And in any case the question was about filters not the signal.
There we agree. The problem is that practical narrow crystal or
mechanical filters have substantial group delay characteristics in
the transition region from passband to stopband (when the tones are
on the skirts). That group delay creates inter symbol interference
(ISI). Narrow filters work *only* with strong signals - when the
decrease in noise/QRM is greater than the increase in ISI. In other
words, one trades off one source of SNR reduction for the other.
If one looks at *practical* filters, the group delay begins to rise
at around the -1 dB points and becomes quite severe as the slope of
the filter skirts increase. A single Inrad "250 Hz" 8 MHz crystal
filter is only about 200 Hz wide at -1 dB ... they give no data on
the 455 KHz filters but the combination will certainly have severe
group delay issues. While the net effect of improved QRM SNR and
the degraded ISI SNR may work for strong contest signals, it does
not work to one's advantage with moderate to weak DX signals.
Unless one's only interest is strong signal contesting, it is wise
to have at least 270 if not 350 Hz of "flat" response to avoid the
severe effects of group delay. Filters that significantly less
than 300 Hz are simply not suitable for moderately to very weak
signals due to filter induced ISI.
73,
... Joe, W4TV
On 8/23/2013 11:19 AM, Jay WS7I wrote:
Joe-
You are just simply wrong. And in any case the question was about
filters not the signal. Been using stacked 250 Hz filters for over 30
years on Icom, Kenwood, Yaesu radio's. They simply work in RTTY
contesting always have and always will. Theory, math, which I trust
Chen on far more than I believe you may indicate something else but
experience tells me narrow is better for contesting, perhaps wider for
weak signals but if they are that weak most won't hear them at all
during a contest which is why DXing if different from Contesting.
On 8/23/2013 8:05 AM, Joe Subich, W4TV wrote:
And most FSK signals are 370 Hz wide or more
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