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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