Building a RC filter

Discussion in 'General Electronics Chat' started by resilient, Mar 3, 2009.

  1. resilient

    Thread Starter Member

    Feb 2, 2009
    23
    0
    Hi all,

    I'm trying to build a low-pass RC filter, but I'm not sure if I've picked out the right values for the resistor and capacitor. I'm trying to extract the sine wave from a sine wave modulated PWM. The PWM switches at 20kHz and the sine wave is anywhere between 1 to 100 Hz. I want to use a 10k ohm resistor and a 20nF capacitor. Do these values sound ok?

    Thanks!
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    The formula for a RC filter is :

    [​IMG]

    If you use the 10K and 20 nF you will get a cutoff frequency of about 800 Hz.

    Greetings,
    Bertus
     
    gems_07 likes this.
  3. resilient

    Thread Starter Member

    Feb 2, 2009
    23
    0
    Right, so will 800Hz be sufficient to extract the sine wave?
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    A normal RC filter has a decay of 6 dB / octave.
    The 20 kHz will be damped with about 30 dB. (estimated).

    The resulting sinewave will be fair enough.

    Greetings,
    Bertus
     
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