*IF* strong signals exist within the IF bandwidth but not the DSP
bandwidth they can cause blocking and IMD effects.  This is most
apparent with narrow bandwidth modes like CW and RTTY where the
interfering bandwidth can be quite small and the interfering power
concentrated on a single frequency.  The issue is not nearly as
acute on SSB where the "channels" are wider and the power density
is lower.

73,

    ... Joe, W4TV

On 7/19/2010 12:39 AM, Jim Brown wrote:
> On Sun, 18 Jul 2010 16:33:40 -0400, Joe Subich, W4TV wrote:
>
>> I don't know that "benefit" is necessarily the correct word to use
>> ton describe cascading.  The better term would be "effect."  The K3
>> still exhibits benefits from the distributed filter design - the
>> roofing filter protects the 2nd IF and DSP from overload, AGC
>> pumping and IMD from strong signals outside the passband of the
>> DSP filter.
>
> Of course. But as contesters and DXers, we always look for more, and
> the cascading of filter responses can certainly be beneficial.
>
>> However, the roofing filter is not intended to contribute significant
>> selectivity within the DSP passband.  That's not the function of a
>> roofing filter
>
> Yes, but the laws of physics does result in cascaded response.   Just
> because a tool is designed for one purpose doesn't necessarily mean
> that it can't be used for others. :) Repeating what I stated earlier
> -- when 100 big signals like W8JI, W4ZV, N6RO, and K1TTT are crammed
> into 15-25 kHz to work EU or JA, you can never have enough front end!
> And thankfully, I think all those stations are running K3s (I'm not
> sure about TTT).
>
> 73, Jim K9YC
>
>
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