[Elecraft] RTTY filter for K3

[Joe Subich, W4TV]

> I was the first to respond to your question. I told you that 
> I was listening to RTTY through the 200 Hz, 5 pole filter and 
> it worked fine.  Then the experts came on here and using many 
> words, explained why it couldn't work because it is too narrow. 

Using a excessively narrow filter on RTTY is a balancing act. 

If the bandwidth is too narrow the tones tend to become "smeared" 
- the transitions become less easy to detect (in other words, 
the "eye" begins to close).  However, narrowing the bandwidth 
also reduces the amount of QRM/QRN making it through the filter 
and to the decoder/detector.  In some cases the benefits of 
reduced interference are greater than the losses caused by 
data smearing in the "too narrow" filter.  This is particularly 
true if the desired signal is strong enough in the absence of 
QRM/QRN to be decoded even with compromised bandwidth. 


73, 

   ... Joe, W4TV 
 



> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of n4lq
> Sent: Sunday, May 18, 2008 8:42 AM
> To: Axel Kaiser; Elecraft Reflector
> Subject: Re: [Elecraft] RTTY filter for K3
> 
> 
> Axel
> I was the first to respond to your question. I told you that 
> I was listening 
> to RTTY through the 200 Hz, 5 pole filter and it worked fine. 
> Then the 
> experts came on here and using many words, explained why it 
> couldn't work 
> because it is too narrow. Maybe they do not actually have the 
> filter but I 
> do.
> Looking at the waterfall on MixW, 200Hz filter engaged, a 
> RTTY signal easily 
> fits within the waterfall's passband with a little room on 
> either side. The 
> vertical stripes of the RTTY signal do not quiet reach the 
> outer edge of the 
> waterfall which tells me that the "data" is not being chopped 
> off. Both weak 
> and strong RTTY signals seem perfectly copyable.
> No I would not use this in a contest unless I had a bandscope 
> so I could 
> find signals. I have ordered the LP-PAN which should take 
> care of that 
> little detail.
> 73
> 
> Steve Ellington
> N4LQ at insightbb.com
> ----- Original Message ----- 
> From: "Axel Kaiser" <dl2sca at hotmail.com>
> To: "Elecraft Reflector" <elecraft at mailman.qth.net>
> Sent: Sunday, May 18, 2008 4:14 AM
> Subject: [Elecraft] RTTY filter for K3
> 
> 
> 
> 
> 
> 
> To all you filter aficionados,
> 
> first of all thank you very much for all your detailed answers to my 
> original question, which roofing filter ist best for RTTY 
> operation of the 
> K3. Lots of very detailed and informative answers were given 
> and even a 
> general discussion on this issue among several others was 
> started. But to 
> tell you the truth, I am still somewhat uncertain, which 
> filter to order 
> with my K3.
> One of my theory books about HF technology, modulation, 
> bandwidth and so 
> forth, tells me, that the necessary bandwidth for a 170 Hz 
> shift RTTY signal 
> should be around 320 Hz. Now some of you guys write, that you 
> are using a 
> 250 Hz filter for RTTY operation. A 250 Hz filter though has 
> this 250 Hz 
> (ok, a few Hz more in reality) bandwidth at - 6dB 
> attenuation, which means 
> that the actual bandwidth at about 0 db is even less.
> So my understanding for a 170 Hz shift RTTY operation is, to 
> use a filter of 
> at least 400 - 500 Hz (at -6dB) in order to get all the 
> information through 
> the filter. This I can confirm also with my current OMNI VI+ 
> which does not 
> allow me to copy RTTY signals with a 250 Hz filter . So 
> please, where is the 
> point which I am missing with the K3, that you can work weak 
> RTTY signals 
> with a 250 Hz roofing filter??
> 
> Best 73 de
> 
> Axel, DL3ZH
> 
> 
> 
> _________________________________________________________________
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