Band-Stop Filters

Discussion in 'Filters' started by Tony R. Kuphaldt, Jun 24, 2010.

  1. Tony R. Kuphaldt

    Tony R. Kuphaldt

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    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. (Figure below)

    [​IMG]

    System level block diagram of a band-stop filter.

    Constructed using two capacitive filter sections, it looks something like (Figure below).

    [​IMG]

    "Twin-T" band-stop filter.

    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:

    [​IMG]

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

    [​IMG]

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

    Code:
    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    
    
    [​IMG]

    Response of "twin-T" band-stop filter.

    • 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.

     
    Tony R. Kuphaldt, Jun 24, 2010
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