Larry,
You are quite correct that all filters are not created equal. For
digital modes, the group delay is just as important (if not more
important) than a flat passband and steep filter skirts. In fact, those
filters with steep skirts often sacrifice group delay to achieve the
steep skirts - but I am generalizing here, and that may not be a
universal truth.
Unfortunately, group delay plots are not common for filters since steep
skirts seem to be the 'criteria of choice' for most amateurs. A
Gaussian to 6 dB filter has a nice rounded nose and gentle skirts, but
has a great group delay characteristic, OTOH, Cohn filters usually have
great skirts and poor group delay in the passband. Group delay will
make a difference in the ability to decode digital signals.
73,
Don W3FPR
Larry Molitor wrote:
--- "Joe Subich, W4TV" <[EMAIL PROTECTED]> wrote:
For a died in the
wool
RTTY DXer, a steep sided 270 - 300 Hz filter would
be nice
instead of 370 Hz from the "250 Hz" filter.
Over the years I've spent a lot of time looking at
this issue. My interest is primarily digging weak RTTY
DX out of the noise but I do occasionally get into a
contest.
I've played with a lot of radios and filters but most
of my real testing was done with a FT-980, a TS-2000,
and a FT-2000.
While I have seen good weak signal performance with a
250 Hz filter, it was the exception not the rule. As
an example, the improvement on the FT-980 going from
the stock SSB filter to the dual CW filter was
dramatic. But going to the 250 Hz CW filter lost about
6 dB in ability to properly decode weak RTTY signals.
It was even worse on signals that had polar flutter. I
no longer have the plots of these FT-980 filters but
as I recall, the 250 Hz filter was a bit peaky in the
middle and had poor group delay characteristics
extending well in from the corners.
As you say Joe, the determining factor is the passband
ripple/group delay. In a typical bandpass filter the
group delay goes to heck at the corners. But the shape
of the "corners" varies from filter to filter. I've
run a number of "ham filters" on a network analyzer
and plotted group delay. Seems like no two filters are
the same even if the same part number. This I believe
is due to ham filters being so cheap and manufacturing
process control being minimal to keep the sell price
down to what we can afford.
So I would say, if you have a 250 Hz filter, no matter
where it's placed in any radio, give it a try on very
weak signals. Switch between a wider filter and the
narrow filter and see if there is any degradation. If
not, use the narrow filter.
Other wise, plan on not using any filter less than 300
Hz in passband width (note - this is NOT the 6 dB down
width!!! very important!!) if you want optimum weak
signal performance.
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