W4TV:
>However, the 250 Hz filter is actually 370 Hz at the - 6dB
points according to previous e-mail here.
>With the K3, the point of diminishing returns for IMD (and
close in rejection) is perhaps - 30dB instead of -60dB or
more in a conventional crystal filter only radio.
>If we use a straight line (linear slope) from the -6dB to
-60dB points the 200 and 250 Hz filters look like this:
200 250
-------------------------------------------------
- 6dB 224 Hz 370 Hz
-60dB 896 Hz 777 Hz
slope 6.22 Hz/dB 3.77 Hz/dB (one side)
-10dB 274 Hz 400 Hz
-20dB 398 Hz 475 Hz
-30dB 522 Hz 550 Hz
-40dB 647 Hz 626 Hz
-50dB 771 Hz 702 Hz
>By this simple calculation (or by plotting on graph paper) one
can see that the 200 Hz filter should do a BETTER job as the
most "narrow" filter. It remains "tighter" to about -35 dB
and the five pole design should result in less pulse stretching
(ringing in the presence of static) than the 250 (really 350 Hz)
filter.
Excellent post Joe (as usual). Another reason one
might want the 200 versus the 250 is the relative difference
between the 400 and 500 filters. From the published data:
Filter BW(-6dB) Shape Factor
200 224 4.0
250 370 2.1
400 435 2.1
500 565 3.1
Practically there is little BW difference in the 250 and
400 8-pole filters. If one wanted both a very narrow
and moderately narrow filter, I would choose the 500
and 200 5-pole filters. They have at least an octave
difference in total bandwidth...plus the group delay
issues you cited in your last paragraph above.
BTW I see very little insertion loss in either of
the 200 Hz or 500 Hz filters. Using an XG1 source, I find
1 dB compensation is all that is needed for both.
73, Bill W4ZV
P.S. I retract my engineers vs marketeers statement.
The marketeers won in naming the filters, especially
calling the 370 Hz filter above a "250 Hz"! :-)
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