Lattice filter dynamic range is limited by the linearity of the crystals
and the transformer cores. Extremely tiny cores may saturate far more
easily than large cores, so that miniature filters may not have a great
IMD. Crystals may be subjected to more voltage in lattice filters
depending on the turns ratio of the transformers used.
Ladder filters are limited by the linearity of the crystals alone. There
need not be any transformers except for input and output impedance
matching though most filters don't because the filter impedance is
appropriate for the solid stages before and after. Given the same
quality of crystals and frequency, ladder filter impedance is a function
of bandwidth.
Then the crystal's IMD limit depends on the size of the quartz slab.
Nearly all these filters operate in thickness mode so there's the design
temptation to minimize the area of the quartz that limits its thermal
capacity and likely its power limit for linearity. Especially in the
lattice filter, large area crystal electrodes have a lot of capacitance
that interferes with filter design. It limits the ultimate rejection and
can easily require several more filter poles for usable rejection in
practical bandwidths.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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