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
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