[Elecraft] K3s AFSK frequencies

[Joe Subich, W4TV]

Too narrow a filter results in "smearing" one bit into the next -
equivalent to severe multi-path - which causes a degraded signal
to noise ratio.  While a narrow IF filter may reduce interference
from AGC capture, there is a fine line between the improvement
in sensitivity due to keeping the strong adjacent frequency signal
from "turning down the RF gain" and the loss of SNR due to self-
induced multi-path (inter symbol interference).

In general one wants a filter that is at least 375-400 Hz wide at the
-1 dB points with flat phase delay across the passband. Filters less
than 370 Hz (@ -1dB), especially multi-pole crystal lattice/ladder
filters, have significant phase non-linearities particularly in the
"knee" region that create inter-symbol interference.

73,

    ... Joe, W4TV


On 4/13/2016 9:56 AM, john at kk9a.com wrote:
> The optimal band width may depend on how crowded the band is. Wide
> filtering may work well for a DXer trying to copy a super weak signal on a
> quiet band. I only use RTTY in contests when the bands are packed and I
> have had great results using very narrow roofing filters.
>
> John KK9A
>
>
> from: Dave Hachadorian
> Tue Apr 12 23:54:57 EDT 2016
>
> Current thinking is to use a fairly wide filter for RTTY.  The
> software filters that are applied in the sound card are much
> narrower, about 50 Hz wide.  Lately I've been using the 2.7 KHz
> roofing filter, with DSP bandwidth set to about 600 Hz.
>
>
> Dave Hachadorian, K6LL
> Yuma, AZ
>
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