[RTTY] Crystal filter width preferences for RTTY contesting

[Kok Chen]

On Aug 25, 2009, at 8/25    2:27 PM, John Barber GW4SKA wrote:

> Simple maths for this used to be shift plus twice the baud rate; 170  
> + (45 x
> 2) = 260Hz


But why did you use 45 * 2?

The fundamental RTTY keying waveform is 45.45/2 Hz, not 45 Hz.   
(Imagine a signal that switches between mark and space at 22  
millisecond intervals; the period of the square wave waveform is 44  
ms, not 22 ms).

Secondly, even if you had intended the equation to represent 2*22.5 Hz  
(twice the fundamental keying signal) on each side of the two keyed  
carriers (thus a total of 2*45), the third harmonic (and arguably even  
up to the fifth harmonic) of the keying signal still has sufficient  
energy to be very useful for an "ideal" RTTY matched filter (i.e.,  
something that convolves the input with a 22 ms rectangular pulse).

The minimum bandwidth I would use for a very weak RTTY signal under  
AWGN is 170 * ( 22.725 * 3 )*2, or a tad over 300 Hz.

When Doppler spreading exists, you would require an even wider  
passband to capture most of the energy from the pulse.  The CCIR  
profile (using the Watterson model) for high latitude flutter has a 2- 
sigma Doppler spread of 10 Hz.  This is why copy of a fluttered signal  
usually improves when you open the received passband up, even when the  
demodulator is not matched to the fluttered signal's pulse profile  
(probabilistically, a rectangular pulse that is convolved with a  
Gaussian).

In today's sound card world, all that the I.F. filter provides is a  
"roofing filter" to keep the sound card (and the receiver stages  
between the I.F. filter and the audio output) from saturating.  If a  
signal, however wide, reaches the sound card without clipping it, good  
software filters are much better than any crystal filters at rejecting  
QRM.

73
Chen, W7AY