> 270 x 2 = 540, so the difference is 356Hz. ;-)
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.
73,
... Joe, W4TV
> -----Original Message-----
> From:
[hidden email]
> [mailto:
[hidden email]] On Behalf Of David Lankshear
> Sent: Monday, December 10, 2007 6:24 PM
> To:
[hidden email]
> Subject: [Elecraft] K3 Roofing Filters - Zerobeat
>
>
> 270 x 2 = 540, so the difference is 356Hz. ;-)
>
> 73, Season's Greetings all. DaveL G3TJP
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