A notch filter, sometimes called a band-stop filter, helps to blank or "notch" out troublesome interference. Most modern radio transceivers have one incorporated, and some allow the notch to be shifted to zone in on the interfering signal. I personally like to use them to block out a morse code signal that is close to a signal I am trying to listen to.

Overall, this type of filter is simple to make. I would recommend that you first read an excellent writeup under band-pass filter for some of the physics and math behind the theory of operation.

 1db  |  __________          __________
  O   |            .|      |.
  U   |            .|      |. 
  T   |            .|      |.
  P   |            . \    / .
  U   |            .  \__/  .
  T   |____________.________.____________
                   ^        ^
                  f1        f2    
         < LOW      FREQUENCY      HIGH >
The object is to get the objectional signal between the lower and upper cutoff frequencies (f1 and f2, respectively). This will cause the signal to be reduced by at least 3 decibels, or effectively half the power of the desired signal. To build one, use the following example circuit:
           |          |
       O---+   LC1    +------------+----O
           |___@@@@___|            |
             Inductor            -----
                                 /```\ Capacitor
                                   @             LC2
                                   @   Inductor
The LC1 circuit is tuned to the frequency you want to block. Once it is set, the LC1 circuit will become resonant at the offending frequency and block the signal from passing. Additionally, the LC2 circuit would actually shunt the signal to ground, effectively blocking the offending signal. Any signal that was not at or within the range (f1 to f2) will be blocked. Any signal outside of the two frequencies would be passed through with few problems.

There are better filters (I personally like crystal filters), but this circuit is very easy and cheap to make. The capacitors may be variable, so one can adjust the notch to move the band-stop function.

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