Low pass filter

Discussion in 'General Electronics Chat' started by Peytonator, Oct 28, 2009.

  1. Peytonator

    Thread Starter Active Member

    Jun 30, 2008
    105
    3
    Hi,

    I have been given a task to design a low pass filter for a PWM based on these requirements:

    1) 10 kHz carrier
    2) 100 Hz base signal
    3) Less than 5% ripple from the carrier in the pass band.

    I'm lost on how to solve this. Where do I start? I do know that

    (V0/Vin)dB = -20log(1 + jwRC), using the Laplace definition.

    Thanks :)
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,163
    1,797
    There are several approaches that you could take. There are analog filters and there are digital filters. I'm guessing that we are looking at an analog filter here. The next thing is to try to describe the requirements in terms of what you expect to happen. That will tell you where the corner frequency might be placed. The number of elements (poles) will tell you how steep the rolloff is going to be. Then you must decide if this is going to be an active filter (involving operational amplifiers and possibly gain), or a passive filter with some insertion loss.

    Since you mention ripple in the passband, I going out on a limb here and thinking that the filter will be a passive filter of the Butterworth, Chebyshev or elliptical variety. These filters are usually designed from tables using a corner frequency, ω of 1 radian/sec or 0.159 Hz. Once the relative values of the components are obtained the results are scaled up to the actual frequency of interest. See below for an example

    http://www.crbond.com/papers/btf2.pdf
     
  3. Peytonator

    Thread Starter Active Member

    Jun 30, 2008
    105
    3
    Ok thanks :)
     
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