> 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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