I know that the ideal notch filter passes all frequencies but one, but is there a filter that passes only one frequency and blocks all others? What is do you call such a filter and what is it used for (if anything)?

  • \$\begingroup\$ Parallel LC tuned circuits pass all but one. Series LC tuned circuits block all but one. \$\endgroup\$ – Optionparty Nov 7 '19 at 2:20
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    \$\begingroup\$ It is possible to use a notch filter (I'm thinking of the twin-T notch filter: 3R, 3C) as a feedback element in an active circuit that does the function you're outlining...a single-frequency pass filter. There are other RC-type bandpass filters that are not quite as fussy about component tolerances. \$\endgroup\$ – glen_geek Nov 11 '19 at 20:52

A high Q Bandpass filter will do this .Tuned circuits were used a lot and are sometimes still used today.

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