There are some many practical problems with the holder capacitance,
stray capacitance and the like that would make such a filter challenging,
even if someone were to deliver a box of 45 MHz crystals with measured Qs
of 2 million to my doorstep. And if the box of crystals arrive, to obtain
frequency stability might require stabilizing the filter assembly in a
temperature controlled oven.
The typical roofing filters at 45 MHz have a bandwidth of 20 KHz or so.
Thus the fractional bandwidth is 50 times larger and the Qx is down into
the 100K range, making these filters relatively easy to realize.
--------------------------------------------------------------------------
An interesting 42.5 MHz filter design appeared several years ago using just
four crystals in ladder configuration. The designer was not looking for
narrow bandwidth nor good shape factor, but illustrating a "method". The
bandwidth was 5 kHz, the shape factor was 4, stopband better than 80db and
the response was symmetrical. In addition to the usual inductors across each
crystal to take care of holder capacitance, as inspired by Zobel, two other
other modifications were incorporated. (1) Parallel tuned circuits were
placed at each end to prevent degeneration into a poor Cauer lowpass filter.
(2) Parallel tuned circuits were used as coupling elements, presumably to
tweak mesh frequencies. Insertion loss unknown, but a fair number of
elements to compensate for temperature change.
73,
Geoff
GM4ESD
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