[RTTY] RTTY Rx filter bandwidth

Joe Subich, W4TV lists at subich.com
Thu Jan 10 12:46:50 EST 2013


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 at 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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