Band-stop filters

Also called band-elimination, band-reject, or notch filters, this kind of filter passes all frequencies above and below a particular range set by the component values. Not surprisingly, it can be made out of a low-pass and a high-pass filter, just like the band-pass design, except that this time we connect the two filter sections in parallel with each other instead of in series.

Constructed using two capacitive filter sections, it looks something like this:

The low-pass filter section is comprised of R₁, R₂, and C₁ in a "T" configuration. The high-pass filter section is comprised of C₂, C₃, and R₃ in a "T' configuration as well. Together, this arrangement is commonly known as a "Twin-T" filter, giving sharp response when the component values are chosen in the following ratios:

Given these component ratios, the frequency of maximum rejection (the "notch frequency") can be calculated as follows:

The impressive band-stopping ability of this filter is illustrated by the following SPICE analysis:

twin-t bandstop filter
v1 1 0 ac 1 sin 
r1 1 2 200      
c1 2 0 2u       
r2 2 3 200      
c2 1 4 1u       
r3 4 0 100      
c3 4 3 1u       
rload 3 0 1k    
.ac lin 20 200 1.5k     
.plot ac v(3)   
.end    

freq       v(3)  1.000E-02  3.162E-02  1.000E-01    3.162E-01  
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
2.000E+02  5.400E-01 .         .           .             .      *.
2.684E+02  4.512E-01 .         .           .             .   *   .
3.368E+02  3.686E-01 .         .           .             . *     .
4.053E+02  2.946E-01 .         .           .            *.       .
4.737E+02  2.290E-01 .         .           .         *   .       .
5.421E+02  1.707E-01 .         .           .      *      .       .
6.105E+02  1.185E-01 .         .           . *           .       .
6.789E+02  7.134E-02 .         .       *   .             .       .
7.474E+02  2.832E-02 .        *.           .             .       .
8.158E+02  1.126E-02 .*        .           .             .       .
8.842E+02  4.796E-02 .         .    *      .             .       .
9.526E+02  8.222E-02 .         .         * .             .       .
1.021E+03  1.144E-01 .         .           . *           .       .
1.089E+03  1.447E-01 .         .           .   *         .       .
1.158E+03  1.734E-01 .         .           .      *      .       .
1.226E+03  2.007E-01 .         .           .       *     .       .
1.295E+03  2.267E-01 .         .           .         *   .       .
1.363E+03  2.515E-01 .         .           .          *  .       .
1.432E+03  2.752E-01 .         .           .           * .       .
1.500E+03  2.980E-01 .         .           .            *.       .
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

• REVIEW:

• A band-stop filter works to screen out frequencies that are within a certain range, giving easy passage only to frequencies outside of that range. Also known as band-elimination, band-reject, or notch filters.

• Band-stop filters can be made by placing a low-pass filter in parallel with a high-pass filter. Commonly, both the low-pass and high-pass filter sections are of the "T" configuration, giving the name "Twin-T" to the band-stop combination.

• The frequency of maximum attenuation is called the notch frequency.