[RTTY] Crystal filter width preferences for RTTY contesting

[Joe Subich, W4TV]

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 at contesting.com 
> [mailto:rtty-bounces at 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 _______________________________________________
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