[RTTY] Bandwidth of a RTTY signal (was:SO2R RTTY Radio)

[Bruce Lifter, WT4I rr]

It is my experience that both 250Hz and 500Hz filters both have their 
strengths. 

I generally run with 500 Hz filters unless the bands get too crowded. 
I like to hear what is close to me. Additionally, I like my TNCs to copy 
wide. If I get called off frequency, I do not want to play with the RIT.
Some of my TNCs will actually copy stations as much as 40-50 Hz off frequency.


If conditions get crowded, I will switch to the 250 Hz filter. If there
is nearby QRM, the narrow filter will copy at times when the 500 Hz filter
won't.

Granted that the 250 Hz filter may trim the edges of the mark or space.
The question is whether there is enough energy left to detect the
tones?

Also consider that it is theoretically possible to copy RTTY with just
one of the tones.

73, Bruce


 

> -----Original Message-----
> From: rtty-admin@contesting.com [mailto:rtty-admin@contesting.com]On
> Behalf Of Ekki
> Sent: Tuesday, August 28, 2001 4:18 PM
> To: _Rtty
> Subject: RE: [RTTY] Bandwidth of a RTTY signal (was:SO2R RTTY Radio)
> 
> 
> > If possible, use a 250 Hz filter for RTTY.  500 Hz is ok for Pactor but
> > unnecessarily wide for RTTY.
> 
> I beg to differ :-)
> 250 Hz is too narrow for RTTY with 170 Hz Shift at 45Bd.
> 
> Practice in commercial RF systems shows that the required baseband signal is
> 1.6 times the keying speed (45.45 Bd). This applies to non rhytmical
> signals, for a perfect square wave (a rhytmic signal) the factor would be
> ~1. The factor 1.6 is a compromise for minimum bandwidth at minimal impulse
> distortions.
> 
> Bb = 45.45 * 1.6 = 72.72 Hz
> 
> 
> Using a FM shift we get a FM bandwidth of
> 
> Bfm = 2 * (shift/2 + Bb)
> 
> Bfm = 2 * (170/2 + 72.72)= 315.44 Hz
> 
> So a 250Hz IF filter is too narrow, except for vy good signals and
> considering that real world filters are not perfect.
> 
> 
> It get's worse when 200Hz shift are used, as it is unfortunately happing
> more and more because it's the standard shift for Pactor-1 and some TUs use
> the same shift for all modes. With 200Hz shift the required bandwidth is
> 345Hz, the gain in S/N ratio vs. 170Hz shift is negligible.
> 
> 
> Maybe some think i may be splitting hairs, but since i have the 756Pro i had
> the chance to play around with various IF bandwidths. Indeed, 250Hz is
> working less good than 350Hz if the signal is noisy or has flutter.
> 
> 73,
> Ekki, DF4OR
> 
> 
> 
> 
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