[RTTY] Crystal filter width preferences for RTTY contesting

[Kok Chen]

On Aug 27, 2009, at 11:31 AM, Jeff Blaine AC0C wrote:

> 1. With a more narrow bandwidth, we have a better ability to  
> suppress adjacent strong stations, especially their sideband  
> amplitude - which will somewhat increase the SNR of our desired  
> station.

I think that you may have misunderstood.  I apologize that I write  
horribly; English is my third language, even though I should not be  
using it as an excuse considering the fact that I have been in this  
country for longer than the majority of people who were born here, and  
have all but forgotten my first two languages! :-).

The point of those plots is to show that a narrower receiver bandwidth  
will NOT help you suppress the keying sidebands from a close by strong  
FSK transmitter. Once the strong sidebands encroaches on the weak  
signal you are trying to copy, it is too late to do anything.   
(Similar situation to SSB signals that are too close to one another,  
no amount of receiving filter will get rid of what people complain as  
"splatter.")   A narrower I.F. filter will not help in that case, and  
yet the narrower I.F. filter will hurt when it comes to copying weak  
signals when there is no QRM.

My personal methodology: use a relatively wide I.F. filter --  
something that is only narrow enough to (1) keep my sound card from  
saturating and (2) keep the AGC from pumping due to very strong close  
in signals (something that has not been brought up yet in this  
thread).  Then find some software that lets me change the demodulation  
bandwidth to match the situation.

I do not tell everyone else that they must, or even should do the same  
thing.  But I do recommend that they do study the problem carefully  
before choosing their own solution.

The advice I usually give is to never ever, ever, ever, ever allow the  
sound card to saturate, not even for a millisecond, while making sure  
that the weak signal is a good 10 or 20 dB above the sound card's  
noise floor.

73
Chen, W7AY