Filter corner frequency

Discussion in 'General Electronics Chat' started by atferrari, Apr 7, 2011.

  1. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
    Cut off / corner frequency.

    Thinking of a DAC, spiting, say 2000 values/sec and the associated LPF at the output:

    According to what I've read up to now, the filter should have a (-3dB) cut off frequency of 2KHz.

    My question: why from the beginning I accept to design with an atenuated output since my interest is to have the signal restored with the right level, that is with 0dB attenuation.

    In other words, shouldn't I design for a corner frequency where my highest signal of interest is not attenuated at all?

    I do not discard misunderstand things or plainly ignoring many.
    Last edited: Apr 7, 2011
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    With a 2kHz sample rate, the maximum signal frequency you can reconstruct is 1kHz, so a 2kHz filter (with a high enough order) will cause minimal attenuation.
  3. Papabravo


    Feb 24, 2006
    You can put the corner anywhere you want. It does not have to be an integer multiple of the highest frequency you want to reconstruct. In fact across the passband with an analog filter there will always be some attenuation depending on the type of filter and its order. The engineering part is to specify how much you are willing to accept.
  4. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
    Thanks to you both for replying.

    I omited to say I was oversampling with 2KHz rate a signal around 286 Hz.

    My first two tries with corner frequencies of 2000 Hz (4 poles) and 2250 HZ (6 poles) had awful results.

    Since I am leaving for a day of work, for the moment this is just food for thought: shouldn't have I tried for something like a corner frequency of 1000 Hz or maybe less?

    What PB says, You can put the corner anywhere you want, made me think that I moved it in the wrong sense, that is, TOO high a cut off.

    Will try when coming back afternoon. (Building a new filter for a new test every time IS a bit discouraging :().

    MOre and more I think of this, I feel that I did wrong.

    What do you think?
  5. t06afre

    AAC Fanatic!

    May 11, 2009
    What kind of signal do you try to reconstruct. The sample rate required to reconstruct a pure sine wave. Is very different from that is needed to reconstruct a square wave of the same frequecy. Can you post some signal snap shoot
    Last edited: Apr 9, 2011
  6. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
    For the first tests it is a sine wave.

    Could you briefly say the difference in sampling rate for sine-like waves, pulses and signals like music / voice?