bandpass Filters

Discussion in 'General Electronics Chat' started by Leighton, Mar 6, 2009.

  1. Leighton

    Thread Starter New Member

    Mar 6, 2009
    2
    0
    I can see how the low pass and high pass filters work. When it comes to bandpass, putting a low pass followed by a high pass. I would have thought that the low pass would only allow the low order frequencies to pass, how come then does the high pass have anything to filter? I know it works because I have built several to try it out.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The low-pass filter has a higher cutoff frequency than the high-pass filter.

    For example, you might build a filter with a low-pass cutoff frequency of 8kHz (attenuate everything above 8kHz), and a high-pass cutoff frequency of 1kHz (attenuate everything below 1kHz). The passband would then be 1kHz to 8kHz.
     
  3. flat5

    Active Member

    Nov 13, 2008
    403
    17
    It depends on the cutoff frequencies.
    If the low pass cuts off at 400hz you don't need a high pass filter that cuts off (the low end) at 1000hz.

    If you want a bandpass of, say, 300hz to 3000hz, (just to annoy AG) you make the low pass cut off at 3khz and the high pass cut off at 300hz. Sorta backwards from the way you were thinking? :)
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    Here I have tried to visualize what SgtWookie told you.

    [​IMG]

    Greetings,
    Bertus
     
  5. Leighton

    Thread Starter New Member

    Mar 6, 2009
    2
    0
    many thanks, got it now
     
  6. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    When you cascade the lowpass and highpass filters, you put the highpass at the input and follow it with the lowpass. The lowpass filter defines your high frequency roll off and the highpass filter defines your low roll on frequency. The result is a bandpass filter.
     
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