[RTTY] Crystal filter width preferences for RTTY contesting

[Jeff Blaine AC0C]

Thanks Joe for that great detail.

I did not realize the 250 hz K3 filter ran close to 370 @ -6db.  The 400 hz 
is only 50 hz wider at the -6db point - quite close to the named value.

The SSB filters look nice and are (by %) much closer to the named widths.

I wonder if this width has proven 100% satisfactory - or if guys have ever 
ran into a case where they would like to have had a bit more narrow filter 
in the front end?

I suppose the answer here is to crank the DSP bandwidth down a bit in the 
cases where a bit more narrow filter is needed...

73/jeff/ac0c


--------------------------------------------------
From: "Joe Subich, W4TV" <lists at subich.com>
Sent: Tuesday, August 25, 2009 4:33 PM
To: "'Jens Petersen'" <oz2jp at c.dk>; "'RTTY contesting'" 
<rtty at contesting.com>
Subject: Re: [RTTY] Crystal filter width preferences for RTTY contesting

>
> 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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