250 Hz is really too narrow unless one has a high s/n and
the interference is from a strong adjacent signal that is
activating the AGC.
As a first approximation the RTTY signal can be thought of
as a 22.7 Hz square wave ... to reproduce that square wave
with minimal distortion requires at least the fundamental,
3rd and 5th harmonics (22.7*5 or approximately 115 Hz) for
285 Hz. Adding the 7th harmonic of the bit clock one gets
22.7*7 + 170 or approximately 330 Hz.
Most amateur filters are specified at -6 dB corner frequencies
where we really need to be flat or down no more than 1 dB for
the 285 Hz bandwidth so the argument would be for a -6dB width
of roughly 300 to 325 Hz. Again, much of this will depend on
the filter behavior at the knee - how sharply it transitions
from passband to skirt - and the steepness of the filter
skirts.
Many have had luck in using "250 Hz" filters with shape
factors in the 1.8 - 2.0 range but these 250 Hz filters
generally measure significantly wider than nominal (see:
http://www.inrad.net/product.php?productid=214 or
http://www.elecraft.com/K3/K3_8_pole_plots.htm) so the
"250 Hz" reports should be taken with a grain of salt.
73,
... Joe, W4TV
> -----Original Message-----
> From: rtty-bounces@contesting.com
> [mailto:rtty-bounces@contesting.com] On Behalf Of Jens Petersen
> Sent: Tuesday, August 25, 2009 4:21 PM
> To: RTTY contesting
> Subject: Re: [RTTY] Crystal filter width preferences for RTTY
> contesting
>
>
> On Tue, 25 Aug 2009 12:23:26 -0500, you wrote:
>
>
> >Q1. If you only were to get 1 filter, what width would it be?
>
> 250Hz
>
> I use 250Hz roofing filter on my K3 and it works super.
>
> --
> OV1A Jens
>
> Good decisions come from experience. Experience comes from
> making bad ones _______________________________________________
> RTTY mailing list
> RTTY@contesting.com http://lists.contesting.com/mailman/listinfo/rtty
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