I haven't done any rigorous experiments but since the 400 Hz filter is
about 430 Hz wide with a "rounded" top see:
(http://www.elecraft.com/K3/K3_filter_plots.htm)
an optimal configuration might be the 400 Hz roofer with DSP set to
350-370 Hz.
The K3 is unique in its filtering as the DPS skirts are the same for
all bandwidths at something like 20 dB/100 Hz (I don't remember the
exact number). They are quite steep and "hard" so one doesn't want
to make the DSP bandwidth too tight due to avoid "smearing" (ISI) due
to group delay when the corners get into the keying sidebands.
The INRAD/Elecraft KFL3A-400 is rather interesting in that it rounded
profile leads to lower group delay (and thus less ISI) than a filter
with a sharper transition from passband to stopband (skirt region).
73,
... Joe, W4TV
On 1/10/2013 12:19 PM, RLVZ@aol.com wrote:
Wow- a lot of terrific RTTY information on this Reflector!
I agree with Joe that "400 Hz is nearly the minimum bandwidth necessary
for proper decoding". When contesting under heavy QRM condx, I try and try
and try using the 250hz roofing filter in my K3 to reduce adjacent channel
interference and the radio simply does not decode the average RTTY signal as
well as the 400hz roofing filter does. By the way, when using the 250hz
roofing filter I have the DSP filter set to 350hz. I've been told that the
K3's 250hz roofing filter has a bandwidth of aprx. 370 hz... so I thought
these 2 settings would provide for maximum selectivity and RTTY decoding...
but that simply is not the case as when I switch over to the 400hz roofing
filter the copy on the average RTTY signal is significantly better.
Perhaps someone has found a better overall RTTY receive filter combination for
the K3 than my settings of: 250hz roofing filter with 350hz DSP filter?
Such as, perhaps the 250hz roofing filter with the DSP filter set to 400 or
450hz?
73,
Dick- K9OM
400 Hz is nearly the minimum bandwidth necessary for proper decoding
(minimum intersymbol interference). Narrower filters work by cutting
adjacent signals more than the desired signal but they impose a much
higher group delay than a filter that is "flat" across the necessary
350 to 370 Hz bandwidth.
73,
... Joe, W4TV
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