[Elecraft] RTTY and filters

[Jerry Flanders]

I just checked the filter plot at http://www.elecraft.com/ . Strange 
that they call it a 250 Hz filter because -6 dB BW is actually 370 
Hz, compared to 435 for the 400 Hz filter. At -60, the "250" is 785 
Hz wide, compared to 935 for the "400" one.

IOW, the 250 should pass all the 170 shift sidebands OK, because it 
is not really that narrow. Not really much difference between the 250 and 400.

Jerry W4UK

At 02:20 PM 1/7/2008, J. Edward (Ed) Muns wrote:
> > I previously mentioned in an email to the group that I was looking forward
> > to trying out the 200hz filter in my K3 for RTTY contesting.  I had one
> > reply wondering if that filter was going to be too tight for RTTY.
> >
> > Any comments from the group on this.
> >
> > The other person that emailed was planning on using the 400hz filter.
>
>I only operate RTTY with narrow filtering, all of the following
>simultaneously in the K3:
>
>- 250Hz crystal filter
>- 200Hz DSP filter
>- Dual-Tone Filter
>
>In the 756ProIII, I use the 250Hz RTTY DSP filter and the Twin-Peak Filter.
>The only roofing filter "choice" in the ProIII is the standard 15kHz,
>although the INRAD 4-5kHz can be added.
>
>I want the filtering as tight as I can get it so the RTTY decoders have the
>best chance to do their job.  Stations need to be good at zero-beating,
>which is easy in RTTY because your decoder display is a perfect tuning
>indicator.  Alternatively, on my side I have to ride the RIT a lot to pull
>in the off-frequency signals.  This weekend there were a lot of stations
>calling off frequency, some as much as 200 Hz away.  I suspect that those
>using AFSK with AFC were responsible for a lot of that.  In other words,
>their receiver was zero beat with me but their "smart" encoder skewed their
>Mark tone away from where they were receiving.
>
>In a sense these off-frequency folks are helpful because they don't QRM the
>ones who are zero beat with me!  Once I work the zero-beaters out of the
>pile-up, then I can grab the others with the RIT ... if they're still there.
>
>73,
>Ed - P49X (W0YK)
>
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