[Elecraft] RTTY Filter for K3

[Joe Subich, W4TV]

> I am watching RTTY right now and using the 200 Hz 5 pole 
> filter. The 170 HZ shift fits perfectly within the passband.

After making extensive tests and plotting the selectivity curves 
of the 200 Hz filter in my K3, I'm convinced that it is too 
narrow for reliable weak signal RTTY operation, particularly in 
conjunction with the dual-tone filter.  

The measured -6dB bandwidth of the 200 Hz crystal filter was 
209 Hz and the composite bandwidth of the 200 Hz crystal filter, 
200 Hz DSP and dual tone fitter was less than 170 Hz.  A standard 
170 Hz shift 45.45 baud RTTY signal needs at least 235 Hz 
(170 + 3*22.7) and the dual tone filter represents a 220 Hz (two 
50 Hz filters with center frequencies separated by 170 Hz). 

After extended discussion with a well know RTTY op and K3 beta 
tester, I believe the combination of a 300 Hz 5-pole crystal filter,
300 Hz DSP and the dual tone filter will have a composite -6 dB 
bandwidth of about 250 Hz - about optimum for narrow RTTY.  The 
currently available, "250," "400" or 500 Hz filters in combination 
with a 300 Hz DPS bandwidth and dual tone filter are very close to 
optimum.  

73, 

   ... Joe, W4TV 
     



> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of n4lq
> Sent: Thursday, May 15, 2008 5:29 PM
> To: Axel Kaiser; Elecraft Reflector
> Subject: Re: [Elecraft] RTTY Filter for K3
> 
> 
> Axel:
> I am watching RTTY right now and using the 200 Hz 5 pole 
> filter. The 170 HZ 
> shift fits perfectly within the passband.
> 
> Steve Ellington
> N4LQ at insightbb.com
> ----- Original Message ----- 
> From: "Axel Kaiser" <dl2sca at hotmail.com>
> To: "Elecraft Reflector" <elecraft at mailman.qth.net>
> Sent: Thursday, May 15, 2008 3:52 PM
> Subject: [Elecraft] RTTY Filter for K3
> 
> 
> 
> Hi K3-folks,
> 
> What would be the best RTTY roofing filter for the K3? Is the 
> 400Hz 8-pole 
> filter too small and the 500Hz 5-pole filter the better choice?
> 
> Thanks in advance for your answers.
> 
> 73 de
> 
> Axel, DL3ZH
> 
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