> I own a scanner and one of the frequencies I want to hear is close 
> to another of the internal ones it uses, so a continue audible tone 
> can be heard (about 3KHz).
>
> As I do not want to put my hands inside the scanner, a passive or 
> active notch filter could be added in the earphones cable to 
> attenuate the annoying tone while hearing through them.
>
> My questions are, should a L-C filter be enough?, shoud I use an 
> active filter?, with op-amps?, with a specific filter chip?, a PIC?

Some thing like a tunable "twin T" is likely to be what you want. This 
gives a super sharp audio notch that can be tuned to the frequency of 
interest. More generally the terms is "tunable notch filter"

Another possible solution, if the problem is input frequency specific 
and not just generally bad design, is to shift the input signals 
somewhat with an input converter. This can be very simple and cheap. 
This sort of thing is used to eg move the FM band on cars imported to 
this country whose radios do not cover all the band used in this 
country. There are circuits on the web. AFAIK they are as simple as a 
1 transistor oscillator and a simple mixer or even an IC as an 
oscillator.

Googling for "twin T" gives many examples.

Simple version - last cct on page

        http://ourworld.compuserve.com/homepages/Bill_Bowden/page9.htm

With op amps
Comments on tunabilitty

        http://www-k.ext.ti.com/SRVS/Data/ti/KnowledgeBases/analog/document/faqs/cfttnhq.htm

With op amps and Q adjustment

        http://www.radio-electronics.com/info/circuits/opamp_notch_filter_2/opamp_twin_t_notch_filter.php

More

    http://sound.westhost.com/project52.htm




        RM 

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