Re: resonators/crystals as filters ?
From: Joe McElvenney (ximac_at_btinternet.com)
Date: 07/27/04
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Date: Tue, 27 Jul 2004 19:43:55 +0000 (UTC)
Hi,
IMHO the simplest (and sharpest) crystal filter is the one
developed during the 30's (possibly by Lamb) for use as a CW
filter in radio communication receivers. You should find it in
standard texts on radio-communications - both professional and
amateur - but not perhaps under that name.
It consists of a single, series resonant crystal fed by a
medium/high impedance source but with a low (and this is vital)
impedance load. As there is an inevitable 'parasitic' capacitance
across the crystal, which results in a null on one side of the
response curve, this is usually cancelled by feeding an anti-
phase signal of the same amplitude through a capacitor of similar
value to the top of the load. The low-Z load results in a fair
insertion loss but a little thought will show the need for this
if one wishes to maintain the steepness of the filter's skirts.
Also, gain is quite cheap these days.
The 'in-' and 'anti-' phase signals can easily be obtained by
a trifilar wound toroid with the signal going into one winding,
the centre of the other two grounded and the outputs taken from
the two loose ends. This has also been done using an equi-valued
RC/RE common-emitter stage with one output taken from the
collector and the other from an un-decoupled emitter.
The windings need not be tuned but occasionally you will see
that the load itself is. This was the selectivity control but
also helped tune out any residual capacitance. It could even
recover some of the lost signal voltage by the step-up effect of
its 'Q' or by auto-transformer action should it be tapped.
From what you imply, you are not tied to a particular
frequency so any available crystal should suffice. There are lots
of them around but I would perhaps suggest something in a HC6/U
case as usually the bigger the crystal, the greater the 'Q'. I
haven't tried any of the $1 computer crystals (HC18/U) but a 1MHz
or 500kHz rock might be worth a go.
Cheers - Joe
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