It is also conceivable that these "gentle" qualities
of the TT filters may be more desirable in the role of
roofing filters than a sharper filter which may have
more ripple and/or ringing. I would also have to
believe that for roofing filter purposes it?s the
"cleanliness" of the filter passband shape which is
most important, not its ultimate skirt selectivity.
Since the Orions?s true and final RX selectivity is
achieved in the DSP-IF the important roofing filter
qualities and requirements are somewhat different than
they are for an analog only RX design. As such I
wouldn?t automatically assume that a ?superior? 3rd
party crystal filter will make the Orion better, it
might actually degrade the Orion?s overall
performance.
Duane
N9DG
--- "Dr. Gerald N. Johnson, electrical engineer"
<geraldj@isunet.net> wrote:
> In my opinion, the relatively gentle slopes and the
> rounded shoulders of
> the standard TT filters are major secrets to the
> great performance of TT
> receivers under adverse conditions. True, the gentle
> slopes reject
> SIGNALs a little poorer (but when you have 16 poles
> cascaded its not too
> bad), but the shape also means the TT filters ring a
> great deal less
> when hit by spike noise, such as lightning static,
> ignition noise, and
> power line noise. Most crystal lattice filters and
> all the Collins
> mechanical filters ring so much that they fill in
> the gaps between power
> line noise spikes and for long periods after
> lightning spikes to make
> the receiver unusable without a wideband noise
> blanker. Then if there is
> a STRONG unwanted signal in the passband of the
> noise blanker it is
> turned to wideband splatter by the blanker.
>
> I KNOW from using multiple receivers over the past
> 40 years, that the
> rounded filters are far more useful in Iowa in the
> summertime, than any
> steep skirted and square cornered crystal or
> mechanical filter.
>
> 73, Jerry, K0CQ
> --
> Entire content copyright Dr. Gerald N. Johnson,
> electrical engineer.
> Reproduction by permission only.
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