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
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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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